Distribution of Caveolin–1 Isoforms in Bovine Ocular Lens Membrane Fractions

Abstract

Abstract: : Purpose: Epithelial and fiber cell membranes of the bovine ocular lens contain abundant caveolin–1. Our goal is to describe the isoforms of caveolin–1 in these cell compartments and the structural basis for the isoforms. Methods: Membrane protein fractions from cultured bovine lens epithelial cells (BLEC), fresh epithelium and cortical fiber cells were fractionated by 2–D electrophoresis and Western blotted with antibodies to caveolin–1, phosphoserine and phosphotyrosine. BLEC were also cultured in the presence of inorganic ³²P, caveolin–1 immunoprecitiated and recovered protein subjected to 2–D electrophoresis. Membranes were Western blotted with anticaveolin–1 antibody and ³²P detected by radioautography. Results: Twelve or more isoforms of alpha caveolin–1 (about 24 kDa) were detected in bovine fiber cell membrane. Beta–caveolin–1 (31 amino acids shorter, from the amino terminal, than the alpha form) was not evident. BLEC membrane appeared to contain fewer isoforms of alpha–caveolin–1, perhaps 4 to 6, and some beta caveolin–1 was detected. Antiphosphoserine antibody ,but not antiphosphotyrosine, reacted with several of the caveolin–1 isoforms from the cultured BLEC. A preliminary ³²P incorporation study indicated some labeling of immunoprecipitated BLEC caveolin–1. Conclusions: Based on these results, we hypothesize that caveolin–1 isoforms reflect variable phosphorylation of serine residues and speculate that differences in the isoforms expressed in membrane of lens epithelial cells and fiber cells reflect different functions for this protein in these cell populations.

Keywords: cell membrane/membrane specializations • protein structure/function • phosphorylation