Dynamic subunit exchange and the regulation of microtubule assembly by the stress response protein human alphaB crystallin.

BACKGROUND: The small heat shock protein (sHSP), human alphaB crystallin, forms large, polydisperse complexes that modulate the tubulin-microtubule equilibrium using a dynamic mechanism that is poorly understood. The interactive sequences in alphaB crystallin for tubulin are surface exposed, and correspond to interactive sites for the formation of alphaB crystallin complexes. METHODOLOGY/PRINCIPAL FINDINGS: There is sequence homology between tubulin and the interactive domains in the beta8-strand of the core domain and the C-terminal extension of alphaB crystallin. This study investigated the hypothesis that the formation of tubulin and alphaB crystallin quaternary structures was regulated through common interactive domains that alter the dynamics of their assembly. Size exclusion chromatography (SEC), SDS-PAGE, microtubule assembly assays, aggregation assays, multiple sequence alignment, and molecular modeling characterized the dynamic response of tubulin assembly to increasing concentrations of alphaB crystallin. Low molar ratios of alphaB crystallin:tubulin were favorable for microtubule assembly and high molar ratios of alphaB crystallin:tubulin were unfavorable for microtubule assembly. Interactions between alphaB crystallin and unassembled tubulin were observed using SEC and SDS-PAGE. CONCLUSIONS/SIGNIFICANCE: Subunits of alphaB crystallin that exchange dynamically with the alphaB crystallin complex can interact with tubulin subunits to regulate the equilibrium between tubulin and microtubules.