Coexistence of desmin and the fibroblastic intermediate filament subunit in muscle and nonmuscle cells: identification and comparative peptide analysis.

Extraction of chicken embryo fibroblasts (CEF) or baby hamster kidney (BHK) cells with 1% Triton X-100 and 0.6 M KCl leaves an insoluble cytoskeletal residue composed primarily of the 52,000 Mr subunit of intermediate filaments (F-IFP). In addition, CEF cytoskeletons exhibit a minor component with Mr of 50,000, identified as alpha-desmin, one of the two major isoelectric variants of the intermediate filament subunit from smooth muscle. BHK cytoskeletons contain the 50,000 Mr mammalian desmin variant. Cytoskeletons prepared from chicken embryonic myotubes contain F-IFP and both alpha- and beta-desmin. These data suggest that two distinct 10-nm filament subunits coexist in a single cell. One-dimensional peptide analysis of F-IFP and desmin from avian and mammalian cells reveals significant interspecies homology, as well as homology between F-IFP and desmin from the same species. Peptide analyses of 32P-labeled intermediate filament subunits suggest that there is considerable similarity in the phosphorylation sites of these proteins. These results indicate that F-IFP and desmin might be evolutionally related.