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J. Cell Biol.,
Volume 145, Number 4, May 17, 1999 659-672
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
In Saccharomyces cerevisiae, vesicles that
carry proteins from the ER to the Golgi compartment
are encapsulated by COPII coat proteins. We identified
mutations in ten genes, designated LST (lethal with sec-thirteen), that were lethal in combination with the COPII
mutation sec13-1. LST1 showed synthetic-lethal interactions with the complete set of COPII genes, indicating
that LST1 encodes a new COPII function. LST1 codes
for a protein similar in sequence to the COPII subunit
Sec24p. Like Sec24p, Lst1p is a peripheral ER membrane protein that binds to the COPII subunit Sec23p.
Chromosomal deletion of LST1 is not lethal, but inhibits
transport of the plasma membrane proton-ATPase
(Pma1p) to the cell surface, causing poor growth on media of low pH. Localization by both immunofluorescence microscopy and cell fractionation shows that the export
of Pma1p from the ER is impaired in lst1
mutants.
Transport of other proteins from the ER was not affected by lst1, nor was Pma1p transport found to be
particularly sensitive to other COPII defects. Together, these findings suggest that a specialized form of the
COPII coat subunit, with Lst1p in place of Sec24p, is
used for the efficient packaging of Pma1p into vesicles
derived from the ER.
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