GTP-BINDING PROTEINS ASSOCIATED WITH THE HUMAN PLACENTAL SYNCYTIOTROPHOBLAST PLASMA-MEMBRANE

The nature of GTP-binding components associated with isolated human term placental syncytiotrophoblast microvillous plasma membrane vesides (SPMV) was determined; these are relevant to elucidation of intracellular signal transduction mechanisms. Four proteins were identified, with molecular weights of 29, 27, 23 and 21 kDa, which specifically bound [α-32P]GTP in the presence of Mg2+. Studies employing anti-p21c-ras monoclonal antibodies indicated these four GTP-binding components were ras-related and one, the 21 kDa component, may be p21c-ras. In addition, SPMV were also found to express the α subunits of three separate G proteins. A 45 kDa SPMV GTP-binding protein was identified as a substrate for Vibrio cholera toxin and was recognized by a rabbit antibody to the α subunit of the adenylate cyclase stimulating G protein, Gs. A 41 kDa SPMV GTP-binding protein substrate of Bordetella pertussis toxin was also recognized by rabbit antibodies to the α subunits of the adenylate cyclase inhibiting G proteins, Gi-1 and Gi-3. No evidence was found to support the presence of the 21 kDa Gp, a G protein previously associated with membranes prepared from whole placental tissue homogenates. © 1994, W. B. Saunders Company Ltd.. All rights reserved.