SummaryActin-binding protein (ABP) is degraded into fragments of 190 and 90 kDa by calpain. A monoclonal antibody (MAb TI10) against the 90 kDa fragment of ABP coprecipitated with the glycoprotein lb (GP lb) peak observed on crossed immunoelectrophoresis of Triton X-100 extracts of platelets prepared without calpain inhibitors. MAb PM6/317 against the 190 kDa fragment was not coprecipitated with the GP lb peak under such conditions. The 90 kDa fragment was adsorbed on protein A agarose from extracts that had been preincubated with antibodies to GP lb. This supports the idea that the GP Ib-ABP interaction resides in the 90 kDa region of ABP. GP lb was sedimented with the Triton-insoluble actin filaments in trace amounts only, and only after high speed centrifugation (100,000 × g, 3 h). Both the 190 kDa and the 90 kDa fragments of ABP were sedimented with the Triton-insoluble actin filaments.

Summary1. Platelet fibrinogen has been purified from washed bovine platelets. The procedure was based on the methods for purification of plasma fibrinogen by fractionated precipitations and extractions with ethanol and glycine below 0°, and precipitation of proteins by dimethylformamide at 0°.2. The platelet extract obtained by freezing and thawing of the cells, freed from insoluble material by centrifugation at 23,000 x g for 30 min, contained 0.22 ±0.003mg fibrinogen per 109 platelets. Total protein of this fraction was 0.77 ±0.08 mg per 109 platelets whereas that of the insoluble fraction was 0.79 ±0.09 mg per 109 platelets.3. The most purified platelet fibrinogen fraction contained 91-98% of the protein in a thrombin-clottable state. The yield was approx. 20%. It showed homogeneity in analytical ultracentrifugation, in immunoelectrophoresis using an antiserum produced by immunization of rabbits against platelet extract, and in starch gel electrophoresis using a discontinuous system of Tris HCl and borate buffers offering a high resolution power towards the platelet proteins. Polyacrylamide disc electrophoresis revealed two to three faint lines behind the main fibrinogen line. At least one such line was also observed with purified plasma fibrinogen.

Summary1. The protein composition of bovine platelet extracts has been investigated by immunoelectrophoresis and polyacrylamide disc electrophoresis. The information obtained is discussed as a basis for study on platelet fibrinogen.2. With antiserum to platelet proteins 11 precipitin lines were observed 3 of which corresponded electrophoretically to plasma albumin, fibrinogen and γ-globulin. These lines were not seen using the same antiserum absorbed with bovine plasma. The 8 additional lines were still present indicating that they represented specific platelet components. Antiserum to plasma produced the 3 above-mentioned lines, but no others.3. With antiserum to purified bovine plasma fibrinogen 3 precipitin lines were observed. The fibrinogen line was the dominant one. The 2 additional lines did not disappear by absorption of the antiserum with bovine serum nor by incubation of the extracts with thrombin. The latter treatment totally removed the fibrinogen line.4. A non-fibrinogen precipitin line, observed only with the antiserum to platelet extract and positioned in the β2-globulin region, disappeared by the incubation of platelet extracts with thrombin.