Localization of Secretion-Related Proteins in Pancreatic

涉及内分泌特异性蛋白Resp18的调节胞吐的分子机制尚不完全清楚。在寻找受多巴胺能药物调控的基因时，首次从大鼠垂体cDNA文库中分离到Resp18。以前认为，RESP18主要定位于神经内分泌组织的内质网（endoplasmic reticulum， ER）。由于RESP18的部分序列与IA-2的管腔区同源，IA-2是一种参与囊泡稳定和生物合成的致密核囊泡膜蛋白，我们已经验证了RESP18参与致密核囊泡分泌调节的假设。通过免疫电镜观察，我们发现RESP18主要在DCVs的管腔中表达，仅在内质网和高尔基网络中表达较少。此外，双标记实验表明，RESP18与胰腺β细胞囊泡中的胰岛素和胰腺α细胞囊泡中的胰高血糖素共定位。结果表明，RESP18是一种新型的调节分泌囊泡的腔内蛋白，参与调节胰岛素分泌信号通路。

Molecular mechanisms of regulated exocytosis involving the endocrine-specific protein Resp18 are not yet fully understood. Resp18 was first isolated from a rat pituitary cDNA library in a search for genes that are regulated by dopaminergic drugs. Previously, it has been believed that RESP18 is localized primarily in the endoplasmic reticulum (ER) of neuroendocrine tissues. Since part of RESP18 shares a sequence homology with the luminal region of IA-2, a dense-core vesicle membrane protein involved in vesicle stabilization and biosynthesis, we have tested the hypothesis that RESP18 is implicated in the regulation of dense-core vesicle secretion. Using immuno electron microcopy, we find that RESP18 is mainly expressed in the lumen of DCVs and only to a lesser extent in the ER and Golgi network. Furthermore, double labeling experiments demonstrate that RESP18 is colocalized with insulin in pancreatic beta cell vesicles, as well as with glucagon in pancreatic alpha cell vesicles. The results suggest that RESP18 is a novel luminal protein of regulated secretory vesicles involving the regulated insulin secretion signaling pathway.