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Published online 27 November 2000. doi:10.1083/jcb.151.5.973
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© The Rockefeller University Press, 0021-9525/2000//973 $5.00
The Journal of Cell Biology, Volume 151, Number 5, , 2000 973-984

Original Article

Lst1p and Sec24p Cooperate in Sorting of the Plasma Membrane Atpase into Copii Vesicles in Saccharomyces cerevisiae



Yuval Shimonia, Tatsuo Kuriharaa, Mariella Ravazzolab, Mylène Amherdtb, Lelio Orcib, and Randy Schekmana

a Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, California 94720
b Department of Morphology, University of Geneva Medical School, 1211 Geneva 4, Switzerland
Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, CA 94720.(510) 642-7846(510) 642-5686

Formation of ER-derived protein transport vesicles requires three cytosolic components, a small GTPase, Sar1p, and two heterodimeric complexes, Sec23/24p and Sec13/31p, which comprise the COPII coat. We investigated the role of Lst1p, a Sec24p homologue, in cargo recruitment into COPII vesicles in Saccharomyces cerevisiae. A tagged version of Lst1p was purified and eluted as a heterodimer complexed with Sec23p comparable to the Sec23/24p heterodimer. We found that cytosol from an lst1-null strain supported the packaging of {alpha}-factor precursor into COPII vesicles but was deficient in the packaging of Pma1p, the essential plasma membrane ATPase. Supplementation of mutant cytosol with purified Sec23/Lst1p restored Pma1p packaging into the vesicles. When purified COPII components were used in the vesicle budding reaction, Pma1p packaging was optimal with a mixture of Sec23/24p and Sec23/Lst1p; Sec23/Lst1p did not replace Sec23/24p. Furthermore, Pma1p coimmunoprecipitated with Lst1p and Sec24p from vesicles. Vesicles formed with a mixture of Sec23/Lst1p and Sec23/24p were similar morphologically and in their buoyant density, but larger than normal COPII vesicles (87-nm vs. 75-nm diameter). Immunoelectronmicroscopic and biochemical studies revealed both Sec23/Lst1p and Sec23/24p on the membranes of the same vesicles. These results suggest that Lst1p and Sec24p cooperate in the packaging of Pma1p and support the view that biosynthetic precursors of plasma membrane proteins must be sorted into ER-derived transport vesicles. Sec24p homologues may comprise a more complex coat whose combinatorial subunit composition serves to expand the range of cargo to be packaged into COPII vesicles. By changing the geometry of COPII coat polymerization, Lst1p may allow the transport of bulky cargo molecules, polymers, or particles.

Key Words: ER • cargo packaging • transport vesicle • yeast • coat protein

© 2000 The Rockefeller University Press

Tatsuo Kurihara's present address is Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.

Abbreviations used in this paper: DSP, Dithiobis (succinimidyl propionate); GMP-PNP, 5'-guanylylimidodiphosphate; gp{alpha}F, glycosylated pro-{alpha}-factor; HA, hemagglutinin; MSS, medium speed supernatant; Pma1p, plasma membrane ATPase; SICs, semi-intact cells; VSV-G, vesicular stomatitis virus glycoprotein.


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