ER export of GPI-anchored proteins

Following their synthesis in the ER, many transmembrane proteins are selectively exported via interactions between their cytosolic domains and the Sec24 subunit of coat protein complex II (COPII) vesicles. By contrast, less is known about how GPI-anchored proteins (which lack a cytosolic domain) are exported from the ER in mammalian cells. Although the mechanisms of early secretory transport have been partially worked out in yeast, only recently is the picture becoming clearer for mammalian cells. On page 1705, Hans-Peter Hauri and colleagues add to this story through studying the transport of endogenous GPI-anchored proteins in HeLa cells. They first show that ER-to-Golgi transport of GPI-anchored proteins in human cells requires COPII and, more specifically, the Sec24 isoforms C and D. On the basis of studies in yeast, the authors test whether members of the p24 protein family (comprising p23, p24 and p25 in mammalian cells) are involved in this process: indeed, p23 and p24, but not p25, interact with and are required for the export of GPI-anchored proteins from the ER. By contrast, p24 proteins are not required for the transport of transferrin receptor, a type I transmembrane protein. Finally, the finding that p23 and p24 partially co-partition with GPI-anchored proteins in detergent-resistant membrane fractions suggests that lipid rafts are involved in the selective transport of GPI-anchored proteins from the ER.