The GPCR properties of polycystin-1- A new paradigm

Abstract

Polycystin-1 (PC1) is an 11-transmembrane (TM) domain-containing protein encoded by the PKD1 gene, the most frequently mutated gene leading to autosomal dominant polycystic kidney disease (ADPKD). This large (> 462 kDal) protein has a complex posttranslational maturation process, with over five proteolytic cleavages having been described, and is found at multiple cellular locations. The initial description of the binding and activation of heterotrimeric Gαi/o by the juxtamembrane region of the PC1 cytosolic C-terminal tail (C-tail) more than 20 years ago opened the door to investigations, and controversies, into PC1's potential function as a novel G protein-coupled receptor (GPCR). Subsequent biochemical and cellular-based assays supported an ability of the PC1 C-tail to bind numerous members of the Gα protein family and to either inhibit or activate G protein-dependent pathways involved in the regulation of ion channel activity, transcription factor activation, and apoptosis. More recent work has demonstrated an essential role for PC1-mediated G protein regulation in preventing kidney cyst development; however, the mechanisms by which PC1 regulates G protein activity continue to be discovered. Similarities between PC1 and the adhesion class of 7-TM GPCRs, most notably a conserved GPCR proteolysis site (GPS) before the first TM domain, which undergoes autocatalyzed proteolytic cleavage, suggest potential mechanisms for PC1-mediated regulation of G protein signaling. This article reviews the evidence supporting GPCR-like functions of PC1 and their relevance to cystic disease, discusses the involvement of GPS cleavage and potential ligands in regulating PC1 GPCR function, and explores potential connections between PC1 GPCR-like activity and regulation of the channel properties of the polycystin receptor-channel complex.

Keywords: ADPKD; GPS cleavage; heterotrimeric G proteins; polycystin-1; polycystin-2; receptor-ion channel complex; tethered peptide agonist.

FIGURE 1

The structure-function features of polycystin-1 (PC1). (A) Domains identified in the N-terminal extracellular region (ECR) and within the membrane-associated portion of PC1 are indicated along with identified functional roles. ECM, extracellular matrix; GPS, GPCR proteolysis site; GAIN, GPCR autoproteolysis inducing; TM, transmembrane, S, transmembrane segment; PLAT, polycystin/lipoxygenase/α toxin; TOP, Tetragonal Opening of Polycystins; LRR, leucine-rich repeats; CTL, C-type lectin; WSC, cell-wall integrity and stress-response component; FN, fibronectin-like. The region of PC1 with sequence and structural homology to the ion channel polycystin-2 (PC2) is indicated in green. (B) The cleavage products of PC1. Shown are the N-terminal fragment (NTF) and C-terminal fragment (CTF) that result from auto-catalyzed GPS cleavage and separation from each other. Separation of the NTF and CTF subunits exposes the stalk consisting of the final, 13th beta strand (red arrow) of the intact GAIN domain and a linker, which then constitutes the N-terminus of the CTF. Approximate locations of the protease-mediated cleavage sites within the CTF are indicated by the yellow arrows.

FIGURE 2

The cytosolic C-terminal tail of PC1 is shown schematically with sequence alignment of the 74 aa minimal G protein binding domain (corresponding to human aa 4,111–4,184). Small deletion and missense ADPKD-associated mutations that fall within this region and score as “Pathogenic” or “Likely Pathogenic” (as determined by the ADPKD Variant Database ) are shown above the sequence alignment. Experimentally-generated mutations designed to test effects on G protein signaling are shown below the alignment. Additional references for these ADPKD-associated mutations can be found at (Afzal et al., 1999; Perrichot et al., 1999; Garcia-Gonzalez et al., 2007; Rossetti et al., 2007; Reed et al., 2008; Tan et al., 2009; Audrezet et al., 2012; Rossetti et al., 2012). ND, not determined; †, decreased PC1/PC2 channel activity; ¥, calmodulin-binding disrupted, decreased PC1/PC2 channel activity and flow-dependent channel response, no effect on ciliary localization, decreased energy metabolism; £, decreased nuclear localization; ¢, C-tail nuclear localization unaltered. *Xenopus sequences are from X. tropicalis; **construct expressed as a soluble protein. Sequence accession numbers: human AAC37576; pig CBZ01637; dog AAM22956; rat AAG33986; mouse AAC53207; puffer XP_011610747; Xenopus XP_017952982.

FIGURE 3

Summary of PC1-mediated regulation of heterotrimeric G protein signaling. The minimal G protein binding domain is represented by the membrane-proximal portion of the C-tail, i.e., red line with hatched box representing the G protein activation motif. Other structural domains are as identified in Figure 1. Arrow indicates activation; the bar-headed line indicates inhibition. There is evidence that PC1 both positively and negatively regulates Gαs and Gα12 families.

FIGURE 4

The proposed allosteric mechanism for stalk-mediated activation of signaling by the CTF form of PC1. The model is supported by the results from molecular dynamics simulations and mutagenesis-signaling studies (Pawnikar et al., 2022). The dashed lines indicate regions of the PC1 CTF protein where important correlated residue motions or residue-residue interactions were identified as being involved in activation of signaling by the stalk, i.e., between the N-terminal stalk (red arrow) and TOP domain, and between the TOP domain and the putative pore loop between S5 and S6 TM domains. Domains are as described in Figure 1; Section 1.

FIGURE 5

The PC1/PC2 hybrid receptor-channel complex. PC1 (red) and PC2 (green) are depicted as forming a heteromeric four-subunit complex comprised of three subunits of PC2 and one subunit of PC1 (Yu et al., 2009; Zhu et al., 2011; Wang et al., 2019). The last 6 TM domains of PC1 have homology with PC2. In this model, it is envisioned that PC2, being a transient receptor potential (TRP) channel (TRPP2) conducts a cation current together with PC1 acting as the fourth subunit of the channel. PC1 may function as a ligand-activated or mechanosensitive ionotropic receptor which transduces a signal to the channel subunits via heterotrimeric G protein activation (Parnell et al., 2018). Additionally, PC1 functioning as a G protein-dependent metabotropic receptor may independently activate downstream signal transduction.