Abstract

Prof. Howard Riezman, University of Geneva, Switzerland.

The coat complex COPII forms vesicles at the endoplasmic reticulum exit sites (ERES) to transport an wide variety of secretory proteins to the Golgi apparatus. Such cargo diversity implies different ER exit mechanisms. In yeast, glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are selectively concentrated at specific ERES(1), from where they are transported to the Golgi in distinct vesicles from other secretory proteins(2,3). This concentrative mechanism involves the remodeling of the GPI anchors(1). Furthermore, the ER export of GPI-APs requires the p24 proteins, which are transmembrane proteins assembled into heteromeric complexes that cycle between the ER and the Golgi. Yeast Emp24p has been postulated to act as a cargo receptor for concentration into COPII vesicles(4). We show that EMP24 is not required for concentration of GPI-anchored proteins into ERES. On the other hand we provide evidence that the Emp24 complex binds specifically to GPI-anchored proteins and acts in quality control of GPI-anchored protein biosynthesis. Emp24p is apparently part of a checkpoint mechanism that monitors the proper remodeling of GPI-anchored proteins. Through this dual function, the p24 complex might coordinate GPI anchor remodeling and ER exit of GPI-APs in yeast. If the mechanism of p24 function is conserved in mammalian cells this would imply a function as a cargo receptor for ER exit GPI-anchored proteins in higher eukaryotes.

References
1. Castillon, G. A., Watanabe, R., Taylor, M., Schwabe, T. M., and Riezman, H. (2009) Traffic 10, 186-200 2. Morsomme, P., and Riezman, H. (2002) Dev Cell 2, 307-317
3. Muniz, M., Morsomme, P., and Riezman, H. (2001) Cell 104, 313-320
4. Muniz, M., Nuoffer, C., Hauri, H. P., and Riezman, H. (2000) J Cell Biol 148, 925-930  

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