Molecular characteristics and interactions of the intermediate filament protein synemin - Interactions with alpha-actinin may anchor synemin-containing heterofilaments

Synemin is a cytoskeletal protein originally identified as an intermediate filament (IF)-associated protein because of its colocalization and copurifi cation with the IF proteins desmin and vimentin in muscle cells. Our sequen cing studies have shown that synemin is an unusually large member (1,604 re sidues, 182,187 Da) of the IF protein superfamily, with the majority of the molecule consisting of a long C-terminal tail domain. Molecular interactio n studies demonstrate that purified synemin interacts with desmin, the majo r IF protein in mature muscle cells, and with cy-actinin, an integral myofi brillar Z-line protein. Furthermore, expressed synemin rod and tail domains interact, respectively, with desmin and a-actinin. Analysis of endogenous protein expression in SW13 clonal lines reveals that synemin is coexpressed and colocalized with vimentin Ifs in SW13.C1 vim+ cells but is absent in S W13.C2 vim - cells. Transfection studies indicate that synemin requires the presence of another IF protein, such as vimentin, in order to assemble int o Ifs. Taken in tote, our results suggest synemin functions as a component of heteropolymeric Ifs and plays an important cytoskeletal cross-linking ro le by linking these Ifs to other components of the cytoskeleton. Synemin in striated muscle cells may enable these heterofilaments to help link Z-line s of adjacent myofibrils and, thereby, play an important role in cytoskelet al integrity.