The major CD9 and CD81 molecular partner - Identification and characterization of the complexes

By associating with specific partner molecules and with each other, the tet raspanins are thought to assemble multimolecular complexes that may be espe cially relevant with respect to metastasis. We have previously identified a 135-kDa molecule (CD9P-1) as a major molecular partner of CD9 in cancer ce ll lines. This molecule was identified, after immunoaffinity purification a nd mass spectrometry analysis, as the protein encoded by the KIAA1436 gene and the human ortholog of a rat protein known as FPRP. Cross-linking experi ments detected a complex of the size of CD9 plus CD9P-1, showing that these glycoproteins directly associate with each other, probably in the absence of any other molecule. The use of chimeric CD9/CD82 molecules revealed the role of the second half of CD9, comprising the large extracellular loop and the fourth transmembrane domain. CD9P-1 was also shown to form separate co mplexes with CD81 and with an unidentified 175-kDa molecule. It also associ ated with other tetraspanins under conditions maintaining tetraspanin/tetra spanin interactions. The identification of a protein strongly linked to the tetraspanin web and the production of a specific monoclonal antibody will help to further characterize the role of this "web" under physiological and pathological conditions.