Abstract
QSR1 (quinol-cytochrome c reductase subunit-requiring) is a highly conserved, essential gene in Saccharomyces cerevisiae that was identified through a synthetic lethal screen by its genetic relationship to QCR6, the gene for subunit 6 (Qcr6p) of the mitochondrial cytochrome bc1 complex. The function of the QSR1-encoded protein (Qsr1p) and its relationship to the QCR6-encoded protein are unknown. When yeast cell lysates are fractionated by density gradient centrifugation, Qsr1p separates from organelles and sediments with a uniformly sized population of particles that are similar to eukaryotic ribosomes upon velocity gradient centrifugation. When 40 S and 60 S ribosomal subunits are separated on velocity gradients, Qsr1p is found exclusively with the 60 S subunits, where it is a stoichiometric component. Extracts prepared from qsr1-1 cells are defective in in vitro translation assays relative to the wild type. In yeast cell lysates in which QCR6 rescues an otherwise lethal qsr1-1 mutation, Qcr6p is found only in mitochondria, both in respiratory-competent cells and in rho0 cells in which the bc1 complex is no longer present. These results suggest that suppression of the qsr1-1 mutation by QCR6 occurs by a trans-relationship across the outer mitochondrial membrane.
Highlights
In yeast cell lysates in which QCR6 rescues an otherwise lethal qsr1-1 mutation, Qcr6p is found only in mitochondria, both in respiratory-competent cells and in rho0 cells in which the bc1 complex is no longer present. These results suggest that suppression of the qsr1-1 mutation by QCR6 occurs by a trans-relationship across the outer mitochondrial membrane
Localization of Qcr6p—Since Qcr6p is a subunit of the mitochondrial bc1 complex, located in the inner mitochondrial membrane, and it suppresses the qsr1-1 mutation in a cytoplasmic ribosomal protein, we investigated whether Qcr6p might exist in a soluble cytosolic form, where it could carry out its suppressor role
The subcellular localization of QSR1-encoded protein (Qsr1p) and its possible association with any organelles are of special interest because of the novel genetic relationship between QSR1 and QCR6, which encodes a subunit of the mitochondrial cytochrome bc1 complex [34]
Summary
In yeast cell lysates in which QCR6 rescues an otherwise lethal qsr1-1 mutation, Qcr6p is found only in mitochondria, both in respiratory-competent cells and in rho0 cells in which the bc1 complex is no longer present. We report that Qsr1p is a 60 S ribosomal subunit protein and that Qcr6p is found only inside mitochondria in yeast cells in which QCR6 rescues the qsr1-1 mutation.
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