PROSITE documentation PDOC50221

GAIN-B domain profile

The GPS motif is a cysteine-rich domain of ~40 amino acid, which was identified in adhesion-G-protein-coupled receptors (aGPCRs) (including CIRLs/ latrophilins), polycystic kidney disease 1 (PKD1)-like proteins and in other membrane-associated proteins like the sea urchin receptor for egg jelly protein (REJ). The GPS motif is located immediately N-terminal to transmembrane region (1, 7, or 11 TM helices). For the CIRL-1, CIRL-2, CIRL-3 and CD-97 proteins, it has been shown that they are each made of two non-covalently bound subunits resulting from the endogenous proteolytic cleavage of a precursor protein. Because the cysteine-rich motif of CIRL-1 and possibly other receptors is involved in the endogenous proteolytic processing of CIRL-1 and possibly other receptors, it has been named GPS for GPCR proteolytic site. As the amino acids surrounding the putative cleavage site are the most conserved residues in the GPS domain, it has been suggested that all proteins containing it may be cleaved at this position [1,2,3].

The GPS motif is not an autonomously folded unit but rather part of a much larger evolutionarily conserved domain that has been refered to as the GPCR-Autoproteolysis INducing (GAIN) domain. The ~320-residue GAIN domain is an ancient domain that exists in primitive ancestor organisms, and is conserved in all aGPCRs and all PKD1-related proteins. Functionally, the GAIN domain is both necessary and sufficient for autoproteolysis, suggesting an autoproteolytic mechanism whereby the overall GAIN domain fine-tunes the chemical environment. In contrast to most other autoproteolytic domains that cause the release of the cleaved mature protein, autoproteolysis of the GAIN domain does not lead to the dissociation of the cleaved extracellular sequences containing the GAIN domain from the membrane-embedded regions of the protein. The GAIN domain forms a tightly associated heterodimer upon proteolysis. Cleavage at the GAIN domain is not a feature that all GAIN domain-containing receptors possess and the GAIN domain likely has other functions (such as interacting with ligands or transmembrane helices) in addition to the autoproteolysis function. Cleavage may be essential for receptor activity and its regulation in some receptors only, whereas in others it may not occur or be dispensable [4,5].

The conservation of the primary sequence of the GAIN domain decreases from the C terminus to the N terminus, and the GPS motif is the most conserved region of the domain. The N-terminal subdomain A of the GAIN domain is composed of a variable number of α-helices (6, 5 or 3), while the C-terminal subdomain B consists of a twisted β-sandwich including 13 β-strands and 2 small α-helices (see <4DLO>). The autocatalytic scissile bond in the GPS of the GAIN domain is positioned at a sharply kinked loop between the last two β-strands of the GAIN domain and lies close to the core of the protein away from the surface. Autoproteolysis cleaves the C-terminal β strand from the rest of the domain. The C-terminal β strand of the GAIN domain corresponds to the "Stachel peptide" (also called "tethered agonist" or "stalk") and acts as the tethered agonist that activates the aGPCR. However, autoproteolysis does not cause the dissociation of the Stachel peptide from the rest of the GAIN domain, and it is unclear how it can be exposed to the transmembrane region to activate the receptor. The Stachel peptide is highly conserved and has a hydrophobic nature. It is suggested that mechanical force applied on the extracellular domains may lead to shedding of the extracellular regions and release of the Stachel peptide from the GAIN domain enabling the Stachel peptide to activate the receptor [4,5,6,7,8].

The proposed autoproteolytic mechanism suggests that a general base at position -2 respective to the cleavage site (such as a histidine in rat latrophilin 1 or a water molecule) retracts a proton from the hydroxyl group of a threonine/serine at position +1 to yield a negatively charged oxygen. This oxygen makes a nucleophilic attack on the carbonyl carbon of the residue at position -1 (a leucine in rat latrophilin 1) forming a tetrahedral intermediate followed by an ester intermediate that eventually produces the cleaved protein. In most GAIN domains, the cleavage site is HL|T/S/C. Although autoproteolysis can occur only if the residue at position +1 is a threonine, serine, or cysteine, the residues at positions -2 and -1 may vary in different GAIN domains [5].

Some proteins known to contain a GAIN domain are listed below:

• Mammalian CIRL/latrophilin 1, 2 and 3 (CL1, CL2 and CL3), G-protein-coupled receptors performing a general and ubiquitous function in cellular signaling. These multidomain proteins also contain a SUEL-type lectin domain (see <PDOC50228>),
• Mammalian GPCRs Emr1, Emr2 and Emr3, which contain EGF domains (see <PDOC00021>).
• Mammalian polycystin-1 (PKD-1), a plasma-membrane protein. In human, defects in PKD1 are the cause of autosomal dominant polycystic kidney disease (ADPKD), a common autosomal dominant genetic disease, the cardinal manifestations of which are renal and liver cysts and intracranial aneurysm.
• Mammalian MEGF2, a type I membrane protein of the cadherin family.
• Human brain-specific angiogenesis inhibitor (BAI) 1, 2 and 3, GPCRs, which contain thrombospondin type I repeats (TSP1) (see <PDOC50092>).
• Human leucocyte antigen CD97. It could be a receptor potentially involved in both adhesion and signaling processes early after leukocyte activation.
• Purple sea urchin sperm receptor for egg jelly (REJ). This type I membrane protein induces the acrosome reaction.

The profile we have developed covers the entire GAIN-B domain.