C E L L B I O L O G YP H D C O U R S E W O R K .

P R E S E N T E D B Y

JAHIR AHMED

Protein Transport, Targeting and Sorting.

10-9Folding in the Endoplasmic

reticulum

COP 2 TRANSPORT VESICLE.

Toc components, mediate

translocation (Toc75 is the

translocon); it is unclear how

preproteins are targeted to the

channel; Hsp70/Hsp40 may be

involved

Hsp70 in both the IMS and the

stroma assist the threading of the

preprotein into the chloroplast

an Hsp100 chaperone also called

ClpC (AAA ATPase) also binds

preproteins in the stroma

Hsp70/chaperonin (Cpn60) may

assist folding/assembly of newly-

imported protein

import into thylakoids (used for

respiration) uses the SRP pathway

10-9

Translocation into chloroplasts

targeting of proteins is initiated

post-translationally by Pex5/7

proteins, which bind the peroxisomal

targeting signal (PTS)

translocon not well defined;

possibility of vesicular budding?

gated pore that is regulated by

membrane proteins?

first organelle demonstrated to

import proteins without a PTS, by

virtue of assembly with other

proteins that contained a PTS

various protein oligomers are

imported into peroxisomes

antibodies with PTS, and 9 nm

gold particles could be imported

Other transport mechanisms likely involve

folded proteins, including the twin-arginine (Tat)

transport system of bacteria, and the cytoplasm-

to-vacuole targeting pathway of yeast

10-10

Translocation into peroxisomes

nuclear localization signal (NLS) is typically highly basic; e.g., the SV40 large tumor

antigen (T ag) has the sequence PKKKRKV

a/b1 importin hetero-dimer recognizes and binds the NLS (or b importin alone)

b importin docks with NPC and mediates interaction with Ran (GDP form)

directionality conferred by nature of guanine nucleotide bound to Ran

Ran binding protein (RanBP) is required for b importin binding to RanGDP; Ran

GTPase activating protein (RanGAP) and nucleotide-exchange factor (RCC) are

cytoplasmic and nuclear

cytopl. RanGDP required for import; nuclear RanGTP required for release

conversely, RanGTP binds substrate with NES in the export direction

proteins to be imported can be in a native/near native form

10-12

Import into the nucleus

some nuclear pore proteins (nucleoporins)

contain core FxFG repeats (yellow)

b importin contains ‘heat’ repeats that bind the

FxFG repeats (Heat repeats 5, 6, 7 are shown in

red, green and blue)

the FxFG repeats interdigitate in grooves formed

by the Heat repeats

interaction of b importin with nucleoporins allows

transport across the nuclear pore complex

Core FxFG repeats found in nucleoporins.

Each repeat is separated by a ‘linker’ region:

Bayliss et al. (2000) Cell 102, 99-108.

10-14

Mechanism of import into nucleus

Oligosaccharide processing in Golgi compartments

Protein Targeting in Golgi

Protein Secretion Pathway (E.R – Golgi and Vice Versa)

Thank You…