Coassembly of bovine and cod microtubule proteins: the ratio of the different tubulins within hybrid microtubules determines the ability to assemble at low temperatures, MAPs dependency and effects of Ca2+.

Abstract

Cod and bovine microtubule proteins (MTP) differ from each other in many respects, e.g., tubulin isoforms and microtubule-associated proteins (MAPs) but only cod MTP are cold-adapted. We used these differences to determine how tubulin isoform composition affects microtubule properties. Mixtures of cod and bovine MTP coassembled at 30 degrees C as shown by light scattering and immunoelectron microscopy, with no apparent preference for one set of MAPs over the other. Bovine tubulin was, in contrast to cod tubulin, unable to assemble in the absence of MAPs, while 50%/50% mixtures of bovine and cod tubulin, respectively, coassembled readily without exclusion of cod or bovine tubulin isoforms in the hybrids, as shown by two-dimensional gel electrophoresis. Alteration in MAPs dependency was also confirmed by the use of the MAPs-binding microtubule inhibitor estramustine phosphate. Addition of 10 mM Ca2+ to microtubules induced formation of spirals or rings depending on the ratio of the cod and bovine MTP, respectively. Bovine MTP were unable to assemble at low temperatures, while cod MTP are cold-adapted and assembled efficiently at 14 degrees C in the presence of MAPs. Amounts of cod MTP as low as 33% were enough to induce assembly of bovine/cod MTP hybrids. The critical concentration for assembly of a 50%/50% mixture was similar to that of 100% cod MTP. Taken together, the results show that the divergent cod and bovine MTP can coassemble, and that alterations in tubulin isotype/isoform composition above certain thresholds significantly modulate microtubule properties such as MAPs dependency, effects of Ca2+, and ability to assemble at low temperatures.