Molecular interactions in intermediate-sized filaments revealed by chemical cross-linking. Heteropolymers of vimentin and glial filament protein in cultured human glioma cells.

Certain glia cells, notably astrocytes and tumor cells derived therefrom, express simultaneously two types of proteins of intermediate-sized filaments, vimentin and glia filament protein (GFP). We have used an established human glioma (astrocytoma) cell culture line (U 333 CG/343 MG) in which both proteins are seen in partly overlapping fibrillar structures by immunofluorescence microscopy, to examine the possible existence of heteropolymer filaments of these two proteins by using reversible oxidative cross-linking facilitated by the 1,10-phenanthroline-cupric ion complex. Dimeric cross-link products are characterized by one-dimensional and two-dimensional gel electrophoresis under non-reducing and reducing conditions as well as by peptide mapping. The relatively large proportions of heterodimers of vimentin and GFP obtained in cytoskeletal filaments cross-linked in this way, demonstrate the frequency of heteropolymer filaments in this cell as well as the frequency of face-to-face 'pairs' of GFP and vimentin in such filaments. Together with our related observations on heteropolymer filaments between vimentin and desmin in some smooth muscle cells [Quinlan, R. A. and Franke, W. W. (1982) Proc. Natl Acad. Sci. USA, 79, 3452-3456], we discuss this as evidence for common principles of molecular arrangements of vimentin, GFP and desmin, at least in the cysteine-containing surface domains. The results are also discussed in relation to cytoskeletal changes during glial differentiation.