Lee C W, Lee K H, Lee S B, Park D, Rhee S G
Laboratory of Biochemistry, NHLBI, National Institutes of Health, Bethesda, Maryland 20892.
J Biol Chem. 1994 Oct 14;269(41):25335-8.
The fourth member of mammalian beta-type phospholipase C isozymes, PLC-beta 4, was recently purified from bovine retina, and the corresponding cDNA was cloned from rat brain and sequenced. PLC-beta 4 has now been shown to differ from the other three mammalian beta-type isozymes (PLC-beta 1, -beta 2, and -beta 3) in that it is selectively inhibited by ribonucleotides. The inhibition requires the 5'-phosphate and 2'-hydroxyl groups of ribose as well as the base moiety. Thus, deoxyribonucleotides and ribose 5-phosphate were not inhibitory. The monophosphate, diphosphate, and triphosphate nucleoside derivatives were all inhibitory, whereas cyclic nucleotides were ineffective. Purine nucleotides were more potent inhibitors than pyrimidine nucleotides; the 50% inhibitory concentrations were 20-30 microM for AMP and GMP, and 100-200 microM for UMP and CMP. Unlike the other beta-type isozymes, PLC-beta 4 contains the GX4GKS consensus sequence for the recognition of the phosphoryl group of nucleotides. In the absence of ribonucleotides, the specific activity of PLC-beta 4 toward phosphatidyl-inositol 4,5-bisphosphate was four to five times the average specific activity of PLC-beta 1 and PLC-beta 3. Thus, nucleotide-dependent inhibition may serve to reduce the activity of PLC-beta 4 in the absence of a hormonal signal. The regulation of PLC-beta 4 by G-proteins was also studied. Similar to the other three PLC-beta isozymes, PLC-beta 4 was activated by the alpha subunit of Gq but not by the transducin alpha subunit. However, unlike other PLC-beta isozymes, PLC-beta 4 was not responsive to activation by G beta gamma subunits.
哺乳动物β型磷脂酶C同工酶的第四个成员，即磷脂酶C-β4（PLC-β4），最近从牛视网膜中纯化出来，其相应的cDNA从大鼠脑中克隆并测序。现已证明，PLC-β4与其他三种哺乳动物β型同工酶（PLC-β1、-β2和-β3）不同，它能被核糖核苷酸选择性抑制。这种抑制作用需要核糖的5'-磷酸基团、2'-羟基基团以及碱基部分。因此，脱氧核糖核苷酸和5-磷酸核糖没有抑制作用。单磷酸、二磷酸和三磷酸核苷衍生物均有抑制作用，而环核苷酸则无效。嘌呤核苷酸比嘧啶核苷酸是更强效的抑制剂；AMP和GMP的50%抑制浓度为20 - 30微摩尔，UMP和CMP的50%抑制浓度为100 - 200微摩尔。与其他β型同工酶不同，PLC-β4含有用于识别核苷酸磷酸基团的GX4GKS共有序列。在没有核糖核苷酸的情况下，PLC-β4对磷脂酰肌醇4,5-二磷酸的比活性是PLC-β1和PLC-β3平均比活性的四到五倍。因此，核苷酸依赖性抑制可能在没有激素信号时用于降低PLC-β4的活性。还研究了G蛋白对PLC-β4的调节作用。与其他三种PLC-β同工酶类似，PLC-β4被Gq的α亚基激活，但不被转导素α亚基激活。然而，与其他PLC-β同工酶不同，PLC-β4对Gβγ亚基的激活没有反应。
