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, 15 900 +/- 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-1 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.