Phosphorylation of lens fiber cell membrane proteins.

Abstract

Two intrinsic membrane proteins of calf lens fiber cells can be phosphorylated by a soluble bovine lens cAMP-dependent protein kinase and rabbit muscle cAMP-dependent protein kinase. After electrophoresis of the phosphorylated membranes, 32P comigrates with the lens main intrinsic protein at 26-27 kDa [kilodalton] and with a minor band of protein that migrates at 19-20 kDa. 32P is also found with proteins that, based on the molecular sizes, are likely multimers of the 19-kDa and 26-kDa proteins. Upon boiling in NaDodSO4 [sodium dodecyl sulfate], all the radioactivity is found at the top of the gel, suggesting that both phosphoproteins are intrinsic membrane proteins. Ser is the only phospho amino acid detected in both proteins regardless of the source of protein kinase. The phosphorylation sites of both proteins are lost upon cleavage with trypsin and chymotrypsin. The smaller phosphoprotein is likely not a crystallin, because antibodies directed against .alpha.-, .beta.-, or .gamma.-crystallins do not cross-react with the 19-kDa protein. The 19-kDa 32P-labeled protein does not migrate coincident with calf .alpha.-crystallin.