Microtubule stabilization by assembly-promoting microtubule-associated proteins: a repeat performance.

Abstract

Microtubules (MTs) are important in a variety of dynamic cellular events such as chromosome segregation during meiosis and mitosis, vesicle and organelle transport, and organization of cellular components during morphogenetic events (e.g., polarization, directed migration, and process outgrowth). In addition, MTs are major components of many stable cellular structures such as ciliary and flagellar axonemes, axons and dendrites, and asymmetric processes in nonneuronal cells. The versatility that allows MTs to participate in such diverse functions may be conferred by several biochemical changes in the constituents of MTs; these include the expression of multiple a and p tubulins encoded by distinct genes [reviewed in Joshi and Cleveland, 19901, the posttranslational modification of a and p tubulin [reviewed in Joshi and Cleveland, 1990; Bulinski and Gundersen, 19911, and the expression or modification of various microtubule-associated proteins (MAPs). A critical feature of MT function is the balance between two antithetical properties of MTs: stability and instability. Stability allows MTs to act as a skeletal framework, while instability allows plasticity in the organization of this framework. MT stabilization is of undisputed importance in the generation and maintenance of asymmetries in differentiating cells such as neurons, myoblasts, and epithelia [reviewed in Bulinski and Gundersen, 19911. However, inappropriate or excessive stabilization of MTs can be deleterious to cellular structure and function. Evidence has accumulated that implicates MAPs as major effectors of MT stability in vivo. Clearly, an understanding of the complexity of MT functions will require elucidation of the role of MAPS in the modulation of MT dynamics.