Proteins that define a new family, termed membrane-associated guanylate kinases, have recently been identified as structural components of epithelial tight junctions and neuronal synapses, and as cell signaling proteins in Drosophila and Caenorhabditis elegans. In particular, the lin-2 gene has been shown to encode a membrane-associated guanylate kinase and to act in the let-23 receptor tyrosine kinase/let-60 ras signaling pathway that controls vulval induction in C. elegans. The combined data from recent biochemical and genetic analyses of membrane-associated guanylate kinases suggest that certain tight junction proteins can play an important role in cell signaling pathways. One possibility is that asymmetric segregation of signaling receptors to the basolateral membrane domain of polarized epithelial cells is crucial for proper cell signaling, and that membrane-associated guanylate kinases may be required for this localization.