PTB or not to be: Promiscuous, tolerant and bizarro domains come of age

PTB domains are protein modules that usually interact with the cytoplasmic tail of a wide variety of growth factor receptors. In so doing, they mediate the transduction of extracellular information to specific downstream targets within the cell that ultimately determine the fate of a number of important biological processes such as cell growth and differentiation, cell cycle regulation and apoptosis. Recent structural and functional studies of PTB domains from a variety of cellular proteins have begun to shed light on the molecular mechanisms of action of these important protein modules. In the present review, we provide an account of such studies and suggest that PTB domains can be subdivided into three distinct categories on the basis of their topological differences. We also discuss the various mechanisms employed by the PTB domains in recognition of a diverse set of ligands without a consensus sequence. Finally, we discuss the role of molecular plasticity as a possible determinant of functional versatility of PTB domains.