CELL BIOLOGY - CLUTCH CH. 13 - INTRACELLULAR PROTEIN...

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CELL BIOLOGY - CLUTCH

CH. 13 - INTRACELLULAR PROTEIN TRANSPORT

CONCEPT: MEMBRANE ENCLOSED ORGANELLES

Eukaryotic Cellular Components Function

Organelle (membrane bound)

Not an Organelle

1 Nucleolus Ribosome biogenesis

2 Nucleus Stores the genome; DNA/RNA synthesis

3 Ribosome Translation of mRNA/Protein creation

4 Vesicle Transportation within the cell

5 Rough Endoplasmic Reticulum Coated with ribosomes for protein synthesis and transportation

6 Golgi Apparatus Modifications, sorting, and packaging of proteins/lipids

7 Cytoskeleton Intermediate filaments, actin filaments, and microtubules used for support, transportation, and cellular division

8 Smooth Endoplasmic Reticulum Synthesizes lipids, phospholipids, and steroids and creates vesicles for transport

9 Mitochondria ATP synthesis by oxidative phosphorylation (powerhouse)

10 Vacuole Storage of nutrients, wastes, water, or other substances

11 Cytosol Aqueous fluid where metabolic pathways and chemical reactions occur

12 Lysosomes Intracellular degradation

13 Centrosome Microtubule organizing center

14 Cell Membrane Phospholipid bilayer separating the interior of the cell from its surroundings

NOT SHOWN IN DIAGRAM

Chloroplasts (plants) ATP synthesis and carbon fixation by photosynthesis

Peroxisomes Oxidation of toxic molecules



● Many organelles are held in place, or move throughout the cell using the cytoskeleton

□ Motor proteins can move them through energy provided from ATP hydrolysis

● Creation of organelles occurred through two main methods

□ Endosymbiont theory describes the formation of mitochondria and chloroplasts

□ All other organelles were created via invagination of the plasma membrane into small internal compartments

- Endomembrane system organelles including Golgi, ER, peroxisomes, endosomes and lysosomes

- Communication between these organelles occurs through small budding vesicles



PRACTICE:

1. True or False: Organelles must remain stationary within the cell

a. True

b. False

2. Which of these cellular components is not considered a membrane bound organelle?

a. Vacuole

b. Microtubules

c. Golgi apparatus

d. Chloroplast



CONCEPT: PROTEIN SORTING

● Proteins in cells are sorted through three mechanisms: gated transport, transmembrane transport, and vesicular transport

□ Sorting signals (15-60 amino acids long) located on the _________________________ direct its transport

- Signal peptidases remove sorting signals after the protein arrives at the intended location

- The protein remains in the cytosol if it lacks sorting signals

1. Gated Transport ● Gated transport refers to the transport of proteins and molecules between the cytosol and the nucleus

□ The nuclear pore is responsible for this type of transport

- Extends through the _____________________________ envelop

- Internal pore structure is a meshwork of unstructured regions that prevent large molecules from diffusing

□ Nuclear localization signals located on large molecules allow nuclear entrance or exit

- Signal is recognized by import receptors which facilitate moving through the pore using energy from GTP

EXAMPLE: Basic structure of a nuclear pore

2. Transmembrane Transport via Protein Translocators ● Protein translocators translocate proteins across ___________________________ membranes □ Translocates from the cytosol into the ER, mitochondria or chloroplasts

□ Proteins must unfold in order to be translocated into their new location



- Signal sequences direct the protein to its location

- Chaperone proteins within the organelle facilitate pulling the protein through and folding it once it’s arrived

EXAMPLE: Transport of unfolded protein in the mitochondria

3. Vesicular Transport ● Transport vesicles are small membrane enclosed compartments that move proteins throughout the cell □ Pinch off from one compartment and _______________________ with another

- EX: Leave the ER and fuse with the plasma membrane

□ Internal environment can hold many different types of proteins or molecules

- Soluble or membrane attached

EXAMPLE: Vesicular transport from the ER to the Golgi and the plasma membrane



PRACTICE: 1. Which of the following is not a type of transport?

a. Gated transport b. Organelle transport c. Vesicular transport d. Transmembrane transport

2. The nuclear pore is classified as which of the following types of transport? a. Gated transport b. Organelle transport c. Vesicular transport d. Transmembrane transport



