The Requirement For Rab Proteins And Their Effectors In DCV Exocytosis In C. Elegans

Rab GTPases is a large family of small GTPases. Rab GTPases regulate vesicle traffic which is fundamental to the existence of eukaryotic cells. Rab GTPases recruit their effector proteins including sorting adaptors, tethering factors, kinases, phosphatases and motors, control membrane identity and vesicle budding, uncoating, motility and fusion. Model organism C. elegans have many advantages, such as short life span, convenient genetics, transparent body, etc, these make them a good research system for exploring the molecular basis of neurotransmission. In this paper, we demonstrate the requirement for Rab proteins and their effectors in DCV exocytosis in C. elegans.The molecular mechanisms by which dense core vesicles (DCVs) translocating, tethering, docking and priming remain to be elucidated. In current study, we have employed C. elegans model system to study the function of Rab proteins and their effectors in DCV exocytosis. We quantified neuropeptide release by monitoring the uptake of GFP-tagged neuropeptides into coelomocytes in vivo. Furthermore, we employed total internal reflection fluorescence microscopy (TIRFM) to track the movements of single DCVs approaching plasma membrane. Through a combination of these methods, we show that RAB-27/AEX-6, but not RAB-3 which is a close homologue of RAB-27, is required for peptide release from neurons. We demonstrate that RAB-27/AEX-6 is involved in the translocation and tethering of DCVs. We further show that different Rab effectors act at distinct steps in DCV exocytosis:whereas rabphilin/RBF-1 is involved in an initial tethering step and stabilizing the docking, and RIM/UNC-10 is required for stabilizing the docking and making the DCVs fusion competent. Our data suggest that RAB-27 interacts with its effector rabphilin/RBF-1 regulate the tethering and docking of DCVs, while active zone protein RIM/UNC-10 acts a role in the docking and priming of DCVs.