title: The effect of multivalent cations and Tau on paclitaxel-stabilized microtubule assembly, disassembly, and structure.
abstract: In this review we describe recent studies directed at understanding the formation of novel nanoscale assemblies in biological materials systems. In particular, we focus on the effects of multivalent cations, and separately, of microtubule-associated protein (MAP) Tau, on microtubule (MT) ordering (bundling), MT disassembly, and MT structure. Counter-ion directed bundling of paclitaxel-stabilized MTs is a model electrostatic system, which parallels efforts to understand MT bundling by intrinsically disordered proteins (typically biological polyampholytes) expressed in neurons. We describe studies, which reveal an unexpected transition from tightly spaced MT bundles to loose bundles consisting of strings of MTs as the valence of the cationic counter-ion decreases from Z=3 to Z=2. This transition is not predicted by any current theories of polyelectrolytes. Notably, studies of a larger series of divalent counter-ions reveal strong ion specific effects. Divalent counter-ions may either bundle or depolymerize paclitaxel-stabilized MTs. The ion concentration required for depolymerization decreases with increasing atomic number. In a more biologically related system we review synchrotron small angle x-ray scattering (SAXS) studies on the effect of the Tau on the structure of paclitaxel-stabilized MTs. The electrostatic binding of MAP Tau isoforms leads to an increase in the average radius of microtubules with increasing Tau coverage (i.e. a re-distribution of protofilament numbers in MTs). Finally, inspired by MTs as model nanotubes, we briefly describe other more robust lipid-based cylindrical nanostructures, which may have technological applications, for example, in drug encapsulation and delivery.
authors.affiliation: Materials Department, University of California, Santa Barbara, CA 93106, USA; Physics Department, University of California, Santa Barbara, CA 93106, USA; Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106, USA. Electronic address: safinya@mrl.ucsb.edu.
authors.forename: Cyrus R
authors.initials: CR
authors.lastname: Safinya
authors.affiliation: Materials Department, University of California, Santa Barbara, CA 93106, USA; Physics Department, University of California, Santa Barbara, CA 93106, USA; Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106, USA.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Chung
authors.affiliation: Materials Department, University of California, Santa Barbara, CA 93106, USA; Physics Department, University of California, Santa Barbara, CA 93106, USA; Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106, USA.
authors.forename: Chaeyeon
authors.initials: C
authors.lastname: Song
authors.affiliation: Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA.
authors.forename: Youli
authors.initials: Y
authors.lastname: Li
authors.affiliation: Materials Department, University of California, Santa Barbara, CA 93106, USA; Physics Department, University of California, Santa Barbara, CA 93106, USA; Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106, USA.
authors.forename: Kai K
authors.initials: KK
authors.lastname: Ewert
authors.affiliation: Department of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.
authors.forename: Myung Chul
authors.initials: MC
authors.lastname: Choi
mesh_terms: D000818:Animals; D006801:Humans; D008870:Microtubules / Q000737:chemistry*; D017239:Paclitaxel; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction; D016875:tau Proteins / Q000737:chemistry*
chemical_list: D016875:tau Proteins; D017239:Paclitaxel
keywords: Correlated ion density fluctuations; Intrinsically disordered proteins; Ion specific effects; Microtubule; Microtubule-associated protein Tau; Multivalent counter-ions