CONCEPT: ER PROCESSING AND TRANSPORT

Two types of ER import

● Co-translational import is the process of ____________________ proteins into the ER as they’re being translated □ ER signal sequence directs the mRNA and ribosome to the ER during translation (located on N-terminus)

- Signal recognition particle (SRP) recognizes the ER signal sequence

- Signal recognition particle receptor (SRPR) is located on the ER and binds to the SRP

□ Translocon is a pore in the ER membrane that binds SRP and SRPR to translocate the protein into ER

- Uses GTP hydrolysis for energy

- Signal peptidase cleaves the ER signal sequence off once it is inside the ER

EXAMPLE: Co-translational import of a protein

● Post-translational import is the process of importing proteins into the ER _______________ they’ve been translated □ The chaperone protein BiP helps pull the protein across the membrane and fold it once its inside

● ER-retention signals are located on the C-terminus and keeps proteins within the ER EXAMPLE: Posttranslational import

Cytosol

ER Lumen

BiP



Inserting Proteins into the Membrane

● Single pass transmembrane proteins have one insertion in the ______________________________ □ Start transfer signal is a signal sequence which opens a translocon and the protein is translocated through it

- Stop transfer signal signals to anchor the protein in the membrane

- The ER signal sequence is then released from the translocon and cleaved off

□ Start transfer signal can be on the N-terminus or located within the middle of the polypeptide chain

EXAMPLE: Insertion of single pass transmembrane protein

● Multi pass transmembrane proteins have more than one insertion in the membrane □ Polypeptide chains contain multiple start and stop ______________________ sequences

- Order of sequence determines whether it is a start or stop sequence (the start sequence is first)

- Multiple start transfer sequence initiate translocation at different sections along the polypeptide chain

StartTransferSequence

StopTransferSequence

SignalPeptidase

COOH

NH3

Single PassTransmembrane Protein

Cytosol

ER Lumen



EXAMPLE: Insertion of a multi pass transmembrane protein

ER Protein Modifications

● The ER is a major hub of protein __________________________________ □ Glycosylation of proteins occurs in the ER

- Dolichol is a precursor oligosaccharide which is added to proteins for N-linked glycosylation

- Oligosaccharides are important tags to mark the state of protein folding – chaperons bind them

□ Glycosylphosphatidyl-inositol (GPI) anchor is added to proteins destined for the plasma membrane

- Can be easily cleaved off in order to release proteins into the extracellular environment

EXAMPLE: Glycosylation in the ER



Multi PassTransmembrane Protein

Cytosol

ER Lumen

NH3COOH



□ Protein disulfide isomerase assists with protein disulfide bond formation

□ Unfolded protein response detects misfolded proteins

- ER-associated degradation (ERAD) proteins recognize misfolded proteins and transports them to cytosol

EXAMPLE: Unfolded Protein Response in the ER



PRACTICE:

1. Match the following term with its definition I. Co-translational import ______________________

II. Post-translational import ______________________ III. ER retention signal ______________________ IV. Translocon ______________________

A. Pore in the ER membrane that binds SRP and SRPR to translocate the protein into ER B. Signal sequence located on the C-terminus and keeps proteins within the ER C. Process of importing proteins into the ER as they’re being translated D. importing proteins into the ER after they’ve been translated

2. Which of the following is responsible for recognizing the ER signal sequence? a. Signal Recognition Particle b. Signal Recognition Particle Receptor c. Translocon d. Stop Transfer Sequence



3. A protein contains 5 start/stop transfer sequences. How many times will this protein cross the membrane? a. 2 b. 5 c. 10 d. 3

4. Glycosylation of proteins in the ER is associated with which of the following molecules or responses? a. Unfolded protein response b. Protein disulfide isomerase c. Glycosylphosphatidyl-inositol (GPI) anchor d. Dolichol



CONCEPT: GOLGI PROCESSING AND TRANSPORT

Proteins Leaving the ER

● Proteins leaving the endoplasmic reticulum arrive at the Golgi complex (apparatus) □ Proteins are marked for exit by sorting signals, which sort them into special COPII vesicles (COPII protein coat)

□ Only properly _____________________________ proteins can exit the ER for the Golgi

- Chaperone proteins (BiP and calnexin) control folding

- Improperly folded proteins are transported out of the ER into the cytosol for proteasome degradation

