ROLE OF THE LIM CLASS HOMEODOMAIN PROTEIN XLIM-1 IN NEURAL AND MUSCLE INDUCTION BY THE SPEMANN ORGANIZER IN XENOPUS

LIKE all known LIM class homeobox genes, Xlim-1 encodes a protein with two tandemly repeated cysteine-rich LIM domains upstream of the homeodomain(1). In Xenopus laevis, Xlim-1 is specifically expressed in the Spemann organizer, whose major functions include neural induction and dorsalization of ventral mesoderm(2-4). From RNA injection experiments we conclude here that: (1) the LIM domains behave as negative regulatory domains; (2) LIM domain mutants of Xlim-1 elicited neural differentiation in animal explants; (3) mutant, and to a lesser extent wild-type, Xlim-1 enhanced muscle formation after coinjection with Xbra; (4) both of these activities are mediated by extracellular signals as seen in combined explant experiments;. (5) Xlim-1 mutants activated goosecoid (gsc) expression in animal explants, but not expression of noggin or follistatin; (6) mutant Xlim-1 elicited formation of partial secondary axes, and cooperated with gsc in notochord formation. Thus Xlim-1 has latent activities, implicating it in organizer functions.