The PTB domain: a modular domain with multiple function

The convergence of cell biology, virology, biochemistry, molecular biology and genetics, has resulted in an increased understanding of the mechanisms by which cells communicate with each other and their environment. It has become clear that signal transduction is not just a linear, stepwise activation of enzymatic cascades. Rather, signal transduction involves the specific temporal and spatial assembly of multiprotein complexes within a cell, thereby leading to specific outcomes such as growth, differentiation and apoptosis. Abrogation of these complexes can lead to diseases such as cancer. The discovery of modular protein recognition domains, eg, Src homology 2 (SH2) and SH3 domains, led to the notion that these domains promote the assembly of multiprotein signaling complexes. The phosphotyrosine binding (PTB) domain, also known as a phosphotyrosine interaction domain, was originally described as an alternative to the SH2 domain for recognition of tyrosine phosphorylated ligands. However, recent experiments suggest that this domain is more versatile than originally described and is capable of binding a broad range of ligands including phospholipids and nonphosphorylated proteins. Thus, PTB domains possess a high degree of flexibility in ligand binding, thereby allowing for a wider range of interactions with cellular targets.