□ Arrival at the Golgi depends on vesicle ____________________

- Heterotypic fusion: membrane fusion from two different compartments (ER vesicle fusing with Golgi)

- Vesicular tubular clusters: ER vesicles fuse together to create a big compartment that fuses with the Golgi

□ ER resident proteins contain retrieval sequences that direct them back to the ER if they leave

- KDEL sequence bind to KDEL receptors which direct them back to the ER via COPI vesicles (COPI coat)

EXAMPLE: Types of arrival at the Golgi

Golgi

Nucleus

EndoplasmicReticulum

Vesicular Tubular ClustersVesicular

HeterotypicFusion



Golgi Structure

● The Golgi Complex (apparatus) has a ________________ structure made up of flattened membrane-enclosed sacs □ Cisternae are the flattened membrane enclosed stacks that make up the Golgi

- The number of cisternae varies between 3-20 depending on the cell type

□ The Golgi is ordered into three sections

1. The cis-golgi faces the endoplasmic reticulum and takes in proteins from the ER

2. The trans-golgi (TGN) faces the plasma membrane and is the location of protein exit

3. The medial-golgi sits between the cis and trans faces, and is the place where proteins are modified

EXAMPLE: Structure of the Golgi complex



Glycosylation and Other Protein Modifications

● The Golgi complex is a major location where proteins are _________________________ □ Glycosylation or the addition of carbohydrates occurs in two forms on proteins

- N-linked glycosylation links an oligosaccharide to a nitrogen atom on an asparagine

- O-linked glycosylation links an oligosaccharide to a hydroxyl group on a serine or threonine

□ Protein glycosylation helps in protein folding and stability

EXAMPLE: Two forms of glycosylation

□ Terminal glycosylation is the final modification that removes carbohydrates. It occurs in the Golgi

- N-terminal glycosylation occurs in the ER with removal of glucose and mannose sugars in the Golgi

- Glycosyltransferases and glycosidases are the two main enzymes responsible for this modification

□ Each modification occurs in a different cisternae ________________________________ within the Golgi

- Functional differences between the three Golgi regions (cis, trans and medial)

EXAMPLE: Protein modifications occurring in the Golgi apparatus



Golgi Maturation and Protein Transport

● Molecules move through the Golgi in two main ways □ Vesicular transport model: Golgi cisternae are stationary, and proteins move via vesicles between golgi stacks

□ Cisternal maturation model: Golgi cisternae move upwards while maturing, disappearing due to vesicle budding

- Replaced by the cisternae below it

□ Evidence suggests proteins move using a combination of the two pathways

EXAMPLE: Two methods of protein travel through the Golgi

● Protein transport occurs in two ________________________________ □ Anterograde transport: Moves from ER through Golgi and towards the plasma membrane

□ Retrograde transport: Moves from the plasma membrane to the Golgi and then to the ER

□ Golgi acts as a sorting hub: recognizes sorting sequences and sorts them into vesicles for transport

- Protein receptors on the TGN bind to sorting sequences and trigger proper sorting and packaging



EXAMPLE: Anterograde vs. Retrograde Transport

PRACTICE:

1. Which of the following is not a method of Golgi transport? a. Retrograde transport b. Vesicular transport model c. Cisternal transport model d. Medial golgi transport

PlasmaMembrane

Golgi

EndoplasmicReticulum

Nucleus

AnterogradeTransport

RetrogradeTransport



2. Which side of the Golgi faces the endoplasmic reticulum? a. Cis-Golgi b. Trans-Golgi c. Medial-Golgi

3. True or False: Each Golgi cisternae matures by moving upwards through the Golgi. a. True b. False



4. Which of the following transport moves molecules from the plasma membrane to the Golgi? a. Cisternal transport b. Vesicular transport c. Retrograde transport d. Anterograde transport



CONCEPT: VESICULAR BUDDING, TRANSPORT, AND COATS

Overview

● Transport vesicles carry molecules between organelles and the plasma membrane □ Secretory pathway beings in the ER, moves to the Golgi, and ends at the cell surface

□ Endocytic pathway beings at the plasma membrane, and brings molecules into different organelles

EXAMPLE: Secretory pathway in the cell

Vesicular Coats

● Many times, vesicles bud from organelles or other cellular structures surrounded by a ___________________ coat □ There are three main types of vesicular coats

