Synemin and vimentin are components of intermediate filaments in avian erythrocytes

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Abstract
Synemin, a high-MW protein associated with intermediate filaments in muscle, and vimentin, an intermediate-filament subunit found in many different cell types, were identified by immunologic and electrophoretic criteria as components of intermediate filaments in mature white leghorn chicken erythrocytes. Desmin, the predominant subunit of intermediate filaments in muscle, was not detected in these cells. Two-dimensional immunoautoradiography of proteolytic fragments of synemin and vimentin demonstrates that the erythrocyte proteins are highly homologous, if not identical, to their muscle counterparts. Double immunofluorescence reveals that erythrocyte synemin and vimentin co-localize in a cytoplasmic network of sinuous filaments that extends from the nucleus to the plasma membrane and resists aggregation by Colcemide. Erythrocytes attached to glass cover slips can be sonicated to remove nuclei and nonadherent regions of the plasma membrane; this leaves elliptical patches of adherent membrane that retain mats of vimentin- and synemin-containing intermediate filaments, as seen by immunofluorescence and rotary shadowing. Similarly, mechanical enucleation of erythrocyte ghosts in suspension allows isolation of plasma membranes that retain a significant fraction of the synemin and vimentin, as assayed by electrophoresis, and intermediate filaments, as seen in thin sections. Both synemin and vimentin remain insoluble, along with spectrin and actin, in solutions containing nonionic detergent and high salt. However, brief exposure of isolated membranes to distilled water releases the synemin and vimentin together in nearly pure form, before the release of significant amounts of spectrin and actin. Avian erythrocyte intermediate filaments apparently are somehow anchored to the plasma membrane; erythrocytes may thus provide a simple system for the study of intermediate filaments and their mode of interaction with membranes. Synemin apparently is capable of associating with either desmin or vimentin and may thus perform a special role in the structure or function of intermediate filaments in erythrocytes as well as in muscle.