LOCALIZING THE ADHESIVE AND SIGNALING FUNCTIONS OF PLAKOGLOBIN

Plakoglobin (PKG) is a major component of cell-cell adhesive functions . I; is also closely related to the Drosophila segment polarity gene p roduct armadillo and can induce a WNT-like neural axis duplication (NA D) phenotype in Xenopus [Karnovsky and Klymkowsky, 1995]. To define t he regions of PKG involved in cell adhesion and inductive signaling, w e examined the behavior of mutated forms of PKG in Xenopus. Deletion o f amino acids 22 through 39 (in the Xenopus PKG sequence) increased th e apparent stability of the polypeptide within the embryo and increase d its ability to induce a WNT-like, NAD phenotype when expressed in th e vegetal hemisphere. The N-terminal ''head'' and first 6 ''ARM'' repe als of PKG, or the C-terminal ''tail'' and the last 3 ''ARM'' repeats, could be removed without destroying the remaining polypeptide's abili ty io induce a NAD phenotype. The nuclear localization of mutant PKGs, however, was not strictly correlated with the ability io induce a NAD phenotype, i.e., some inactive polypeptides still accumulate in nucle i. Removal of PKG's head and first ARM repeat, which includes its alph a-catenin binding site, resulted in a polypeptide that, when expressed in the embryo, generated a dramatic cell adhesion defect. Removal of the next three ARM repeats abolished this adhesion defect, suggesting that the polypeptide no longer competes effectively with endogenous ca tenins for binding io cadherins. Expression of a Form of PKG truncated after the 5th ARM repeat produced a milder cell adhesion defect, wher eas expression of a polypeptide truncated after the 8th ARM repeat had little apparent effect on cellular adhesion. Based on these observati ons, we conclude that functions related io stability and cellular adhe sion reside in the N-terminal region of the polypeptide, whereas the a bility io induce a NAD phenotype ies within repeats 6-10 of the centra l region. The function(s) of the C-terminal domain of PKG remain uncer tain at this lime. (C) 1997 Wiley-Liss, Inc.