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J. Cell Biol.,
Volume 142, Number 1, July 13, 1998 39-49
Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403
Three previously identified genes from Saccharomyces cerevisiae, VMA12, VMA21, and VMA22,
encode proteins localized to the endoplasmic reticulum
(ER). These three proteins are required for the biogenesis of a functional vacuolar ATPase (V-ATPase), but
are not part of the final enzyme complex. Subcellular
fractionation and chemical cross-linking studies have
revealed that Vma12p and Vma22p form a stable membrane associated complex. Cross-linking analysis also
revealed a direct physical interaction between the Vma12p/Vma22p assembly complex and Vph1p, the
100-kD integral membrane subunit of the V-ATPase.
The interaction of the Vma12p/Vma22p complex with
Vph1p was transient (half-life of ~5 min), reflecting trafficking of this V-ATPase subunit through the ER en
route to the vacuolar membrane. Analysis of these protein-protein interactions in ER-blocked sec12 mutant
cells indicated that the Vph1p-Vma12p/Vma22p interactions are quite stable when transport of the V-ATPase out of the ER is blocked. Fractionation of solubilized membrane proteins on a density gradient revealed
comigration of Vma22p and Vma12p, indicating that
they form a complex even in the absence of cross-linker. Vma12p and Vma22p migrated to fractions separate from Vma21p. Loss of Vph1p caused the
Vma12p/Vma22p complex to sediment to less dense
fractions, consistent with association of Vma12p/
Vma22p with nascent Vph1p in ER membranes. This is
the first evidence for a dedicated assembly complex in
the ER required for the assembly of an integral membrane protein complex (V-ATPase) as it is transported through the secretory pathway.
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