Building the Microtubule Cytoskeleton Piece by Piece

Abstract

The microtubule (MT) cytoskeleton gives cells their shape, organizes the cellular interior, and segregates chromosomes. These functions rely on the precise arrangement of MTs, which is achieved by the coordinated action of MT-associated proteins (MAPs). We highlight the first and most important examples of how different MAP activities are combined in vitro to create an ensemble function that exceeds the simple addition of their individual activities, and how the Xenopus laevis egg extract system has been utilized as a powerful intermediate between cellular and purified systems to uncover the design principles of self-organized MT networks in the cell.

Keywords: cell division; cytoskeleton; microtubule; microtubule-associated protein (MAP); mitotic spindle.

FIGURE 1.

Organization of the microtubule cytoskeleton by MAPs. A, representative classes of MAP activities are depicted schematically. B, different combinations of MAPs drive the self-organization of MTs into functional networks, which determine the organization of the cell, differ between cell cycles states, and change during cell differentiation.

FIGURE 2.

In vitro reconstitutions of microtubules and more than one MAP. A, C, E, and G depict in vitro reconstitutions with the MT depolymerase MCAK. B, F, H, and J display in vitro studies with the MT polymerase XMAP215. D, E, F, H, I, and J show how EBs modulate MT dynamics by recruiting factors to the MT plus-end. Each of the in vitro studies (A–J) are discussed in the main text.

FIGURE 3.

Spindle scaling. A and B, different regulation of katanin activity has been traced to make X. tropicalis spindles smaller than X. laevis spindles. TPX2 concentrations modulate the spindle architecture and lead to a concentration of MTs the poles of the X. tropicalis spindle. B and C, spindle length of the X. laevis spindle is determined by MT growth velocity, which can be modulated via XMAP215 activity. X. laevis spindles become smaller throughout development and between stage 3 and stage 8; the observed size difference can be contributed to differential regulation of kif2a.