Yoo Seung Hyun, Nam Seong Won, Huh Seong Kwon, Park Seon Young, Huh Yang Hoon
National Creative Research Initiative Center for Secretory Granule Research and Department of Biochemistry, Inha University College of Medicine, Jung Gu, Incheon 400-712, Korea.
Biochemistry. 2005 Jun 28;44(25):9246-54. doi: 10.1021/bi047427t.
Although the inositol 1,4,5-trisphosphate (IP(3)) induced nuclear Ca(2+) releases have been shown to play key roles in nuclear functions, the presence and operation of the IP(3)-dependent Ca(2+) control mechanism in the nucleoplasm have not been shown. Recently, we found the presence of a high-capacity, low-affinity Ca(2+)-storage protein chromogranin B (CGB) and all three IP(3) receptor (IP(3)R) isoforms in the nucleoplasm, localizing widely in both the heterochromatin and euchromatin regions. In view of the essential role of CGB-IP(3)R coupling in IP(3)-dependent Ca(2+) release in the endoplasmic reticulum, the potential coupling between CGB and the IP(3)Rs in the nucleoplasm was investigated. Hence, we found in the present study the presence of a nucleoplasmic complex, which is composed of the IP(3)R, CGB, and phospholipids, with an estimated molecular mass of approximately 2-3 x 10(7) Da, suggesting the possibility of the presence of an IP(3)-sensitive Ca(2+) store in the nucleoplasm. Moreover, double-labeling immunogold electron microscope studies showed the colocalization of all three IP(3)R isoforms with CGB to the extent that the majority of each IP(3)R isoform-labeling gold particles found in the nucleoplasm was literally next to the CGB-labeling gold particles. In line with the potential existence of an IP(3)-dependent vesicular nucleoplasmic Ca(2+) store, our preliminary results indeed showed a sudden release of Ca(2+) from a putative nucleoplasmic Ca(2+) store in response specifically to IP(3) but not to inositol 1,4-bisphosphate or inositol 1,3,4,5-tetrakisphosphate.
尽管已证明肌醇1,4,5 - 三磷酸（IP(3)）诱导的核钙释放（Ca(2+)）在核功能中起关键作用，但尚未证明核质中存在依赖IP(3)的钙调控机制及其运作情况。最近，我们发现核质中存在一种高容量、低亲和力的钙储存蛋白嗜铬粒蛋白B（CGB）以及所有三种IP(3)受体（IP(3)R）亚型，它们广泛定位于异染色质和常染色质区域。鉴于CGB - IP(3)R偶联在内质网中依赖IP(3)的钙释放中起重要作用，我们研究了核质中CGB与IP(3)R之间潜在的偶联情况。因此，我们在本研究中发现了一种核质复合物，它由IP(3)R、CGB和磷脂组成，估计分子量约为2 - 3×10(7) Da，这表明核质中可能存在对IP(3)敏感的钙储存库。此外，双标记免疫金电子显微镜研究表明，所有三种IP(3)R亚型与CGB共定位，以至于在核质中发现的每种IP(3)R亚型标记的金颗粒大部分实际上都紧邻CGB标记的金颗粒。与依赖IP(3)的核质囊泡钙储存库的潜在存在一致，我们的初步结果确实表明，假定的核质钙储存库会因特异性响应IP(3)而非肌醇1,4 - 二磷酸或肌醇1,3,4,5 - 四磷酸而突然释放钙。
