Abstract
The complete amino acid sequence of a 55-kDa erythrocyte membrane protein was deduced from cDNA clones isolated from a human reticulocyte library. This protein, p55, is copurified during the isolation of dematin, an actin-bundling protein of the erythrocyte membrane cytoskeleton. Fractions enriched in p55 also contain protein kinase activity that completely abolishes the actin-bundling property of purified dematin in vitro. The predicted amino acid sequence of p55 does not contain any consensus sequence corresponding to the catalytic domains of protein kinases but does contain a conserved sequence found in the noncatalytic domains of oncogene-encoded tyrosine kinases. This conserved src homology 3 (SH-3) motif appears to suppress the tyrosine kinase activity of various oncoproteins and has also been found in several plasma membrane associated proteins involved in signal transduction. Northern blot analysis indicated that p55 mRNA was constitutively expressed during erythropoiesis and underwent 2-fold amplification after induction of K562 erythroleukemia cells toward the erythropoietic lineage. The abundant expression of p55 mRNA, along with protein 4.1 mRNA, was evident in terminally differentiated human reticulocytes. Although p55 has many features consistent with known peripheral membrane proteins, its tight association with the plasma membrane is reminiscent of an integral membrane protein. This fact may be partly explained by the observation that p55 is the most extensively palmitoylated protein of the erythrocyte membrane.