XMAP215: a tip tracker that really moves

Abstract

XMAP215 is a microtubule plus-end binding protein implicated in modulating microtubule dynamics. In this issue, Brouhard et al. (2008) propose a new mechanism to explain how XMAP215 promotes microtubule growth. They report that XMAP215 moves with the growing microtubule plus ends where it catalyzes the addition of tubulin subunits.

Figure 1. Accelerating growth at microtubule plus ends and tip tracking

(A) The templating model for XMAP215 function. In this model, XMAP215 accelerates filament growth by acting as a template for the head-to-tail oligomerization of multiple tubulin dimers and enhancing delivery of these pre-assembled protofilaments to the microtubule tip (Gard and Kirschner, 1987; Kerssemakers et al., 2006; Slep and Vale, 2007). (B) Two classes of tip tracking behavior: treadmilling and surfing. During treadmilling, individual tip-binding proteins continuously bind to newly added tubulin subunits at a growing tip and later dissociate from the lattice, without moving along the microtubule. In principle treadmilling might also occur on disassembling tips, by continuous binding to tubulin subunits just before they detach from the filament, although this has not been observed. EB1 and its yeast homolog Mal3p appear to track growing tips by treadmilling (Tirnauer et al., 2002; Bieling et al., 2007). During surfing, the individual tip-binding proteins move with the assembling (or disassembling) tip. Brouhard and co-workers (2008) report that XMAP215 surfs on assembling and disassembling tips. In addition, they argue that XMAP215 accelerates microtubule growth not by templating, but rather by processively adding individual tubulin subunits to the ends of microtubules.