Mechanisms and Physiology of COPI Transport

We have been at the forefront in elucidating the mechanisms and physiology of Coat Protein I (COPI) transport. We are proposing four lines of future investigation to maintain this track record of achievement. First, following up on our recent discovery that COPI generates not only vesicles but also tubules, and the small GTPase Cdc42 promotes COPI tubule formation through an intrinsic ability to bend membrane, we will elucidate how Cdc42 achieves this remarkable feat. In addition, as Cdc42 belongs to the Rho family of small GTPases, and we have found that other Rho members also affect COPI transport, we will elucidate how they could affect COPI vesicle and tubule formation. Second, we will address a current controversy regarding how COPI bends membrane. Whereas coat proteins are predicted to assemble into protein lattices with regular geometry in bending membrane, COPI has been found recently to assemble into lattices with irregular geometry. We will examine whether this apparent exception is due to the reconstitution of COPI vesicles that has thus far not accounted for all the factors needed for a physiology reconstitution of COPI vesicles. Third, we will follow up on our recent discovery that has identified a novel role for a ciliary protein, known as IFT20. We have found that IFT20 exists at the Golgi, where it promotes COPI tubular transport. Thus, we will elucidate how IFT20 exerts this novel role. Fourth, we will elucidate how a point mutation in a core component of the COPI complex, known as yl-COP, leads to immunodeficiency in affected individuals. We have already elucidated one explanation, which involves defect in COPI binding to the KDEL receptor, leading to stress in the endoplasmic reticulum (ER) to impair the function of T and B cells. However, because COPI binds to other cargo proteins, including a large family of proteins that promote exit from the ER, known as ER cargo receptors, we will identify those ER cargo receptors affected by the yl-COP mutation and then elucidate how their defective binding by COPI leads to altered cellular functions. In addition, as we have found that the y l -COP mutation also impairs COPI tubular transport, we will elucidate a mechanistic explanation for this additional effect of the mutation. We anticipate that the completion of these four aims will advance a basic understanding of how COPI acts to generate transport carriers, as well shed physiologic insights into cellular processes that requires this transport.