Background: Molecular mechanisms underlying target of rapamycin complex 2 (TORC2) signaling are poorly understood. Results: The TORC2 component Avo1 directly interacts with a downstream substrate Ypk2. Conclusion: Avo1-Ypk2 interaction is essential for TORC2-Ypk2 coupling and activation of the downstream effector pathway(s) regulating cell integrity and actin organization. Significance: Physical associations between TORC2 components and specific downstream effectors provide the molecular basis for selective downstream signaling.
In Saccharomyces cerevisiae, Cdc13p is a singlestranded TG 1-3 DNA binding protein that protects telomeres and maintains telomere length. A mutant allele of CDC13, cdc13-1, causes accumulation of singlestranded TG 1-3 DNA near telomeres along with a G 2 /M cell cycle arrest at non-permissive temperatures. We report here that when the single-stranded TG 1-3 DNA is masked by its binding proteins, such as S. cerevisiae Gbp2p or Schizosaccharomyces pombe Tcg1, the growth arrest phenotype of cdc13-1 is rescued. Mutations on Gbp2p that disrupt its binding to the single-stranded TG 1-3 DNA render the protein unable to complement the defects of cdc13-1. These results indicate that the presence of a single-stranded TG 1-3 tail in cdc13-1 cells serves as the signal for the cell cycle checkpoint. Moreover, the binding activity of Gbp2p to single-stranded TG 1-3 DNA appears to be associated with its ability to restore the telomere-lengthening phenotype in cdc13-1 cells. These results indicate that Gbp2p is involved in modulating telomere length.Telomeres are the structure at the ends of eukaryotic linear chromosomes (1, 2). In most organisms, the telomeric DNA is composed of short, tandem repeated sequences with a strand rich in guanine residues (G-strand) running 5Ј to 3Ј toward the end of telomere. For example, the telomeric sequences in the brewers yeast Saccharomyces cerevisiae are ϳ250 -300 base pair-long TG 1-3 /C 1-3 A repeats. Sequences of ciliate telomeres reveal that the G-strand extends beyond the duplex region, creating a short single-stranded 3Ј-overhang (3-5). In yeast, longer G-strand DNA with varying lengths, presumably an intermediate during telomere replication, is detected during late S phase (6, 7). Telomeres are essential for the maintenance of chromosome integrity. Telomeres protect chromosomes from degradation by nucleases, facilitate complete replication of chromosomes, and differentiate linear chromosome ends from broken ends (1, 2).Several single-stranded telomeric DNA binding proteins, including Oxytricha ␣ and  subunits (8, 9), Cdc13p (10, 11), Gbp2p (12, 13), hnRNPs (14 -16), and Pot1 (17), have been identified in vitro. Among these proteins, Oxytricha ␣ and  subunits have been well characterized because of the abundance of telomeres in this organism. The ␣ subunit binds to the G 4 T 4 G 4 T 4 single-stranded end of the telomere, and the  subunit is required for making the terminus-specific binding (8,18,19). Oxytricha ␣-and -like-binding proteins represent a novel type of DNA-binding protein because their binding is extremely salt-resistant. The binding of other single-stranded telomeric binding proteins to telomeric DNA is relatively salt-sensitive. A protein Pot1 that shares partial sequence homology with the Oxytricha ␣ subunit has been identified from human and Schizosaccharomyces pombe (17). Mutation in S. pombe POT1 causes loss of telomeric DNA and circulation of chromosomes. In S. cerevisiae, Cdc13p binds specifically to single-stranded TG 1-3 DNA in vitro and affects telomere functi...
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