Most mitochondrial precursor polypeptides are imported from the cytosol into the mitochondrion, where they must efficiently undergo folding. Mitochondrial precursors are imported as unfolded polypeptides. For proteins of the mitochondrial matrix and inner membrane, two separate chaperone systems, HSP60 and mitochondrial HSP70 (mtHSP70), facilitate protein folding. We show that LONP1, an AAA+ protease of the mitochondrial matrix, works with the mtHSP70 chaperone system to promote mitochondrial protein folding. Inhibition of LONP1 results in aggregation of a protein subset similar to that caused by knockdown of DNAJA3, a co-chaperone of mtHSP70. LONP1 is required for DNAJA3 and mtHSP70 solubility, and its ATPase, but not its protease activity, is required for this function. In vitro, LONP1 shows an intrinsic chaperone-like activity and collaborates with mtHSP70 to stabilize a folding intermediate of OXA1L. Our results identify LONP1 as a critical factor in the mtHSP70 folding pathway and demonstrate its proposed chaperone activity.

SummaryYak1 is a member of an evolutionarily conserved family of Ser/Thr protein kinases known as dualspecificity Tyr phosphorylation-regulated kinases (DYRKs). Yak1 was originally identified as a growth antagonist, which functions downstream of Ras/PKA signalling pathway. It has been known that Yak1 is phosphorylated by PKA in vitro and is translocated to the nucleus upon nutrient deprivation. However, the regulatory mechanisms for Yak1 activity and localization are largely unknown. In the present study, we investigated the role of PKA and Bmh1, a yeast 14-3-3 protein, in regulation of Yak1. We demonstrate that PKA-dependent phosphorylation of Yak1 on Ser295 and two minor sites inhibits nuclear localization of Yak1. We also show that intramolecular autophosphorylation on at least four Ser/Thr residues in the noncatalytic N-terminal domain is required for full kinase activity of Yak1. The most potent autophosphorylation site, Thr335, plays an essential role for Bmh1 binding in collaboration with a yet unidentified second binding site in the N-terminal domain. Bmh1 binding decreases the catalytic activity of Yak1 without affecting its subcellular localization. Since the binding of 14-3-3 proteins to Yak1 coincides with PKA activity, such regulatory mechanisms might allow cytoplasmic retention of an inactive form of Yak1 under high glucose conditions.

STP1 gene was found to be hypersensitive to rapamycin, and overexpression of STP1 conferred resistance to rapamycin. Interestingly, we found that the rapamycin-induced disappearance of Stp1 from the nucleus resulted from Stp1 degradation, which was dependent on the activity of a protein phosphatase 2A (PP2A)-like phosphatase, Sit4, which is a wellknown downstream effector of TORC1. Taken together, our findings highlight an intimate connection between the aminoacid-sensing pathway and the rapamycin-sensitive TOR signaling pathway.Key words: TORC1, Rapamycin, Stp1, Sit4, Saccharomyces cerevisiae Summary TORC1 controls degradation of the transcription factor Stp1, a key effector of the SPS amino-acidsensing pathway in Saccharomyces cerevisiae

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The target of rapamycin (TOR) signaling pathway is a conserved pathway that regulates eukaryotic cell growth in response to environmental cues. Chemical genomic approaches that profile rapamycin sensitivity of yeast deletion strains have given insights into the function of TOR signaling pathway. In the present study, we analyzed the rapamycin sensitivity of yeast deletion library strains on synthetic medium. As a result, we identified 130 strains that are hypersensitive or resistant to rapamycin compared with wild-type cells. Among them, 36 genes are newly identified to be related to rapamycin sensitivity. Moreover, we found 16 strains that show alteration in rapamycin sensitivity between complex and synthetic media. We suggest that these genes may be involved in part of TOR signaling activities that is differentially regulated by media composition.