Adhesion GPCRs as a paradigm for understanding polycystin-1 G protein regulation

Abstract

Polycystin-1, whose mutation is the most frequent cause of autosomal dominant polycystic kidney disease, is an extremely large and multi-faceted membrane protein whose primary or proximal cyst-preventing function remains undetermined. Accumulating evidence supports the idea that modulation of cellular signaling by heterotrimeric G proteins is a critical function of polycystin-1. The presence of a cis-autocatalyzed, G protein-coupled receptor (GPCR) proteolytic cleavage site, or GPS, in its extracellular N-terminal domain immediately preceding the first transmembrane domain is one of the notable conserved features of the polycystin-1-like protein family, and also of the family of cell adhesion GPCRs. Adhesion GPCRs are one of five families within the GPCR superfamily and are distinguished by a large N-terminal extracellular region consisting of multiple adhesion modules with a GPS-containing GAIN domain and bimodal functions in cell adhesion and signal transduction. Recent advances from studies of adhesion GPCRs provide a new paradigm for unraveling the mechanisms by which polycystin-1-associated G protein signaling contributes to the pathogenesis of polycystic kidney disease. This review highlights the structural and functional features shared by polycystin-1 and the adhesion GPCRs and discusses the implications of such similarities for our further understanding of the functions of this complicated protein.

Keywords: ADPKD; Cell adhesion; GAIN domain; GPS; Heterotrimeric G protein.

Figure 1.

The major distinguishing structural features of PC1. The >3,000-aa N-terminal extracellular region consists of multiple interaction motifs, 16+1 repeats of the PKD domain, and a region with predicted structural homology to the GAIN domain identified in the ADGR family (18). The GAIN domain harbors a conserved GPS motif that is necessary for autocatalytic cleavage of PC1 into extracellular NTF and intramembrane CTF subunits, which remain non-covalently associated. The N-terminus of the CTF subunit generated by GPS cleavage resembles the stalk region of the ADGRs. Locations of additional cleavages that occur within the membrane-embedded 11-TM portion to generate C-terminal fragments called P100, CTT and P15 are illustrated by dashed arrows. Other notable features within the CTF include the region with homology to PC2, the coiled coil domain, and the G protein activation motif. Key lists the features illustrated.

Figure 2.

Structural features of ADGRs. (A) The distinguishing features, 7TM and large extracellular region with membrane-proximal GAIN domain, of a generic ADGR are illustrated. The N-terminal ECR consists of different types of adhesion-like and other binding domains often in multiple copies. The GAIN domain is composed of two subunits with the GPS motif located in the C-terminal portion of subdomain B. The cytosolic C-tail often contains a PDZ binding motif or other type of protein-protein interaction motif. (B) Dissociation of the extracellular NTF and intramembrane CTF subunits following autocatalytic cleavage at the GPS results in exposure of the stalk, the ‘new’ extracellular, N-terminus of the CTF subunit. The stalk consists of the final, 13^th, β-strand of the GAIN domain, which prior to dissociation composed part of the GPS motif, and a short linker sequence preceding the first TM domain. (C) Illustration of the arrangement and interactions of β-strand 13 with adjacent β-strands within prior to GPS cleavage or dissociation based on the work by D. Arac and colleagues (18). β-strand 13 is involved in multiple bonding interactions with β-strands 6, 7 and 9. The disulfide bond between 2 of the conserved cysteines of the GPS motif links β-strands 9 and 12 that helps to form the kinked loop where GPS cleavage occurs at the peptide bond between the Leu and Thr residues of the conserved cleavage tripeptide sequence, HLT/S. Key lists the features illustrated in (A)-(C).

Figure 3.

Activation mechanisms of ADGRs. Illustration of 3 main models (A-C) proposed for activation of G protein signaling by ADGRs. The central image represents the inactive form of a generic ADGR with associated NTF and CTF subunits. (A) The Disinhibition model proposes that the NTF subunit inhibits the constitutive signaling activity of the CTF by stabilizing an inactive conformation, which is ‘dis-inhibited’ upon dissociation of the two subunits. Dissociation or removal of the NTF has been proposed to occur via homophilic interactions between NTF subunits as illustrated here. (B) The Tethered cryptic ligand model proposes that the NTF sequesters or encrypts an agonistic element, the Stachel sequence, contained within the stalk region of the CTF, thus preventing its ability to fold back and interact with the extracellular loops/TM core to activate G proteins. β-strand 13 is proposed to comprise the majority of the agonistic Stachel sequence activity. The binding of ECM ligands and/or mechanical forces have been proposed as means of removing the NTF subunit to allow exposure of and interaction by the tethered Stachel ligand. (C) The Tunable model proposes that the binding of extracellular region partners, an agonistic antibody (as illustrated here), or the application of mechanical forces induces conformational changes within the extracellular region that result in exposure of a portion of the Stachel sequence or an alteration in interactions between the NTF and CTF subunits such that signaling is activated. This model may explain observations of stalk-independent signaling and the signaling activity of GPS cleavage-deficient ADGRs. These models are not considered to be mutually exclusive, and in fact, the use of more than one mechanism by an ADGR could be a means of biased signaling (i.e., activation of different G protein pathways).