Purification of heterotrimeric G protein alpha subunits by GST-Ric-8 association: primary characterization of purified G alpha(olf)

Non-canonical signaling and localizations of heterotrimeric G proteins

Heterotrimeric G proteins typically transduce signals from G protein-coupled receptors (GPCRs) to effector proteins. In the conventional G protein signaling paradigm, the G protein is located at the cytoplasmic surface of the plasma membrane, where, after activation by an agonist-bound GPCR, the GTP-bound Gα and free Gβγ bind to and regulate a number of well-studied effectors, including adenylyl cyclase, phospholipase Cβ, RhoGEFs and ion channels. However, research over the past decade or more has established that G proteins serve non-canonical roles in the cell, whereby they regulate novel effectors, undergo activation independently of a GPCR, and/or function at subcellular locations other than the plasma membrane. This review will highlight some of these non-canonical aspects of G protein signaling, focusing on direct interactions of G protein subunits with cytoskeletal and cell adhesion proteins, the role of G proteins in cell division, and G protein signaling at diverse organelles.

Ric-8 proteins are molecular chaperones that direct nascent G protein α subunit membrane association

Ric-8A (resistance to inhibitors of cholinesterase 8A) and Ric-8B are guanine nucleotide exchange factors that enhance different heterotrimeric guanine nucleotide-binding protein (G protein) signaling pathways by unknown mechanisms. Because transgenic disruption of Ric-8A or Ric-8B in mice caused early embryonic lethality, we derived viable Ric-8A- or Ric-8B-deleted embryonic stem (ES) cell lines from blastocysts of these mice. We observed pleiotropic G protein signaling defects in Ric-8A(-/-) ES cells, which resulted from reduced steady-state amounts of Gα(i), Gα(q), and Gα(13) proteins to <5% of those of wild-type cells. The amounts of Gα(s) and total Gβ protein were partially reduced in Ric-8A(-/-) cells compared to those in wild-type cells, and only the amount of Gα(s) was reduced substantially in Ric-8B(-/-) cells. The abundances of mRNAs encoding the G protein α subunits were largely unchanged by loss of Ric-8A or Ric-8B. The plasma membrane residence of G proteins persisted in the absence of Ric-8 but was markedly reduced compared to that in wild-type cells. Endogenous Gα(i) and Gα(q) were efficiently translated in Ric-8A(-/-) cells but integrated into endomembranes poorly; however, the reduced amounts of G protein α subunits that reached the membrane still bound to nascent Gβγ. Finally, Gα(i), Gα(q), and Gβ(1) proteins exhibited accelerated rates of degradation in Ric-8A(-/-) cells compared to those in wild-type cells. Together, these data suggest that Ric-8 proteins are molecular chaperones required for the initial association of nascent Gα subunits with cellular membranes.

An autoinhibitory helix in the C-terminal region of phospholipase C-β mediates Gαq activation

Phospholipase C-β (PLCβ) is a key regulator of intracellular calcium levels whose activity is controlled by heptahelical receptors that couple to G_q. We have determined atomic structures of two invertebrate homologs of PLCβ (PLC21) from cephalopod retina and identified a helix from the C-terminal regulatory region that interacts with a conserved surface of the catalytic core of the enzyme. Mutations designed to disrupt the analogous interaction in human PLCβ3 dramatically increase basal activity and diminish stimulation by Gα_q. Gα_q binding requires displacement of the autoinhibitory helix from the catalytic core, thus providing an allosteric mechanism for activation of PLCβ.

Ric-8B is a GTP-dependent G protein alphas guanine nucleotide exchange factor

ric-8 (resistance to inhibitors of cholinesterase 8) genes have positive roles in variegated G protein signaling pathways, including Gα(q) and Gα(s) regulation of neurotransmission, Gα(i)-dependent mitotic spindle positioning during (asymmetric) cell division, and Gα(olf)-dependent odorant receptor signaling. Mammalian Ric-8 activities are partitioned between two genes, ric-8A and ric-8B. Ric-8A is a guanine nucleotide exchange factor (GEF) for Gα(i)/α(q)/α(12/13) subunits. Ric-8B potentiated G(s) signaling presumably as a Gα(s)-class GEF activator, but no demonstration has shown Ric-8B GEF activity. Here, two Ric-8B isoforms were purified and found to be Gα subunit GDP release factor/GEFs. In HeLa cells, full-length Ric-8B (Ric-8BFL) bound endogenously expressed Gα(s) and lesser amounts of Gα(q) and Gα(13). Ric-8BFL stimulated guanosine 5'-3-O-(thio)triphosphate (GTPγS) binding to these subunits and Gα(olf), whereas the Ric-8BΔ9 isoform stimulated Gα(s short) GTPγS binding only. Michaelis-Menten experiments showed that Ric-8BFL elevated the V(max) of Gα(s) steady state GTP hydrolysis and the apparent K(m) values of GTP binding to Gα(s) from ∼385 nm to an estimated value of ∼42 μM. Directionality of the Ric-8BFL-catalyzed Gα(s) exchange reaction was GTP-dependent. At sub-K(m) GTP, Ric-BFL was inhibitory to exchange despite being a rapid GDP release accelerator. Ric-8BFL binds nucleotide-free Gα(s) tightly, and near-K(m) GTP levels were required to dissociate the Ric-8B·Gα nucleotide-free intermediate to release free Ric-8B and Gα-GTP. Ric-8BFL-catalyzed nucleotide exchange probably proceeds in the forward direction to produce Gα-GTP in cells.