B-chain sequence and in situ hybridization of the rabbit placental relaxin-like gene product

We reported that the nucleotide sequence of a cDNA generated from rabbit pl acental poly(A)(+) RNA using porcine preprorelaxin primers was identical to SQ10, a product of squamous differentiated tracheal epithelial cells. Howe ver, these results did not confirm that SQ10 was the biologically active ra bbit relaxin that had been isolated previously yet not sequenced. In this s tudy, a 7-kDa protein isolated from rabbit placentas exhibited relaxin bioa ctivity and cross-reacted with a porcine relaxin antiserum. A partial amino acid sequence of this protein revealed a sequence identical to that of SQ1 0. Although the amino acid sequence of the putative relaxin receptor-bindin g domain found in the B chain of relaxin was modified in SQ10 from CGRDYVR to CRNDFVR, the placental protein was bioactive. These results suggest that SQ10 is the rabbit relaxin. In situ hybridization, using an SQ10 riboprobe , indicated radiolabeling in the syncytiotrophoblast cells of the rabbit pl acenta. The pattern of labeling corresponded with the immunohistochemical s taining for relaxin observed with use of a porcine relaxin antiserum. These results indicate that the syncytiotrophoblast cells are a site of synthesi s for SQ10 and that the immunostaining is not solely the result of sequeste ring SQ10 through receptor-mediated endocytosis. A potential role for relax in in implantation is discussed.