Gq protein alpha subunit is a family of heterotrimeric G protein alpha subunits. This family is also commonly called the Gq/11 (Gq/G11) family or Gq/11/14/15 family to include closely related family members. G alpha subunits may be referred to as Gq alpha, Gαq, or Gqα. Gq proteins couple to G protein-coupled receptors to activate beta-type phospholipase C (PLC-β) enzymes. PLC-β in turn hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to diacyl glycerol (DAG) and inositol trisphosphate (IP3). IP3 acts as a second messenger to release stored calcium into the cytoplasm, while DAG acts as a second messenger that activates protein kinase C (PKC). Depending on the Gα subunit involved in the complex, the most well-known G-proteins are qualified as Gi, Gs, or Gq. They signal through different pathways. Gq proteins rely on enzymes of the phospholipase C family (PLC), while Gs and Gi proteins respectively stimulate and inhibit adenylate cyclase (AC) and thus act upon the amount of cytosolic cAMP. Contraction of smooth muscle is initiated by a Ca2+-mediated change in the thick filaments, whereas in striated muscle Ca2+ mediates contraction by changes in the thin filaments. In response to specific stimuli in smooth muscle, the intracellular concentration of Ca2+ increases, and this activator Ca2+ combines with the acidic protein calmodulin. This complex activates MLC kinase to phosphorylate the light chain of myosin (Fig. 1). Cytosolic Ca2+ is increased through Ca2+ release from intracellular stores (sarcoplasmic reticulum) as well as entry from the extracellular space through Ca2+ channels (receptor-operated Ca2+ channels). Agonists (norepinephrine, angiotensin II, endothelin, etc.) binding to serpentine receptors, coupled to a heterotrimeric G protein, stimulate phospholipase C activity. This enzyme is specific for the membrane lipid phosphatidylinositol 4,5-bisphosphate to catalyze the formation of two potent second messengers: inositol trisphosphate (IP3) and diacylglycerol (DG). The binding of IP3 to receptors on the sarcoplasmic reticulum results in the release of Ca2+ into the cytosol. DG, along with Ca2+, activates protein kinase C (PKC), which phosphorylates specific target proteins. There are several isozymes of PKC in smooth muscle, and each has a tissue-specific role (e.g., vascular, uterine, intestinal, etc.). In many cases, PKC has contraction-promoting effects such as phosphorylation of L-type Ca2+ channels or other proteins that regulate cross-bridge cycling. Adrenergic receptors that use Gq protein signalling to contract smooth muscles include the alpha-1 receptors. The alpha-1 receptor agonist actions include Smooth muscle contraction, mydriasis, vasoconstriction in the skin, mucosa and abdominal viscera & sphincter contraction of the GI tract and urinary bladder.

Gq Adrenergic Smooth Muscle Contraction References