Association of an Mr 50,000 cap binding protein with the cytoskeleton in BHK cells

A monoclonal antibody (anti-CBP antibody) is shown to be directed against cap binding protein(s) (CBP) by virtue of its ability to inhibit the translation of capped reovirus mRNA in a cell-free system derived from L-cells and inhibit the specific (cap analogue-inhibited) cross-linking of proteins to the oxidized 5' terminal cap structure of reovirus mRNA. Anti-CBP antibody reacts with an Mr 50,000 polypeptide in rabbit reticulocyte polysomes and this polypeptide appears to be associated with the 5' cap structure of mRNA. In BHK-21 cells immunofluorescence microscopy reveals that the antibody reacts with a fibrous network extending through the cytoplasm in a radial arrangement. The network behaves like intermediate filaments in colchicine-treated cells suggesting a direct or indirect linkage of CBP with intermediate filaments. The association of CBP with a cytoskeletal element is further confirmed by isolation of proteins from Triton X-100-extracted cells and identification of CBP in the cytoskeletal fraction with anti-CBP antibody. The major polypeptide reacting with anti-CBP antibody is an Mr 50,000 component. Tryptic peptide mapping shows that this polypeptide is related to an Mr 24,000 polypeptide identified as cap binding protein in earlier experiments [Sonenberg, Morgan, Merrick & Shatkin (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4843-4847].