Chaperone activity of alpha-crystallins modulates intermediate filament assembly.

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Abstract
Intermediate filaments are generally regarded as one of the most insoluble and resilient cytoskeletal structures of eukaryotic cells. In extracts from the ocular lens, we noticed an unusually high level of vimentin in a soluble, non‐filamentous form. Immunoprecipitation of this soluble vimentin resulted in the co‐precipitation of alpha‐crystallins. The alpha‐crystallins are homologous to the small heat shock proteins (sHSPs) and have recently been identified as molecular chaperones, capable of preventing the heat‐induced aggregation of proteins. We find that the alpha‐crystallins dramatically inhibit the in vitro assembly of GFAP and vimentin in an ATP‐independent manner. This inhibition is also independent of the phosphorylation state of the alpha‐crystallin polypeptides and each one of the four polypeptides, either alpha A1‐, alpha A2‐, alpha B1‐ or alpha B2‐crystallin, are equally effective in this inhibition. Furthermore, we show that alpha‐crystallins can increase the soluble pool of GFAP when added to preformed filaments. Electron microscopy demonstrated that alpha‐crystallin particles could bind to intermediate filaments in a regular fashion, the spacing coinciding with the molecular length of GFAP. This is the first report, as far as we are aware, of a chaperone being involved in intermediate filament assembly and implicates chaperones in the remodeling of intermediate filaments during development and cell differentiation.