![]() When all four aromatic residues were simultaneously mutated to Ala or Gly the interaction was lost. ![]() Noting that certain separations of aromatic residues were particularly common, the authors identified a 16-residue portion of the bovine Gi2α subunit (the GP peptide) which bound to CSDs from caveolin-1 and 3 and much less so to caveolin 2. The peptides obtained were statistically enriched in tryptophan (decapeptides and 15-mers) or other aromatic amino acids (15-mers). The original definition of the CBM arises from the work of Couet et al., who obtained random peptides binding to the CSD by phage display. Target association with the caveolin scaffolding domain is mainly proposed to occur via the caveolin binding motif (CBM) on the binding partner. Dynamin-2, endothelin-B, connexin-43 and Rab5 also interact with multiple distinct regions of Cav-1 –. For example, interaction with protein kinase A is dependent on either the CSD or C-terminal domain (amino acids 135–178) of Cav-1. Furthermore, in some cases interactions appear to occur via multiple distinct caveolin domains. Association of caveolin with NOSTRIN, cyclooxygenase-2, high affinity nerve growth factor receptor (Trk ), growth factor receptor-bound protein 7 (Grb7 ) and insulin receptor substrate 1 (IRS1 ) are all thought to occur independently of the CSD. Since the original definitions of the CSD and CBM, an increasing number of studies have suggested that interactions between caveolin and target need not necessarily involve both regions. Interactions between caveolin and other proteins, however, remain poorly understood in terms of physiology, modes of binding/suppression and the mechanisms that regulate interaction. This paradox has been largely resolved for eNOS whereby interaction with caveolin under basal conditions maintains an inactive enzyme and compartmentalization of eNOS in caveolae ensures a rapid response upon stimulation. Paradoxically, association with caveolin typically suppresses activity in the targeted protein, , suggesting that recruitment to caveolae might hamper and not enhance signalling efficiency (the so-called ‘caveolar paradox’). Interaction with caveolin, which appears to be important in protein recruitment to caveolar domains and thus the formation of microenvironments rich in interacting signalling molecules, is commonly believed to be mediated via a ∼20 amino acid N-terminal region on the caveolin molecule known as the caveolin scaffolding domain (CSD) and an aromatic-rich caveolin binding motif (CBM) on the associated protein. Aside from roles in caveolae formation and stability, caveolins interact with many caveolae-localized signalling molecules including heterotrimeric G proteins, Src family tyrosine kinases, phosphoinositide 3-kinase, integrins, epidermal growth factor receptor (EGFR), H-Ras, endothelial nitric oxide synthase (eNOS) and a number of ion channels. Together, these findings suggest that the canonical CBM may not be a common characteristic of caveolin-target interactions and that interfaces between caveolin and targets may be more structurally diverse than presently appreciated.Ĭaveolins are a family of cholesterol-binding membrane proteins (caveolin-1, -2 and -3) that coat the intracellular surface of caveolae, small flask-shaped pits (50–100 nm in diameter) that form at the plasma membrane of most cells –. Analysis of the relative solvent accessible area of putative CBMs shows that the majority of their aromatic residues are buried within the protein and are thus unlikely to interact directly with caveolin, but may instead be important for protein structural stability. Furthermore, sequence- and structure-based considerations suggest that CBMs do not have characteristics commonly associated with true interaction motifs. We find that sequences conforming to the CBM occur in 30% of human proteins, but find no evidence for their statistical enrichment in the caveolin interactome. Here we exploit recent improvements in bioinformatics tools and in our understanding of linear motifs to critically examine the role of CBMs in caveolin interactions. The CBM resembles a typical linear motif - a short, simple sequence independently evolved many times in different proteins for a specific function. Caveolin-protein interactions are commonly considered to occur between a ∼20 amino acid region within caveolin, the caveolin scaffolding domain (CSD), and an aromatic-rich caveolin binding motif (CBM) on the binding partner (фXфXXXXф, фXXXXфXXф or фXфXXXXфXXф, where ф is an aromatic and X an unspecified amino acid). Aside from roles in caveolae formation, caveolins recruit, retain and regulate many caveolae-associated signalling molecules. Caveolins are coat proteins of caveolae, small flask-shaped pits of the plasma membranes of most cells.
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