Immunoblotting and immunoprecipitation analysis of platelet and endothelial cell protein extracts. Triton X-100–soluble proteins from platelets (A) or from endothelial cells (B) were analyzed directly by immunoblotting (lanes 1 and 4) or after immunoprecipitation with the MoAb SZ2 to GPIbα (lanes 3), with the irrelevant monoclonal IgG MOPC21 (lanes 2 and 5), or with the MoAb 23C6 to the αvβ3 integrin complex (lane 6). Proteins were separated by SDS-PAGE under reduced conditions, transferred to nitrocellulose membranes, and immunoblotted with a polyclonal antiserum to GPIbα (lanes 1 to 3, 2 days of exposure on film) or with a mixture of two polyclonal antisera to each of the αv and β3 subunits (lanes 4 through 6, 1 day of exposure on film), as detailed in the Materials and Methods. Immunoprecipitation of platelet lysates with SZ2 shows a major band in the position of platelet GPIbα, indicated by an arrowhead (A), whereas no such band is detected in endothelial cell immunoprecipitates. On (B), arrowheads depict the position of endothelial αv heavy chain and β3 subunits. Molecular mass markers are indicated on the left-hand side.