Valentijn J A, LaCivita D Q, Gumkowski F D, Jamieson J D
Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510, USA.
Eur J Cell Biol. 1997 Jan;72(1):1-8.
Rab4 is a small GTP-binding protein that has been implicated in the regulation of membrane traffic and recycling of transferrin receptors and GLUT4 transporters along the endocytic pathway. Here we present data that suggest a novel and very different role for rab4 during development in the rat exocrine pancreas. On immunoblots of pancreatic homogenates, a dramatic increase in rab4 expression occurred over the first 40 h after birth, concomitant with the time of acquisition of stimulus-secretion coupling. Following high-speed centrifugation of postnuclear supernatants prepared from 1-day neonatal pancreatic homogenates, rab4 partitioned into a Triton X-100 insoluble particulate fraction and was partially solubilized upon extraction with 0.1 M Na2CO3, pH 11.5, or 1 M KCl, suggesting that rab4 was not an integral membrane protein. This was confirmed by Triton X-114 extractions of post-nuclear supernatants showing that rab4 partitioned into the aqueous phase of Triton X-114, which is indicative of a lack of isoprenylation. Confocal and electron microscopic immunocytochemistry revealed that rab4 colocalized with the actin terminal web and microvilli in the apical region of the exocrine acinar cells. In view of these findings, we propose that rab4 is involved in the maturation of the regulated secretory pathway in pancreatic acinar cells through an interaction with the apical actin cytoskeleton.
Rab4是一种小GTP结合蛋白，它与膜运输的调节以及转铁蛋白受体和GLUT4转运体沿内吞途径的循环利用有关。在此，我们展示的数据表明，rab4在大鼠外分泌胰腺发育过程中具有一种全新且截然不同的作用。在胰腺匀浆的免疫印迹上，出生后的前40小时内rab4表达急剧增加，这与获得刺激-分泌偶联的时间一致。对1日龄新生胰腺匀浆制备的核后上清液进行高速离心后，rab4分配到Triton X-100不溶性颗粒部分，并用0.1 M Na2CO3（pH 11.5）或1 M KCl提取后可部分溶解，这表明rab4不是整合膜蛋白。通过对核后上清液进行Triton X-114提取证实了这一点，结果显示rab4分配到Triton X-114的水相中，这表明它缺乏异戊二烯化。共聚焦和电子显微镜免疫细胞化学显示，rab4在外分泌腺泡细胞顶端区域与肌动蛋白终末网和微绒毛共定位。鉴于这些发现，我们提出rab4通过与顶端肌动蛋白细胞骨架相互作用参与胰腺腺泡细胞中受调节分泌途径的成熟。
