Structural Protein 4.1 is Located in Mammalian Centrosomes

Structural protein 4.1 was first characterized as an important 80-kDa protein in the mature red cell membrane skeleton. It is now known to be a member of a family of protein isoforms detected at diverse intracellular sites in many nucleated mammalian cells. We recently reported that protein 4.1 isoforms are present at interphase in nuclear matrix and are rearranged during the cell cycle. Here we report that protein 4.1 epitopes are present in centrosomes of human and murine cells and are detected by using affinity-purified antibodies specific for 80-kDa red cell 4.1 and for 4.1 peptides. Immunofluorescence, by both conventional and confocal microscopy, showed that protein 4.1 epitopes localized in the pericentriolar region. Protein 4.1 epitopes remained in centrosomes after extraction of cells with detergent, salt, and DNase. Higher resolution electron microscopy of detergent-extracted cell whole mounts showed centrosomal protein 4.1 epitopes distributed along centriolar cylinders and on pericentriolar fibers, at least some of which constitute the filamentous network surrounding each centriole. Double-label electron microscopy showed that protein 4.1 epitopes were predominately localized in regions also occupied by epitopes for centrosome-specific autoimmune serum 5051 but were not found on microtubules. Our results suggest that protein 4.1 is an integral component of centrosome structure, in which it may play an important role in centrosome function during cell division and organization of cellular architecture.