Glick J, Santoyo G, Casey P J
Department of Molecular Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710-3686, USA.
J Biol Chem. 1996 Feb 9;271(6):2949-54. doi: 10.1074/jbc.271.6.2949.
Gz is a member of the family of trimeric guanine nucleotide-binding regulatory proteins (G proteins), which plays a crucial role in signaling across cell membranes. The expression of Gz is predominately confined to neuronal cells and platelets, suggesting an involvement in a neuroendocrine process. Although the signaling pathway in which Gz participates is not yet known, it has been linked to inhibition of adenylyl cyclase. We have found that arachidonate and related unsaturated fatty acids suppress guanine nucleotide binding to the alpha subunit of Gz. This inhibition of nucleotide binding by cis-unsaturated fatty acids is specific for Gz alpha; other G protein alpha subunits are relatively insensitive to these lipids. The IC50 for inhibition by the lipids closely corresponds to their critical micellar concentrations, suggesting that the interaction of the lipid micelle with Gzalpha is the primary event leading to inhibition. The presence of the acidic group of the fatty acid is critical for inhibition, as no effect is observed with the corresponding fatty alcohol. While arachidonic acid produces near-complete inhibition of both GDP and guanosine 5-(3-O-thio)triphosphate binding by Gzalpha, release of GDP from the protein was unaffected. Furthermore, the rate of inactivation of Gzalpha by arachidonate is essentially identical to the rate of GDP release from the protein, indicating that GDP release is required for inactivation. These observations indicate that the mechanism of inactivation of Gzalpha by unsaturated fatty acids is through an interaction of an acidic lipid micelle with the nucleotide-free form of the protein. Although the physiologic significance of this finding is unclear, similar effects of unsaturated fatty acids on other proteins involved in cell signaling indicate potential roles for these lipids in signal modulation. Additionally, the ability of arachidonate to inactivate this adenylyl cyclase-inhibitory G protein provides a molecular mechanism for previous findings that treatment of platelets with arachidonate results in elevated cAMP levels.
Gz是三聚体鸟嘌呤核苷酸结合调节蛋白（G蛋白）家族的成员，在跨细胞膜信号传导中起关键作用。Gz的表达主要局限于神经元细胞和血小板，提示其参与神经内分泌过程。尽管Gz参与的信号通路尚不清楚，但它与腺苷酸环化酶的抑制有关。我们发现花生四烯酸及相关不饱和脂肪酸可抑制鸟嘌呤核苷酸与Gzα亚基的结合。顺式不饱和脂肪酸对核苷酸结合的这种抑制作用对Gzα具有特异性；其他G蛋白α亚基对这些脂质相对不敏感。脂质抑制的IC50与其临界胶束浓度密切对应，提示脂质胶束与Gzα的相互作用是导致抑制的主要事件。脂肪酸酸性基团的存在对抑制至关重要，因为相应的脂肪醇未观察到作用。虽然花生四烯酸对Gzα结合GDP和鸟苷5-(3-O-硫代)三磷酸均产生近乎完全的抑制，但蛋白质中GDP的释放不受影响。此外，花生四烯酸使Gzα失活的速率与蛋白质中GDP释放的速率基本相同，表明GDP释放是失活所必需的。这些观察结果表明，不饱和脂肪酸使Gzα失活的机制是通过酸性脂质胶束与蛋白质的无核苷酸形式相互作用。尽管这一发现的生理意义尚不清楚，但不饱和脂肪酸对其他参与细胞信号传导的蛋白质的类似作用表明这些脂质在信号调节中具有潜在作用。此外，花生四烯酸使这种抑制腺苷酸环化酶的G蛋白失活的能力为先前的研究结果提供了分子机制，即花生四烯酸处理血小板会导致cAMP水平升高。
