Which is conserved in all PLC isoforms (Additional file 1: Figure S1), and the region between the catalytic TIM barrel and C2 domain of PLC3, providing an interface between PLC3 and Gq for interaction between a series of charged residue pairs. The highly conserved helix-turn-helix segment (H1/H2) at the C-terminus of the Lenvatinib C2 domain of PLC3 resides on the surface region formed by switch II (2-4) and the 3 helix of Gq and allows the formation of various contacts with Gq in the large binding interface (Fig. 1c). More recently, discovery of the full-length structures of both PLC3 and Gq in complex has highlighted additional domains of PLC3 and Gq necessary for activation of lipid hydrolysis and protein interactions [14]. The crystallized full-length PLC3 contains a distal C-terminal domain (CTD) which is considered to be important for activation, membrane localization, and regulation by Gq proteins [15, 16]. The distal CTD adopts an orientation that makes direct contacts with the N helix of Gq and most likely participates in binding with G proteins. Regions of the Gq necessary for PLC interaction (namely, Ile217 to Lys276 which encompass the 2-4-35 regions) have PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/22993420 previously been identified by alaninescanning mutagenesis [17] and they are appropriately positioned for the interaction with PLC3 (Fig. 1a). A total of 33 amino acids in small clusters along the 2 to 4 regions (except Ile21, Ile25, and Leu29, which lies in the N helix, and Lys41 which lies on the 1 strand) of Gq are predicted to form intermolecular bonds with PLC [13, 14]. As expected, most interacting residues in the core regions are conserved in all other Gq members including G11, G14, and G16 (Fig. 1a). However, between 36 and 60 of the identified PLC-interacting residues are also found in other G protein families, with members of the Gi family having the highest homology to Gq (Additional file 1: Figure S2) [18, 19]. For instance, Gz of the Gi subfamily exhibits close to 60 identity with Gq in the core PLC-interacting regions (Fig. 1a). Such a high degree of identity is rather surprising especially when G16, which stimulates PLC, is only 74 identical to Gq in the PLC-interacting regions (Fig. 1a). More interestingly, molecular modeling between Gq andGz predicts that their differences in the PLC-interacting regions form a ring around a central core domain (Fig. 1b, space filled models), with most of the PLC contact points conserved between the two G subunits (Fig. 1c). This calls into question whether the residues identified by molecular replacement [13] are PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/8627573 sufficient to provide PLC binding selectivity to Gq members. Although the identified residues are involved in the formation of the PLC3-Gq complex and are necessary in PLC3 activation as confirmed in IP3 studies [13], there may be additional regions in Gq members that determine selectivity for PLC. It has been well established that constitutively active Gq subunits can efficiently stimulate PLC [20] but has no regulatory effect on adenylyl cyclase [21]. Early studies have employed chimeric Gq/Gs and G16/Gz constructs to map the PLC and receptor interacting domains on the Gq and G16 subunits [17, 22]. It has not been demonstrated whether other Gq members such as G14 (with over 80 sequence similarity with Gq) utilize the same regions to interact with PLC. Likewise, it remains to be determined if other PLC isoforms such as PLC2 (with the highest resemblance to PLC3; Additional file 1: Figure S1) employ similar structural.
