Abstract
The small G proteins including Rab5 and Rheb, which cycle between active (GTP-bound) and inactive (GDP-bound) states, play essential roles for membrane budding and trafficking in cells. Here we show novel Rab5-binding proteins (RIN family), which contain many functional domains shared with other RIN members and additional Pro-rich domains. RIN3 displays the same biochemical properties as RIN2, the stimulator and stabilizer for GTP-Rab5. RIN3 was also capable of interacting via its Pro-rich domain with amphiphysin II, which contains SH3 domain and participates in receptor-mediated endocytosis. Interestingly, cytoplasmic amphiphysin II was translocated into the RIN3-positive vesicles when co-expressed with RIN3. These results indicate that RIN3 biochemically characterized as the stimulator and stabilizer for GTP-Rab5 plays an important role in the transport pathway from plasma membrane to early endosomes. Rheb appears to be involved not only in cell growth but also in nutrient uptake. We identified that Rheb expression in cultured cells induces the formation of large cytoplasmic vacuoles. The vacuole formation required the GTP form of Rheb, but not the activation of the downstream mTOR kinase. These results suggest that Rheb regulates endocytic trafficking pathway independent of mTOR pathway. The physiological roles of the two Rheb-dependent signaling pathways are discussed in terms of nutrient uptake and cell growth or cell cycle progression. [J Physiol Sci. 2007;57 Suppl:S43]
