CHAPERONE ACTIVITY OF ALPHA-CRYSTALLINS MODULATES INTERMEDIATE FILAMENT ASSEMBLY

Intermediate filaments are generally regarded as one of the most insol uble and resilient cytoskeletal structures of eukaryotic cells. In ext racts from the ocular lens, we noticed an unusually high level of vime ntin in a soluble, non-filamentous form. Immunoprecipitation of this s oluble 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, ca pable 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 t his inhibition. Furthermore, we show that alpha-crystallins can increa se the soluble pool of GFAP when added to preformed filaments. Electro n microscopy demonstrated that alpha-crystallin particles could bind t o 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 remodelling of intermediate filaments during development and cell differentiation.