The PI/PTB domain: a new protein interaction domain involved in growth factor receptor signaling.

In summary, a new domain called the PI/PTB domain has been identified in the Shc adapter protein. This motif binds the NPXpY motif that is found in a large number of signal transduction molecules. The presence in Shc of both a PI/PTB domain and an SH2 domain presumably gives Shc the ability to interact with a large number of tyrosine-phosphorylated proteins. The structure of the PI/PTB domain has been solved and is very similar to the PH domain. Highly related in binding specificity is the PTB domain of IRS-1 and IRS-2, which also binds an NPXpY motif. Several other PI domains have been identified that may also have binding specificity for the NPXY motif. The terminology used at present to define these domains is unclear. The original terms PI and PTB domain stood for phosphotyrosine interaction and phosphotyrosine binding domain, respectively. The name may be inappropriate for some members of this family in which phosphotyrosine may not be essential for binding. Furthermore, these domains are structurally related to the previously named PH domains. At present we feel the use of the name PTB domain should be reserved for Shc and IRS-1/IRS-2, where phosphotyrosine binding has been demonstrated. We place the other proteins we have identified in the PI domain family, with PI now representing the protein interaction rather than the phosphotyrosine interaction domain. The role of several of the PI domains in other proteins is beginning to be studied. It seems clear that our understanding of these domains and their function in cell biology will rapidly expand over the next several years.