- Clathrin coated vesicles travel between the Golgi and plasma membrane

- COPI coated vesicles bud from the Golgi towards the ER

- COPII coated vesicles bud from the ER

EXAMPLE: Coated vesicle



□ Clathrin coats drive vesicular _______________________________________

- Adaptor proteins bind the clathrin, and transmembrane proteins that are being transported

- Cargo receptors are transmembrane proteins that capture soluble cargo

- Dynamin is a cytoplasmic protein that assembles a ring around the neck and uses GTP to pinch it off

EXAMPLE: Clathrin coated vesicle formation

□ GTPases regulate recruitment of _____________________ to the membrane

- A coat protein binds to a cargo molecule or adaptor – which triggers a GDP to GTP transition

- GTP activated coat protein associates with the membrane and recruits more coat proteins

- Rab proteins are GTPases that control specificity of coat proteins and specificity of transport

- GTP form is tightly associated with membranes

- Each vesicle contains unique combination of Rab proteins



EXAMPLE: Rab proteins in various organelles

SNARE Proteins and Vesicular Fusion

● SNARE proteins are responsible for catalyzing membrane fusion and providing specificity to membrane fusion □ There are two types of SNARES which reside within the _________________________

- T-snare has 2-3 target snares and resides on the target organelle.

- V-snare is 1 protein that resides on the vesicle

□ The two SNARES come together to form a four helix bundle (trans-SNARE complex)

- Causes fusion of the vesicle to the target membrane

□ To unravel the SNAREs energy from ATP is needed

- NSF (N-ethylmaleimide sensitive factor) releases V and T SNARES using ATP hydrolysis



EXAMPLE: SNARE protein and vesicular fusion

PRACTICE:

1. Which of the following is not a protein coat? a. COPI b. COPII c. COPIII d. Clathrin



2. Which of the following coats is used on vesicles that transport molecules from the Golgi to the ER? a. COPI b. COPII c. COPIII d. Clathrin

3. Vesicle fusion requires all but which of the following? a. T SNARES b. V SNARES c. Trans SNARE complex d. Rab GTP proteins



CONCEPT: TARGETING PROTEINS TO THE MITOCHONDRIA AND CHLOROPLASTS

● Proteins entering into the mitochondria and chloroplast have ____________ sequences that direct them to the organelle □ A chaperone protein (Hsc/Hsp70) carries the protein to the organelle

- ATP hydrolysis removes the protein so it can pass through the TOM protein

□ The TOM (TOC- for chloroplast) protein complex on the outer membrane recognizes a signal sequence

- The protein is unfolded and led into the intermembrane space

□ The TIM (TIC – for chloroplast) protein complex on the inner membrane recognizes a second signal sequence

- A H+ gradient drives the passage of the protein through the TIM complex

□ _______________________ proteins within the organelle bind and help it refold

EXAMPLE: Protein feeding through the TOM/TIM complexes to enter the mitochondria

● Proteins are directed to many ______________________________________ within the organelles □ There are many sub compartments within the mitochondria and chloroplasts

- Signal sequences direct the protein to each compartments

□ Some proteins need to insert into various organelle membranes

- Inserts the same way as other transmembrane proteins

EXAMPLE: Insertion into a thylakoid membrane

TOM/TOC

TIM/TIC

Stroma

Cytosol

Intermembranespace

Hsc/Hsp70

ATP ADP + Pi

H+

H+H+

H+H+

H+

H+

H+

H+

H+

H+

1.

2.

3.

4.Chaperone

Protein



PRACTICE: 1. True or False: Proteins must remain intact in order to cross the membrane and enter the mitochondria or

chloroplast? a. True b. False



SignalPeptidase

COOH

NH3

Stroma

ThylakoidLumen



2. The energy to insert proteins through the TIM/TIC complex comes from what reaction? a. ATP hydrolysis b. H+ gradient c. GTP hydrolysis d. Ca2+ influx into the mitochondria or chloroplast



CONCEPT: LYSOSOMAL AND OTHER DEGRADATIVE PATHWAYS

Lysosomal Characteristics and Sorting

● The lysosome contains many enzymes that ________________________ intracellular and extracellular materials □ The lysosomal lumen is extremely acidic

- ATP H+ pump controls the acidity by pumping protons into the lysosome

- Lysosomal membrane is protected from the acidity due to glycosylation on the lipids and proteins

