The Characterization of Pig Platelet Membrane Proteins: Studies on Membrane-Associated Actin and the Surface-Associated Phosphodiesterase Activity

Abstract

Analysis of the polypeptides of a pig platelet surface membrane fraction by SDS-polyacryl-amide gel electrophoresis revealed the presence of approx. 12 components (molecular weight range 12000–200000).A component of 43–45,000 daltons was particularly prominent and this has been identified as actin, and shown to be similar in many respects to the cytoplasmic protein. The membrane-associated actin has been isolated as a pure band by preparative SDS-polyacrylamide gel electrophoresis and analysis of its amino acid composition, although very similar to that of actin purified from whole platelets, showed a much lower content of 3-methylhistidine (approx. 0.05 residues/mole compared with 0.62 residues/mole). The surface membrane-associated phosphodiesterase (towards bis-(p-nitropheny1) phosphate) has also been partially characterised. This enzyme activity can be completely solubilised from the membrane using 0.5% Triton X-100, and can then be isolated as a single peak on Sepharose 4B. The separated enzyme fraction showed considerable purification over the original membrane as illustrated by SDS-polyacrylamide gel electrophoresis with the prominent component being the 95,000 dalton polypeptide. Our studies have also shown that this phosphodiesterase is inhibited competitively by ADP and ATP.