Van Der Sluijs P, Hull M, Zahraoui A, Tavitian A, Goud B, Mellman I
Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510.
Proc Natl Acad Sci U S A. 1991 Jul 15;88(14):6313-7. doi: 10.1073/pnas.88.14.6313.
Small GTP-binding proteins of the rab family have been implicated as playing important roles in controlling membrane traffic on the biosynthetic and endocytic pathways. We demonstrate that a distinct rab protein, rab4p, is associated with the population of early endosomes involved in transferrin-receptor recycling. An antibody to human rab4p was found to detect a doublet of approximately 24-kDa proteins on immunoblots from various cell types. Seventy-five percent of these proteins were tightly membrane bound and could be released only by detergent treatment. Upon isolation of early endosomes, late endosomes, and lysosomes, by free-flow electrophoresis and Percoll density-gradient centrifugation, most (70%) of the rab4p was found to cofractionate with early endosomes and endocytic vesicles containing 125I-labeled transferrin. The rab proteins previously localized to the endoplasmic reticulum and/or Golgi apparatus were not found in these fractions. We also localized rab4p to transferrin-receptor-containing early endosomes by immunofluorescence after expression of rab4 cDNA. The association of rab4p with early endosomes and other vesicles involved in the intracellular transport of transferrin receptor suggests that rab4p may play a role in regulating the pathway of receptor recycling.
rab家族的小GTP结合蛋白被认为在控制生物合成和内吞途径中的膜运输方面发挥重要作用。我们证明，一种独特的rab蛋白rab4p与参与转铁蛋白受体循环的早期内体群体相关。在来自各种细胞类型的免疫印迹上，发现一种针对人rab4p的抗体可检测到约24 kDa蛋白质的双峰。这些蛋白质中有75%与膜紧密结合，只有通过去污剂处理才能释放。通过自由流动电泳和Percoll密度梯度离心分离早期内体、晚期内体和溶酶体后，发现大部分（70%）的rab4p与早期内体和含有125I标记转铁蛋白的内吞囊泡共分离。先前定位于内质网和/或高尔基体的rab蛋白在这些组分中未被发现。在rab4 cDNA表达后，我们还通过免疫荧光将rab4p定位于含有转铁蛋白受体的早期内体。rab4p与早期内体和参与转铁蛋白受体内细胞运输的其他囊泡的关联表明，rab4p可能在调节受体循环途径中发挥作用。
