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First published online July 13, 2004
In this issue |
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Cargo is transported from the ER to the Golgi in vesicles bearing the coat complex COP-II. Assembly of COP-II complexes is regulated by the GTPase Sar1p, and they are thought to interact with cargo either directly or through specialized receptors. But is all cargo sorted in the same way, or does the COP-II machinery distinguish between different types? To find out, David Stephens and Rainer Pepperkok have compared the trafficking of transmembrane proteins, lipid-anchored proteins, small soluble proteins and large soluble proteins, using constructs that have different fluorescent tags (see p. 3635). They find that all use the COP-II machinery and require Rab GTPases and dynein motors for efficient transport out of the ER. When they introduce a mutant form of Sar1p (H79G) that inefficiently hydrolyses GTP, however, differences become apparent. Under these conditions, lipid-anchored cargo (GPI-FP) and large soluble cargo (procollagen) remain in the ER, whereas other cargoes concentrate at ER-exit sites. The authors conclude that certain COP-II cargoes require Sar1p GTP hydrolysis for efficient ER exit but others do not. One possibility, they suggest, is that Sar1p is required for coupling of certain COP-II cargoes to specific export receptors.
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