□ Lysosomal acid hydrolases are enzymes within the lysosomal lumen that breakdown materials

- Around 50 types in each lysosome

EXAMPLE: Lysosome with a variety of lysosomal acid hydrolases

● Proteins destined for the lysosome must contain the appropriate ______________________ □ A mannose 6 phosphate tag is added onto lysosomal proteins in the Golgi

- This tag is recognized by lysosomal receptors in the Golgi

- Proteins with this tagged are sorted into transport vesicles destined for the lysosome

EXAMPLE: Mannose 6-phosphate tag directing proteins to the lysosome

LysosomeM6PR



Other Degradation Pathways

● Autophagy is the process of the cell “eating” itself by _______________________ large molecules or organelles □ Autophagosomes form by enclosing old organelles with a double membrane

- This large membrane fuses with the lysosome (autolysosome) for degradation

- Also called macrophagy (microphagy is a smaller vessel with a single bilayer)

EXAMPLE: Autophagosome and autolysosome formation

● Phagocytsosis is the process of the cell internalizing large particles from the extracellular environment for degradation □ Phagosomes form by enclosing the molecules with a ____________________________ membrane

- This large membrane fuses with the lysosome (phagolysosome) for degradation

EXAMPLE: Overview of Phagocytosis



PRACTICE: 1. Which of the following terms describes the cell eating itself?

a. Lysosomal degradation b. Autophagy c. Phagocytosis d. Pinocytosis

2. The mannose 6 phosphate tag is added onto proteins for what region? a. Targets them to the lysosome for their function b. Targets them to the lysosome for degradation c. Targets them for autophagosomes d. Targets them to the cytoplasm



3. Macrophagy differs from microphagy in which of the following ways? a. Macrophagy degrades proteins and microphagy degrades small molecules b. Macrophagy is involved in cell death, microphagy is involved with taking things into the cell c. Macrophagy uses two membranes, microphagy uses one membrane d. Macrophagy uses autophagosomes, microphagy uses lysosomes



CONCEPT: ENDOCYTIC PATHWAYS

Three types of endocytosis

● There are three methods of endocytosis which bring fluid or other molecules __________________ the cell □ Pinocytosis (cell drinking) brings fluid into cells

□ Phagocytosis (cell eating) brings large molecules into cells

- Phagosome fuse with lysosomes in order to degrade molecules

□ Receptor-mediated endocytosis is a highly specific way to bring molecules into cells

- Clathrin coated pits bind specific receptors which select cargo for entry (LDL and cholesterol)

- Caveolae (small invaginations of the plasma membrane) can also specifically bring molecules into the cell

EXAMPLE: Three types of endocytosis



Endosomal Sorting

● Endosomes are a large organelle sorting ______________ for a variety of materials entering and exiting the cell □ There are two types of endosomes: early and late

- Late endosomes are a more mature form of early endosomes

□ Multivesicular bodies form when invaginations from the endosomal membrane form internal vesicles

- Carries these internalized vesicles to the lysosome for degradation

EXAMPLE: Multivesicular body formation and transport to the lysosome

□ Endosomes are slightly ___________________ compartments which acts to release receptors from their cargo

- Recycling endosomes carry receptors back to the plasma membrane

- Cargo can travel to the lysosome or other organelles

- Transcytosis occurs If cargo remains attached to the receptor and it travels to a different cellular spot

- Cargo that remains bound to receptors ends up wherever the receptor ends up

EXAMPLE: Recycling endosomes and transcytosis

Recycling Endosome

Endosome

Transcytosis



Cholesterol (LDL) Uptake

● Receptor-mediated endocytosis is responsible for taking up _______________________ from the blood stream □ Low-density lipoproteins (LDLs) are the main form of cholesterol found in the blood

- LDL receptors are present on the plasma membrane

□ When the cell needs cholesterol it makes LDL receptors and places them in the plasma membrane

- These receptors then diffuse in the membrane until they come in contact with clathrin-coated pits

- LDL receptors then bind LDL which allows for interaction with adaptor proteins

- This triggers further formation of the clathrin coated pit and internalization of LDL cholesterol

□ High blood cholesterol develops in individuals with mutations in the LDL receptor

