EXPRESSION OF THE ARMADILLO FAMILY MEMBER P120(CAS)1B IN XENOPUS EMBRYOS AFFECTS HEAD DIFFERENTIATION BUT NOT AXIS FORMATION

The Armadillo family is formed by proteins which possess an Arm domain comprising multiple copies of a 42-amino-acid motif, the Arm repeat, initially described for the Drosophila segment polarity gene product A rmadillo. The Arm domain serves in protein-protein interactions which are required for the family members Armadillo, beta-catenin and plakog lobin to mediate cell-cell adhesion and Wnt/Wingless signalling. Simil arily, p120(cas), the Arm domain containing src substrate, also binds to cadherins and becomes tyrosine phosphorylated in response to a vari ety of stimuli. However, a putative function of p120(cas) in adhesion or signalling has not yet been demonstrated. It has also not been show n until now that an Arm domain is a common signal transduction motif. Using Xenopus embryos we show by expression of murine p120(cas)1B (mp1 20(cas)1B) in ventral blastomeres that this catenin cannot replace bet a-catenin function in dorsal axis formation. Thus, the presence of an Arm domain per se is not sufficient to activate the Wnt/Wg pathway. In deed, injection of mp120(cas)1B into dorsal blastomeres led instead to delayed blastopore closure and posteriorized phenotypes with malforme d head structures indicative of disturbed gastrulation movements. Beca use neither convergent extension behaviour nor adhesion to fibronectin was altered in the injected embryos we assume that mp120(cas)1B influ ences motility or orientation of migrating mesodermal cells.