Sun-Wada Ge-Hong, Toyomura Takao, Murata Yoshiko, Yamamoto Akitsugu, Futai Masamitsu, Wada Yoh
Department of Biochemistry, Faculty of Pharmaceutical Sciences, Doshisha Women's College, Kyotanabe 610-0395, Japan.
J Cell Sci. 2006 Nov 1;119(Pt 21):4531-40. doi: 10.1242/jcs.03234. Epub 2006 Oct 17.
Vacuolar-type H(+)-ATPase (V-ATPase) is a multi-subunit enzyme that has important roles in the acidification of a variety of intracellular compartments and some extracellular milieus. Four isoforms for the membrane-intrinsic subunit (subunit a) of the V-ATPase have been identified in mammals, and they confer distinct cellular localizations and activities on the proton pump. We found that V-ATPase with the a3 isoform is highly expressed in pancreatic islets, and is localized to membranes of insulin-containing secretory granules in beta-cells. oc/oc mice, which have a null mutation at the a3 locus, exhibited a reduced level of insulin in the blood, even with high glucose administration. However, islet lysates contained mature insulin, and the ratio of the amount of insulin to proinsulin in oc/oc islets was similar to that of wild-type islets, indicating that processing of insulin was normal even in the absence of the a3 function. The insulin contents of oc/oc islets were reduced slightly, but this was not significant enough to explain the reduced levels of the blood insulin. The secretion of insulin from isolated islets in response to glucose or depolarizing stimulation was impaired. These results suggest that the a3 isoform of V-ATPase has a regulatory function in the exocytosis of insulin secretion.
液泡型H(+)-ATP酶（V-ATP酶）是一种多亚基酶，在多种细胞内区室及一些细胞外环境的酸化过程中发挥重要作用。在哺乳动物中已鉴定出V-ATP酶膜内在亚基（a亚基）的四种同工型，它们赋予质子泵不同的细胞定位和活性。我们发现具有a3同工型的V-ATP酶在胰岛中高度表达，并定位于β细胞中含胰岛素分泌颗粒的膜上。a3基因座发生无效突变的oc/oc小鼠，即使给予高糖，血液中的胰岛素水平也降低。然而，胰岛裂解物中含有成熟胰岛素，oc/oc胰岛中胰岛素与胰岛素原的量的比值与野生型胰岛相似，这表明即使在缺乏a3功能的情况下胰岛素加工也是正常的。oc/oc胰岛的胰岛素含量略有降低，但不足以显著解释血液胰岛素水平的降低。分离的胰岛对葡萄糖或去极化刺激的胰岛素分泌受损。这些结果表明，V-ATP酶的a3同工型在胰岛素分泌的胞吐作用中具有调节功能。
