XENOPUS CADHERINS - THE MATERNAL POOL COMPRISES DISTINGUISHABLE MEMBERS OF THE FAMILY

Three maternal cadherins have been reported to occur in the pregastrul a Xenopus embryo. EP- and XB-cadherin are distinguished by their disti nct cDNA sequences. U-cadherin has been characterized by its reaction with a specific monoclonal antibody (mAb 6D5). Thus far, lack of speci fic probes that discriminate between these molecules has prevented the ir identification as distinct cadherins. We now demonstrate by means o f RNase protection assays that both EP- and XB-cadherin mRNAs are pres ent in oocytes and mature eggs. By use of the Xenopus cadherin protein s expressed in mammalian cell lines,we find that mAb 6D5 crossreacts w ith XB-cadherin, but not with EP-cadherin, The major fraction of the m aternal cadherins does not contain the 6D5 epitope and probably repres ents EP-cadherin, A minor fraction carries the 6D5 epitope indicative for the XB- and U-type of cadherins. We have termed this fraction XB/U -cadherin. The function of maternal cadherins was examined by in vitro cell adhesion assays. A newly developed antiserum with a broad specif icity for various Xenopus cadherins efficiently blocks all calcium dep endent cell adhesion in the early embryo. We conclude that the materna l cadherins play a central role in interblastomere adhesion in the ear ly embryo and comprise at least two discrete cadherin forms, EP- and X B/U-cadherin.