Dissecting the functions of protein-protein interactions: caveolin as a promiscuous partner. Focus on “Caveolin-1 scaffold domain interacts with TRPC1 and IP3R3 to regulate Ca store release-induced Ca entry in endothelial cells”

THE CENTRALITY OF PROTEIN complexes and transient proteinprotein interactions in cellular physiology is becoming increasingly clear. Although one can readily demonstrate proteinprotein interactions with nonphysiological approaches such as immunoprecipitation, it has been more challenging to show transient interactions in the cytoplasm, demonstrate the specific protein regions involved in the interactions, and dissect out the physiological role of the interactions. Although there are a multitude of such protein-protein interactions in the cytoplasm, one particularly promiscuous binding partner that is found in the plasma membrane and that has gained considerable attention is the family of caveolin proteins. Caveolins (-1,-2, and -3) have been extensively studied for their role in binding to components of cell signaling cascades and can have either stimulatory or inhibitory influences on such cascades (6, 13); caveolins also have a central role in regulating cell metabolism (6, 11). Caveolin-1 (Cav-1) has been described as forming mobile signaling platforms within caveolae by sequestration of multiple proteins through interaction via the scaffolding domain (CSD) found at residues 82-101 within the NH2 terminus. In most cases, interaction with the CSD of Cav-1 either maintains the signaling protein in an inactive state until a stimulus is presented or terminates signal transmission after activation. The CSD is critical for caveolin homo-oligomerization and for interaction with a wide variety of caveolaeassociated proteins (4, 13). Although there is a growing catalog of interactions of caveolin with protein partners, to understand the role of those interactions it is essential to identify the regions of interaction between the proteins, the effect of binding on cellular localization, and the effect of binding on cellular function. Transient receptor potential channels (TRPC) have recently been reported to localize within caveolae, particularly the canonical TRPC1 isoform (2, 10, 17). In addition, other molecules crucial for calcium handling have been found to interact with caveolae, such as the plasma membrane calcium pump and the sodium-calcium exchanger (5). However, few studies have been able to elucidate a particular cellular function (e.g., calcium handling) and decipher in detail the specifics of the organization as well as the functional role of the various interactions within caveolae (Fig. 1). Sundivakkam et al. (14) elaborate on recent reports and demonstrate a role for Cav-1 in regulating not only the localization and functional interaction of TRPC1 with other proteins within and outside of the caveolae, but also in terms of the functional role of interactions with the CSD domain of Cav-1. In vascular endothelial cells, the release of internal calcium stores activates extracellular calcium entry that signals varied responses, such as increased endothelial permeability and proinflammatory responses (16). Mammalian homologues of transient receptor potential (TRP) gene family of channels have Address for reprint requests and other correspondence: C. D. Hardin, Dept. of Nutrition and Exercise Physiology, Univ. of Missouri, 217 Gwynn Hall, Columbia, MO 65211 (e-mail: [email protected]). Fig. 1. Schematic diagram depicting the potential interaction between caveolin-1, transient receptor potential channel 1 (TRPC1) within the caveolae, and inositol 1,4,5-trisphosphate (IP3) receptor 3 (IP3R3) in the endoplasmic reticulum/sarcoplasmic reticulum (ER/SR) in the activation mechanism of storeoperated Ca2 entry. Ligand-mediated activation of G protein-coupled receptors (GPCR) results in depletion of ER/SR Ca2 stores via IP3R3 activation. This store depletion process can activate signaling molecules that mediate Ca2 entry through TRPC1. Highlighted is the potential benefit of such tight spatial organization within the caveolae as compared with the rest of the plasma membrane (PM) in the regulation of ligand-mediated activation of store-operated Ca2 entry and overall Ca2 homeostasis. SERCA, sarcoplasmic reticulum Ca2 ATPase; NCX, Na /Ca2 exchanger. Am J Physiol Cell Physiol 296: C387–C389, 2009; doi:10.1152/ajpcell.00663.2008.