Faculty and Research
Faculty by Name
Randy Schekman
|
Randy Schekman
Howard Hughes Investigator and Professor of Cell and Developmental Biology*
*And Affiliate, Division of Biochemistry and Molecular Biology
Research Interests
-
Our research is on the mechanism and control of intracellular protein transport in Saccharomyces cerevisiae and mammalian cells. In particular we are studying the secretory process and its role in the assembly of cellular organelles. Enzymology, genetics, and electron microscopy are employed in this investigation.
Current Projects
-
Protein translocation into the lumen of the endoplasmic reticulum represents the initial step in assembly of the eukaryotic cell surface. Sec61p forms the channel in the ER membrane through which secretory polypeptides pass from the cytoplasm into the lumen. Misfolded secretory proteins are removed from the ER by reverse translocation, though the channel responsible for this process remains to be clearly identified.
Subsequent stages in the secretory pathway involve protein sorting and transport from the endoplasmic reticulum to the Golgi body and from there to the cell surface. An assay that depends on Sec proteins has been reconstituted in vitro. Biochemical requirements for vesicle budding from the ER have been elucidated by fractionation of cytosolic and peripheral membrane associated Sec proteins. Budding requires four proteins: a small GTP-binding protein called Sar1p, a complex (Sec23/Sec24) that is required to stimulate GTP hydrolysis by Sar1p, an integral membrane protein (Sec12p) that facilitates nucleotide exchange by Sar1p, and another complex (Sec13/Sec31). These proteins comprise a novel coat protein complex, COPII, which is distinct from the coat complex, COPI, involved in vesicle budding in the Golgi apparatus. A detailed picture of the mechanism of protein sorting and vesicle morphogenesis has emerged from structured-function studies with the purified proteins. A comparable reaction with mammalian COPII proteins has been reconstituted and is being used to explore the mechanism of transport of proteins implicated in familial forms of Alzheimer's disease and other human diseases affecting secretion.
A model for the binding of coat proteins and capture of membrane molecules in COPII vesicles.
Most new cell surface proteins are assembled in the bud portion of a growing yeast cell. However, some membrane proteins define a ring that separates the bud and mother portions of a cell. We have focused on the enzymes that make chitin, a carbohydrate polymer in the cell wall that grows inward from a ring to form a disk that separates daughter cells at the end of the cell division cycle. Chitin synthases (CSI, CSII, and CSIII) which operate at different stages in the cell cycle, organize division. All three enzymes traverse the secretory pathway en route to the cell surface; however, CSII and III then end up tightly organized at the locus where chitin is deposited.
In contrast to most cell surface and secreted proteins, a sizeable fraction of CSIII is maintained in an intracellular reservoir comprising the trans Golgi network and early endosome. We propose that this station represents a holding station from which CSIII may be mobilized to the mother-daughter cell junction at the beginning of the cell cycle. CSIII transport between the early endosome and the trans Golgi depends on the coat protein clathrin and the adaptor complex, AP-1. Normal traffic to the cell surface depends on a novel coat protein complex, the exomer, which is responsible for traffic of a non-essential subset of plasma membrane proteins. The mechanism of action of this coat is being explored using biochemical, genetic, and high resolution fluorescence and electron microscopy.
Selected Publications
- Please visit the Schekman lab website for a current list of publications.
Last Updated 2006-08-15