Microtubule-Associated Proteins and Their Essential Roles During Mitosis

Abstract

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

Introduction

Mitosis is a fundamental biological process that has captured the imagination of researchers for more than a century. During this process, the previously duplicated genome is structurally reorganized into compact chromosomes, each made up of two sister chromatids, which are then equally segregated into daughter cells. Mitosis was first described by Walter Flemming in the late 1870s and consists of two distinct processes: division of the nucleus, or karyokinesis, and division of the cytoplasm, or cytokinesis. Traditionally, the division of the nucleus occurs in five stages: prophase, prometaphase, metaphase, anaphase, and telophase. In prophase, chromatin starts to condense, to form clearly defined chromosomes (Fig. 1). By this time, centrosomes, which are the major microtubule-organizing centers (MTOCs) in animals (Doxsey, 2001), have already migrated to opposite sides of the nucleus, and a bipolar microtubule-based structure, known as the mitotic spindle, is assembled (Fig. 2). This structure is responsible for chromosome segregation during mitosis (Wittmann et al., 2001). In vertebrates, the end of prophase and the beginning of prometaphase are marked by nuclear envelope breakdown (NEB). During prometaphase, each chromosome initially orients to one pole of the spindle, a process that is mediated by attachment of microtubules (MTs) to the kinetochore, a protein-based structure located on the surface of the primary constriction of each chromatid. Subsequently, MTs derived mostly from the opposite pole bind the other sister kinetochore, and the chromosome becomes bioriented, moving toward the center of the cell and aligning at the spindle equator during metaphase. Later, during anaphase, sister chromatids separate and migrate toward opposite poles of the spindle. This stage can be subdivided into anaphase A and B. During anaphase A, the two chromatids from each chromosome lose their cohesion, split apart, and move toward opposite poles, while during anaphase B, the spindle usually elongates, thereby increasing the distance between the two poles. Finally, during telophase, each set of chromatids decondenses to form two daughter nuclei, and a cleavage furrow forms between them. This cleavage furrow then contracts and eventually gives rise to the midbody, a structure formed by bundled MTs. This structure participates in the division of the cytoplasm and the formation of the two daughter cells during cytokinesis.

MTs are highly dynamic polymers that alternate between states of growth and shrinkage, a phenomenon known as dynamic instability (Mitchison and Kirschner, 1984b). Modulation of MT's dynamic behavior is thought to be due primarily to association with a broad class of proteins known as microtubule-associated proteins (MAPs). However, the definition of MAPs has changed significantly since the time the first of these proteins were isolated (Olmsted, 1986). MAPs were originally identified as proteins that copurified with tubulin through repeated cycles of MT polymerization and depolymerization and were shown to stimulate MT assembly, suggesting a possible role for them in stabilizing the polymer. Subsequently, it was proposed that MAPs be defined as proteins that bind MTs in vivo (Solomon et al., 1979). Since spindle organization and function are now known to be governed by many different types of proteins that associate transiently or constitutively with MTs and can modify their dynamic properties, we have chosen a broader definition for MAPs, to include all of those proteins that are found at least transiently associated with MTs, either in vitro or in vivo, regardless of their functional, regulatory, or motility properties. Of course, we have to be careful, when considering a protein to be a MAP according to this definition, to exclude all of the proteins that may bind to MTs indirectly but yet play no role in modulating MT properties or function.

In this review we will first provide a short description of the major components of the mitotic apparatus and the major mitotic events. We will then focus on the roles of various MAPs as the cell transits from interphase into mitosis (see Table I for general references), including (1) organization and maintenance of the bipolar spindle, (2) attachment of MTs to kinetochores, (3) chromosome movement, and (4) cleavage-furrow formation during cytokinesis. The results obtained to date reveal that MAPs are essential to coordinately regulate the dynamic properties of MTs and their interactions with various cellular structures so that MTs can effectively be used by the cell to move chromosomes during mitosis.