# G\(\gamma\) and G\(\alpha\) Identity Dictate a G-Protein Heterotrim Plasma Membrane Targeting
## 1 Introduction  
G-protein-coupled receptor (GPCR) stimulation results in the activation of a G\(\alpha\) subunit and a G\(\beta\gamma\) complex of the heterotrimeric G-protein. Both the activated components of the heterotrimer modulate the function of downstream effector proteins located on the cytosolic surface of the cell membrane. The intracellular response effect is caused by second messenger molecules after the stimulation of effector proteins such as adenylyl cyclase, phospholipase C-\(\beta\), and ion channels. There are 18 G\(\alpha\), five G\(\beta\), and 12 G\(\gamma\) genes in the human genome [1]. The G\(\alpha\) subunit family is the most diverse among G-proteins and is responsible for the specificity of interactions with GPCRs [2]. There is evidence of distinct activity of the G\(\beta\gamma\) dimer in terms of G-protein activation and modulation of effector proteins [3]. Several different lipids are covalently attached to G\(\alpha\) and G\(\gamma\) subunits, and as hydrophobic anchors, lipids promote localization to cellular membranes. Apart from the subunits G\(\alpha\)t and G\(\alpha\)gust, all G\(\alpha\) subunits are palmitoylated, and some are also myristylated. Palmitoylation is a post-translational modification that involves the addition of saturated 16-carbon palmitic acid to a specific cysteine in an amino acid sequence through a thioester bond (S-palmitoylation). This modification is unique among lipidations and can be quickly reversed under in vivo conditions [4; 5]. It is also possible to attachpalmitoleic acid by an amide bond to the glycine residue in the N-terminal fragment of the G\(\alpha\)s protein (N-palmitoylation) [6]. Myristylation is a co-translation or post-translation modification which consists of attaching the myristate (a 14-carbon saturated fatty acid) to the N-terminal glycine residue through a peptide bond [7]. The non-myristylated G\(\alpha\) subunits (G\(\alpha\)s, G\(\alpha\)q, and G\(\alpha_{12/13}\) families) contain the motif of basic residues, arginines and lysines, forming a charged surface on one side of the N-terminal helix [8; 9; 10; 11]. Such a charged protein area is able to interact with negatively-charged lipid head groups. A positively charged surface is also present in the G\(\beta\) structure. Such a region, through electrostatic interactions favors binding to acidic membrane phospholipids [12]. All G\(\gamma\) subunits undergo post-translational isoprenylation. Prenylation is an irreversible multistage modification that involves the transfer of a C\({}_{15}\) farnesyl or C\({}_{20}\) geranylgeranyl group to a cysteine residue within the C-terminal CaaX motif via a thioether bond. Farnesylation occurs in the G\(\gamma_{1}\), G\(\gamma_{9}\), and G\(\gamma_{11}\) subunits; the other subunits of G\(\gamma\) are geranylgeranylated [13]. According to the two-signal hypothesis or kinetic trapping [14; 15; 16; 17; 18] a single membrane binding signal might be insufficient for proper membrane docking. However, a combination of two or more signals such as lipid anchor, poly-basic sequence or other motifs can ensure high-affinity interaction with cellular membranes. Apart from differences in the length of anchors, the literature distinguishes three groups of gamma subunits based on transient translocation to endmembranes. The \(\gamma_{1}\), \(\gamma_{11}\), \(\gamma_{9}\), and \(\gamma_{13}\) belong to a group characterized by rapid translocation, \(\gamma_{5}\) and \(\gamma_{10}\) translocate slowly, while the other G\(\gamma\) subunits, previously identified as non-translocating, are currently described as the slowest translocating [19; 20]. It is noteworthy, that the translocation process is acylation/deacylation cycle dependent and the activation of G-protein is not necessary [21].