Mitochondria-to-nuclear signaling is regulated by the subcellular localization of the transcription factors Rtg1p and Rtg3p

Cells modulate the expression of nuclear genes in response to changes in th e functional state of mitochondria, an interorganelle communication pathway called retrograde regulation. In yeast, expression of the CIT2 gene shows a typical retrograde response in that its expression is dramatically increa sed in cells with dysfunctional mitochondria, such as in rho(o) petites. Th ree genes control this signaling pathway: RTG1 and RTG3, which encode basic helix-loop-helix leucine zipper transcription factors that bind as heterod imer to the CIT2 upstream activation site, and RTG2, which encodes a protei n of unknown function. We show that in respiratory-competent (rho(+)) cells in which CIT2 expression is low, Rtg1p and Rtg3p exist as a complex largel y in the cytoplasm, and in rho(o) petites in which CIT2 expression is high, they exist as a complex predominantly localized in the nucleus. Cytoplasmi c Rtg3p is multiply phosphorylated and becomes partially dephosphorylated w hen localized in the nucleus. Rtg2p, which is cytoplasmic in both rho(+) an d rho(o) cells, is required for the dephosphorylation and nuclear localizat ion of Rtg3p. Interaction of Rtg3p with Rtg1p is required to retain Rtg3p i n the cytoplasm of rho(+) cells; in the absence of such interaction, nuclea r localization and dephosphorylation of Rtg3p is independent of Rtg2p. Our data show that Rtg1p acts as both a positive and negative regulator of the retrograde response and that Rtg2p acts to transduce mitochondrial signals affecting the phosphorylation state and subcellular localization of Rtg3p.