Little is known about the structure and function of integral membrane proteins in secretory storage granules or about the signals which direct these proteins to granules. We have identified a protein, named GMP-140, which is localized to the membranes of storage granules in platelets and vascular endothelial cells. This protein is rapidly translocated to the plasm membrane when these cells are activated and it may be reinternalized from the surface of endothelial cells. We propose to examine the properties of this unique molecule through four approaches: 1) The primary structure of GMP-140 will be determined by sequencing cDNA clones isolated from endothelial cell and megakaryocytic tumor cDNA libraries. The sequence will be compared with known peptide sequence and analyzed for features providing clues as to function. 2) The movements of GMP-140 in human endothelial cells will be studied by a combination of immunocytochemistry and binding of 125I-labeled monoclonal antibodies. 3) Cloned GMP-140 will be expressed in heterologous cells which contain secretory storage granules. If the expressed protein is localized in granule membranes, site-directed mutagenesis will be employed to locate the signals which specify its delivery to these organelles. 4) The function of GMP-140 will be investigated by measuring its binding to candidate ligands and by introducing antibodies to peptides derived from the sequence of the cytoplasmic domain into cells. These studies will clarify the role of GMP-140 in the vascular system as well as the mechanisms by which integral membrane proteins are directed to secretory storage granules.