Dynamics of A-Lattice Microtubules

Microtubules are intrinsically dynamic structures whose polymerisation is subject to extensive spatial and temporal control in cells, partly through the activity of microtubule-associated proteins. Microtubules can potentially assemble with two different lattice arrangements of heterodimers. Lateral contacts of heterodimer subunits may be either alpha-beta, making an A-lattice, or alpha-alpha and beta-beta, forming a B-lattice. 13-protofilament B-lattice microtubules contain a single seam of A-lattice contacts. A-lattice microtubules are composed purely of these seams. Since microtubules assembled in vitro have predominantly the B-lattice arrangement, it has not previously been possible to study the dynamics of the A-lattice. We recently found that Mal3, the EB1-homologue in S. pombe, stabilises the A-lattice and promotes assembly of 13-protofilament A-lattice-containing microtubules. We are now analysing the dynamics of Mal3-induced A-lattice microtubules in vitro. We measured the gliding velocity and the shrinkage rate of GMPCPP stabilised A-lattice and B-lattice microtubules during gliding on a rat kinesin-1 coated-glass surface in the absence of free tubulin. The gliding velocity of A-lattice microtubules was similar to that of B-lattice microtubules. On the other hand, the plus end of A-lattice microtubules shrank 20 times faster than the plus end of B-lattice microtubules, and 10 times faster than the minus end of A-lattice microtubules. This suggests that tubulin heterodimer subunits dissociate faster from the A-lattice than from the B-lattice and that the A-lattice is therefore less stable than the B-lattice. The A-lattice seam in B-lattice microtubules may therefore be a zone of unusual structural weakness that would provide an opportunity for regulating microtubule dynamics via MAPs that suppress the dissociation of subunits from the A-lattice.