The phosphotyrosine interaction (PI) domains (also known as the PTB, or phosphotyrosine binding, domains) of Shc and IRS-1 are recently described domains that bind peptides phosphorylated on tyrosine residues. The PI/PTB domains differ from Src homology 2 (SH2) domains in that their binding specificity is determined by residues that lie amino terminal and not carboxy terminal to the phosphotyrosine. Recently, it has been appreciated that other cytoplasmic proteins also contain PI domains. We now show that the PI domain of X11 and one of the PI domains of FE65, two neuronal proteins, bind to the cytoplasmic domain of the amyloid precursor protein (␤APP). ␤APP is an integral transmembrane glycoprotein whose cellular function is unknown. One of the processing pathways of ␤APP leads to the secretion of A␤, the major constituent of the amyloid deposited in the brain parenchyma and vessel walls of Alzheimer's disease patients. We have found that the X11 PI domain binds a YENPTY motif in the intracellular domain of ␤APP that is strikingly similar to the NPXY motifs that bind the Shc and IRS-1 PI/PTB domains. Protein-protein interactions are important in signaling by many cell surface receptors. For example, activated growth factor receptors bind several signaling proteins in a phosphotyrosine-dependent fashion. Many of these signaling proteins contain Src homology 2 (SH2) domains that mediate the interaction with growth factor receptors by binding to specific phosphopeptide sequences present on the receptors (40, 45). Our laboratory has been cloning receptor-binding proteins by screening bacterial expression libraries with the tyrosine-phosphorylated epidermal growth factor (EGF) receptor. In general, this approach has led to the cloning of genes that encode SH2-domain proteins (33,36,39,47). However, recently, our group and others have identified a new phosphotyrosine interaction/phosphotyrosine binding (PI/PTB) domain in the protein Shc (3, 23, 28). Shc has an important role in tyrosine kinase signal transduction. Shc becomes tyrosine phosphorylated upon activation of many growth factors, cytokines, and G-protein-coupled receptors (12,14,32,41,43,55). The tyrosine phosphorylation of Shc allows it to bind Grb2, which begins a cascade leading to the activation of Ras and mitogenactivated protein kinase pathways (43). The PI/PTB domain found in the amino terminus of Shc appears to be crucial for the interaction of Shc with EGF receptor, nerve growth factor receptor, and insulin receptor (1,3,13,16,52,54).Although both PI/PTB domains and SH2 domains bind to phosphotyrosine-containing peptides, their binding specificities and tertiary structures are completely different (48,56,59). While SH2 domain binding specificity is determined by residues that lie carboxy terminal to the phosphotyrosine, the Shc PI/PTB domain specificity is determined by residues amino terminal to the phosphotyrosine. The binding specificity for the Shc PI/PTB is the sequence ⌿XNPXpY (where ⌿ is a hydrophobic residue, N is asparagine, P is proline, X...

Apolipoprotein E, ␣ 2 -macroglobulin, and amyloid precursor protein (APP) are involved in the development of Alzheimer's disease. All three proteins are ligands for the low density lipoprotein (LDL) receptor-related protein (LRP), an abundant neuronal surface receptor that has also been genetically linked to Alzheimer's disease. The cytoplasmic tails of LRP and other members of the LDL receptor gene family contain NPxY motifs that are required for receptor endocytosis. To investigate whether these receptors may have functions that go beyond ligand internalization, e.g. possible roles in cellular signaling, we searched for proteins that might interact with the cytoplasmic tails of the receptors. A family of adaptor proteins containing protein interaction domains that can interact with NPxY motifs has previously been described. Using yeast 2-hybrid and protein coprecipitation approaches in vitro, we show that the neuronal adaptor proteins FE65 and mammalian Disabled bind to the cytoplasmic tails of LRP, LDL receptor, and APP, where they can potentially serve as molecular scaffolds for the assembly of cytosolic multiprotein complexes. FE65 contains two distinct protein interaction domains that interact with LRP and APP, respectively, raising the possibility that LRP can modulate the intracellular trafficking of APP. Tyrosinephosphorylated mammalian Disabled can recruit nonreceptor tyrosine kinases, such as src and abl, to the cytoplasmic tails of the receptors to which it binds, suggesting a molecular pathway by which receptor/ligand interaction on the cell surface could generate an intracellular signal.

Protein-protein interactions are key elements in building functional protein complexes. Among the plethora of domains identified during the last 10 years, PDZ domains are one of the most commonly found protein-protein interaction domains in organisms from bacteria to humans. Although they may be the sole protein interaction domain within a cytoplasmic protein, they are most often found in combination with other protein interaction domains (for instance, SH3, PTB, WW) participating in complexes that facilitate signaling or determine the localization of receptors. Diversity of PDZ-containing protein function is provided by the large number of PDZ proteins that Mother Nature has distributed in the genome and implicates this protein family in the wiring of a huge number of molecules in molecular networks from the plasma membrane to the nucleus. Although at first sight their binding specificity appeared rather monotonous, involving only binding to the carboxyl-terminus of various proteins, it is now recognized that PDZ domains interact with greater versatility through PDZ-PDZ domain interaction; they bind to internal peptide sequences and even to lipids. Furthermore, PDZ domain-mediated interactions can sometimes be modulated in a dynamic way through target phosphorylation. In this review, we attempt to describe the structural basis of PDZ domain recognition and to give some functional insights into their role in the scaffolding of protein complexes implicated in normal and pathological biological processes.