Cannabinoid Receptor Interacting Protein 1a (CRIP1a): Function and Structure

Abstract

Cannabinoid receptor interacting protein 1a (CRIP1a) is an important CB₁ cannabinoid receptor-associated protein, first identified from a yeast two-hybrid screen to modulate CB₁-mediated N-type Ca²⁺ currents. In this paper we review studies of CRIP1a function and structure based upon in vitro experiments and computational chemistry, which elucidate the specific mechanisms for the interaction of CRIP1a with CB₁ receptors. N18TG2 neuronal cells overexpressing or silencing CRIP1a highlighted the ability of CRIP1 to regulate cyclic adenosine 3',5'monophosphate (cAMP) production and extracellular signal-regulated kinase (ERK1/2) phosphorylation. These studies indicated that CRIP1a attenuates the G protein signaling cascade through modulating which Gi/o subtypes interact with the CB₁ receptor. CRIP1a also attenuates CB₁ receptor internalization via β-arrestin, suggesting that CRIP1a competes for β-arrestin binding to the CB₁ receptor. Predictions of CRIP1a secondary structure suggest that residues 34-110 are minimally necessary for association with key amino acids within the distal C-terminus of the CB₁ receptor, as well as the mGlu_8a metabotropic glutamate receptor. These interactions are disrupted through phosphorylation of serines and threonines in these regions. Through investigations of the function and structure of CRIP1a, new pharmacotherapies based upon the CRIP-CB₁ receptor interaction can be designed to treat diseases such as epilepsy, motor dysfunctions and schizophrenia.

Keywords: CP55940; G protein coupled receptor (GPCR); G proteins; WIN55212-2; adenylyl cyclase; computational chemistry; cyclic adenosine 3′,5′monophosphate (cAMP); extracellular signal-regulated kinase (ERK); β-arrestin.

Figure 1

(A). Overview of the known structural and biochemical features of the CB₁ receptor. The complex between full-length CB₁ receptor, agonist MDMB-fubinaca (FUB), and G protein complex, as determined by cryoelectron microscopy (PDB code 6N4B) [32]. The intracellular region interactions are highlighted. Importantly, similar to available crystal structures, no electron density was observed for ICL3 and the C-terminus (residues 412–472). The nuclear magnetic resonance study by Ahn and colleagues supports that residues 440–461 form a second amphipathic helix that can interact with the membrane [33]. Cys415 can be palmitoylated to help anchor the C-terminus to the membrane [34,35]. (B). Primary sequence of the human CB₁ receptor ICL3 loop, and C-terminal central and distal regions. The sequence is depicted in the same coloring as Figure 1A. Residues that can be phosphorylated are indicated in bold and underlined. Negatively-charged amino acids are indicated in bold. Five residues in the distal regions of the CB₁ and mGlu_8a receptors important for CRIP1a binding are boxed in black (see text for details).

Figure 2

Primary sequence of CRIP1a from model organisms. Scientific names and protein codes: Rattus norvegicus (Q5M7A7), Mus musculus (Q5M8N0), Homo sapiens (Q96F85), Bos taurus (Q17QM9), Xenopus laevis (NP_001087998), Danio rerio (NP_001314699). The sequence identity relative to the top sequence proceeds down the alignment: 99%, 95%, 95%, 67%, and 59%.