Plexin D1 is a type I transmembrane glycoprotein that is the prototype of the plexin D subfamily of semaphorin receptors (1, 2). Human Plexin D1 contains a 46 amino acid (aa) signal sequence, a 1225 aa extracellular domain (ECD), a 21 aa transmembrane domain, and a 633 aa cytoplasmic domain that includes features common to other plexins (1). The human Plexin D1 ECD shares 89% identity with mouse Plexin D1, and ~84-92% aa identity based on incomplete sequences of rat, bovine, porcine and canine Plexin D1. It contains a sema domain, two plexin-semaphorin-integrin (PSI) or Met-related sequence (MRS) cysteine-rich motifs, and three glycine/proline-rich IPT/TIG domains which are immunoglobulin-like domains found in plexins, transcription factors, and the scatter factor receptors Met and Ron (1, 2). Isoforms of 1787 and 1747 aa have been sequenced; these contain a 178 aa N-terminal deletion with or without a longer alternate C-terminus (3). Like other Sema/plexin interactions, Plexin D1 interacts with Sema3C or Sema4A via neuropilins. Interaction with Sema3E, however, is direct (4). Plexin D1/Sema3E interaction mediates vascular guidance during development or angiogenesis; deletion of either molecule results in similar, profound cardiac abnormalities (4, 5). Plexin D1 is also expressed in lymphocytes, osteoblasts, the neural crest and the central nervous system during development (2, 6). In the brain, the presence of neuropilin can change Plexin D1/Sema3E interaction from an attractive to a repulsive signal (7, 8). Plexin D1 directs migration of thymocytes to the thymic medulla, probably through repulsion of Sema3E (9). Endothelial cell Plexin D1 binding to Sema4A can oppose VEGF and suppresses tumor angiogenesis, and expression of Sema3E correlates inversely with tumor metastasis, indicating that Plexin D1 is anti-metastatic in the presence of its ligands (10, 11).
