Abstract
A new cadherin-like protein (CLP) was identified in oocytes, eggs, and cleavage stage embryos of Xenopus laevis. As a probe for detecting new cadherin proteins, an antiserum was raised to a 17 amino acid peptide derived from a highly conserved region in the cytoplasmic domain of all cadherins which have been sequenced to date. This antipeptide antibody recognized Xenopus E-cadherin and a polypeptide in Xenopus brain extracts similar to N-cadherin, which were independently identified by specific mAbs. In extracts of eggs and midblastula stage embryos the antipeptide antibody recognized specifically a 120-kD glycoprotein that migrated faster on SDS gels than the 140-kD E- and N-cadherin polypeptides. This 120-kD polypeptide was not recognized by the mAbs specific to E- and N-cadherin. In fact, E- and N-cadherin were not detectable in eggs or midblastula stage embryos. The possible relationship of CLP to P-cadherin, which has been identified in mouse tissues, has not yet been determined. CLP was synthesized by large, late stage oocytes. When oocytes were induced to mature in vitro with progesterone it accumulated to the same level found in normally laid eggs. It did not accumulate further to any significant extent during the early cleavage stages. CLP was detected on the surface of stage 8 blastomeres by cell surface biotinylation, but only after the tight junctions of the blastula epithelium were opened by removal of Ca2+. We conclude that CLP is a maternally encoded protein that is the major, if not only, cadherin-related protein present in the earliest stages of Xenopus development, and we propose that it may play a role in the Ca2(+)-dependent adhesion and junction formation between cleavage stage blastomeres.