Radial Compression & Buckling of Microtubules under Osmotic Stress: A New Mechanical Probe of Bio-nanotubes

C. R. SAFINYA, UC Santa Barbara, DMR-0503347

Microtubules (MTs) are model nanotubes, involved in a range of cellular functions including intracellular trafficking and cell division (see figure). We have developed a new biophysical method of probing the mechanical properties of individual microtubules. The actual experiments are deceptively simple: One adds a certain concentration of a large enough polymer (yellow in figure) that does not fit within the lumen of MTs to a solution of MTs. The difference in the polymer concentration between outside and inside of the MT wall creates an osmotic pressure which when large enough causes the MT to buckle (Fig. 2, 2nd from left). Conceptually this is similar to subjecting a hollow tube with closed ends to enormous hydrostatic pressure, like a submarine in deep sea. Here, we are doing it to a nano-scale tube.

The critical osmotic pressure for MT buckling, is a measure of the bond strength between neighboring protein units within the microtubule wall. (D. J. Needleman, et al., Biophys. J. 89, 3410, 2005)

Illustration of nanoscale microtubules (MTs) under osmotic pressure due to added polymer (yellow). Top and bottom are cross section and side views respectively. For osmotic pressures < 600 Pa, the MTs are undistorted (left). Above this pressure the MTs buckle to a noncircular cross-section and form bundles with rectangular symmetry (2nd from left). The MTs distort further as the osmotic pressure increases (3rd from left). At very high pressures > 25,000 Pa the polymer is forced inside the lumen of the MTs, and they are converted to hexagonal bundles of undistorted MTs as the pressure inside and outside the MT is equalized (right).

This new technique is broadly applicable in measuring the wall strength of individual bio-nanotubes, which form a large group of intensively studied materials in nanotechnological applications, including as chemical carrying tubules and as templates for nanowires.

We are currently probing the biomechanical properties of MTs coated with microtubule-associated-proteins (MAPs) to elucidate the role of MAPs in mechanically stabilizing tracks responsible for the trafficking of signaling molecules between neurons.

Phase Behavior and Interactions of Biomolecular MaterialsC. R. SAFINYA, UC Santa Barbara, DMR- 0503347

Education: Multidisciplinary teams comprised of physics, chemistry, biology, and materials science students and postdocs are educated in methods to discover nature’s rules for assembling the molecular building blocks in distinct shapes and sizes for particular functions. The learned concepts enable development of advanced nanoscale materials for broad applications in electronic, chemical, and pharmaceutical industries.

Outreach: Nate Bouxsein (materials science student), Kai Ewert (Project scientist & synthetic chemist), Chris McAllister (biology student), and Raha Shirazi (chemistry student) form an interdisciplinary group working on the physical, chemical, and biological aspects of the phase behavior of lipid-DNA complexes and its applications in gene delivery (Top photo, left to right). Nate mentored Lisa Boyer, the Science Department Head at Cuyama Valley High School (New Cuyama, CA) as part of a summer 2006 internship program in the PI’s group. Kai and postdoctoral MC Choi (from the Korean Advanced Institute of Science & Technology) are mentoring Heike Schirmer, a physics exchange Diploma (Masters) graduate student from the Technical University of Munich, on the phase behavior of lipid-DNA complexes (Middle photo). Kelsey Gorter (bottom photo, right), a 2005 Summer intern from Allan Hancock Community College (Internships in Nanosystems Science and Engineering Technology) joined UCSB as an undergraduate chemistry student and continued her research in the PIs group during the 2005-2006 school year. She and Mentor Jayna Jones (materials science student, bottom photo, left) studied the phase behavior of neurofilaments derived from nerve cells. The PI’s group in collaboration with Dr. Youli Li of the Materials Research Laboratories provides service to other campus researchers requiring x-ray diffraction.