EXAMPLE: Diagram of LDL Uptake

LDL

LDLReceptor

Clathrin CoatedPit

Clathrin

Separation of LDL from LDLR

LDL ReceptorRecylcing



PRACTICE: 1. Which of the following is not an endocytic pathway?

a. Receptor-mediate endocytosis b. Pinocytosis c. Fluid uptake d. Phagocytosis

2. Which vesicle is responsible for carrying receptors back to the plasma membrane? a. Early endosomes b. Late endosomes c. Recycling endosomes d. Multivesicular bodies



3. High blood cholesterol develops when what happens to the LDL receptor? a. Too much LDL receptor is produced b. Too little LDL receptor is produced c. LDL binds the LDL receptor d. The cell takes in too much LDL in clathrin coated pits



CONCEPT: EXOCYTOSIS

● Exocytosis is the process of _______________________________ molecules outside of the cell □ Constitutive secretory pathway – is the default pathway that operates continuously

- Supplies the plasma membrane with newly synthesized lipids and proteins

□ Regulated secretory pathway occurs in specialized secretory cells that produce a lot of a released molecule

- Secretory vesicles store this molecule for later release

- Bud from Golgi, but dock near the plasma membrane for release

- Releases material in response to a chemical signal (calcium)

- Common in nerve cells

□ Microtubules ___________________ vesicles to the cell surface

EXAMPLE: Regulated secretory pathway in a neuron



● Exocytosis occurs in four steps 1. Vesicle carrying cargo moves towards the plasma membrane

- Polarized membranes have different domains

- Vesicles are targeted to one domain via C-terminal sorting signals

2. Vesicle fuses with the plasma membrane

3. Vesicular contents are released into the extracellular environment

4. Membrane fusion causes the membrane to become larger

- Endocytosis (molecules entering the cell) reduces membrane size

EXAMPLE: Vesicular fusion steps

1.

2.

3.



PRACTICE: 1. Which of the following is not a pathway of exocytosis?

a. Constitutive secretory pathway b. Regulated secretory pathway c. Continuing secretory pathway

2. When a vesicle fuses to the plasma membrane what happens to the lipids in the vesicles? a. They are released with the contents of the vesicle b. They are degraded c. They are incorporated into the plasma membrane



CONCEPT: PEROXISOMES

● Peroxisomes are organelles that contain _____________________ chemical reactions

□ Uses oxygen to produce hydrogen peroxide (H2O2)

- Catalase is the enzyme that uses H2O2 is then used to oxidize other toxic molecules (ex: ethanol)

□ Around 50 enzymes function in various biochemical pathways

- Lipid synthesis including plasmogens (phospholipids with glycerol attached with a ether bond)

- In plants it is the place of the glyoxylate cycle which converts fatty acids to carbohydrates

- Beta oxidation (breakdown of fatty acids) occurs here

EXAMPLE: Oxidizing toxic molecules using hydrogen peroxide

□ Peroxisomes are __________________________ in the ER

- Bud off and fuse with other buds to form new peroxisomes

□ SKL sorting signal exists on peroxin proteins that reside in the peroxisomes

- Defects in import of perixosomal proteins can cause severe diseases (Zellweger syndrome)



PRACTICE:

1. What is the function of a peroxisome? a. Maintain acidity of the cell b. Contain toxic reactions c. Protein synthesis d. Protein glycosylation



CONCEPT: PLANT VACUOLE

● Plant vacuoles are large, ___________________________ enclosed vacuoles with many functions □ Functions for the plant vacuole include:

- The membranes are acidified and can act similarly to lysosomes

- Maintain turgor pressure to prevent cellular collapse

- Regulate cytosolic pH through ATP dependent proton pumps

- Stores nutrients and other solutes

□ The plant vacuole is ______________ from a pro-vacuole (similar to an endosome) which matures to a vacuole

- All vacuole components are produced in the ER, and transferred to the Golgi for processing

- After processing, vesicles are released that eventually fuse to form the pro-vacuole

- Tonoplast is the membrane that bounds the vacuole

EXAMPLE: Overview of a plant cell



PRACTICE: 1. Which of the following is not a function of the plant vacuole?

a. Maintain turgor pressure b. Regulate cytosolic pH c. Support protein transfer in the plant cell d. Store nutrients and solutes



CELL BIOLOGY - CLUTCH CH. 13 - INTRACELLULAR PROTEIN...

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