Identification and Structure of Activated‐Platelet Protein‐1, a Protein with RNA‐Binding Domain Motifs that is Expressed by Activated Platelets

Beyond their critical role in thrombosis, platelets perform important functions in vascular remodeling, inflammation, and wound repair. Many of these functions are executed by molecules expressed by activated platelets. A novel molecule, activated‐platelet protein‐1 (APP‐1), was identified by a monoclonal antibody against activated rabbit platelets. When platelets were stimulated by thrombin, A23187 or ADP, APP‐I was expressed on the platelet surface. APP‐1 was also detected in whole cell lysates of platelets, but not on the external surfaces of resting platelets. With maximal activation by thrombin, 15900±2800 molecules APP‐1 were expressed/platelet. A 2.3‐kb cDNA fragment containing a partial coding sequence for APP‐1 was isolated from a rabbit bone marrow library by expression cloning with the anti‐APP‐l monoclonal antibody. When expressed as a recombinant fusion protein in bacteria, APP‐1 bound specifically to poly(A)‐Sepharose. The full‐length cDNA coding for human APP‐1, obtained by DNA hybridization techniques, showed 98.7% amino acid sequence identity with the rabbit protein. Northern analysis with human APP‐1 identified a 3.7‐kb mRNA transcript in megakaryocytic lines that express transcripts for platelet proteins. Human APP‐1 has four ribonucleotide binding domains with ribonucleoprotein 1 and 2 motifs. By virtue of its ribonucleotide binding domains, APP‐1 is structurally related to polyadenylate‐binding protein, which regulates translation initiation and polyadenylate shortening, and to nucleolysin, a specific effector molecule found in the granules of cytotoxic T lymphocytes.