PxxP

PxxP is a short linear motif in protein sequences consisting of a proline (P), followed by any two amino acids, followed by another proline. The motif is commonly found in proline-rich regions of eukaryotic proteins and is known for mediating protein-protein interactions by binding to SH3 domains.

In signaling proteins, PxxP motifs interact with SH3 (Src Homology 3) domains, forming transient, low-affinity interactions that help assemble signaling complexes. Two widely described SH3-binding orientations exist, often referred to as class I and class II, defined by the arrangement of basic residues relative to the motif and the binding surface orientation. These contextual features influence affinity and partner specificity.

Occurrence and function: PxxP motifs are enriched in cytoplasmic and membrane-associated proteins involved in signal transduction, cytoskeletal organization, endocytosis, and cell adhesion. By binding SH3 domains on kinases and adaptor proteins, PxxP-containing ligands help recruit and organize signaling complexes, regulate enzyme activity, and coordinate membrane trafficking.

Structure and detection: PxxP motifs adopt polyproline II helices, presenting the two proline residues to the SH3-binding groove. Detection and study rely on peptide-binding assays, phage display, mutational analysis, and structural methods such as NMR and X-ray crystallography. Because SH3 binding is influenced by flanking residues and local structure, the presence of a PxxP motif alone is not a guarantee of interaction.

Examples and notes: Notable SH3-containing proteins that recognize PxxP-containing ligands include members of the Src family, Crk, Grb2, and Nck, among others. The motif is a common but nonspecific feature of proline-rich regions and is a component of many signaling networks in eukaryotic cells.