αB-Crystallin-coated MAP microtubule resists nocodazole and calcium-induced disassembly

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Abstract
αB-Crystallin, one of the small heat-shock proteins, is constitutively expressed in various tissues including the lens of the eye. It has been suggested that αB-crystallin provides lens transparency but its function in nonlenticular tissues is unknown. It has been reported that αB-crystallin is involved in the stabilization and the regulation of cytoskeleton, such as intermediate filaments and actin. In this study, we investigate the possibility whether αB-crystallin interacts with the third cytoskeleton component, microtubules (MTs). First, we precisely observed the cellular localization of αB-crystallin and MT networks in L6E9 myoblast cells and found a striking coincidence between them. MTs reconstituted from cell lysate contained αB-crystallin. Electron micrographs clearly showed direct interactions of purified αB-crystallin with the surface of microtubule-associated proteins (MAPs) attached to MTs. Purified αB-crystallin bound to MAP-MTs in a concentration-dependent manner. However, αB-crystallin did not bind MTs reconstituted from purified tubulin. Finally, we observed that αB-crystallin increased the resistance of MTs to depolymerization in cells and in vitro. Taken together, these results suggest that one of the functions of αB-crystallin is to bind MTs via MAP(s) and to give the MTs resistance to disassembly.