A novel intermediate filament-associated protein: further characterization of the G.3.5 antigen.

Several novel intermediate filament-associated proteins (IFAPs) have been discovered using hybridoma technology on multiple sclerosis plaque tissue. One of these, designated the G.3.5 antigen, is a desmin-binding IFAP in skeletal and cardiac muscle. It has been suggested that because of sequence similarities to alpha-actinin the G.3.5 antigen is an alpha-actinin-like protein which may cross-link actin and intermediate filaments in the cytoskeleton. In this study, it is reported that (1) the G.3.5 antigen is present in hepatocytes in addition to the previously described astrocytes, skeletal and cardiac myocytes, fibroblasts, and several other non-nervous tissues; (2) in myocytes and hepatocytes, the G.3.5 and alpha-actinins do not co-localize; (3) by transmission electron microscopy the G.3.5 antigen appears to be a rod-shaped dimer similar to alpha-actinin; (4) isolation of the G.3.5 antigen does not simultaneously isolate alpha-actinin; and (5) limited proteolysis of the G.3.5 antigen and alpha-actinin generates dissimilar maps. In binding studies, alpha-actinin cross-links actin but has no effect on desmin; the G.3.5 antigen does not appear to cross-link actin, desmin or mixtures of both under the assay conditions. These results support the hypothesis that the G.3.5 antigen is a novel IFAP related to alpha-actinin, but do not support a role for the antigen as a cross-linker between intermediate filaments and actin.