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Eukaryotic Cell, August 2008, p. 1328-1343, Vol. 7, No. 8
1535-9778/08/$08.00+0     doi:10.1128/EC.00065-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Involvement of Saccharomyces cerevisiae Avo3p/Tsc11p in Maintaining TOR Complex 2 Integrity and Coupling to Downstream Signaling {triangledown} ,{dagger}

Hsiang-Ling Ho,1 Hsin-Yi Lee,1 Hsien-Ching Liao,1 and Mei-Yu Chen1,2*

Institute of Biochemistry and Molecular Biology, School of Life Sciences,1 Department of Biochemistry, School of Medicine, National Yang-Ming University, Taipei 11221, Taiwan2

Received 18 February 2008/ Accepted 2 June 2008

Target-of-rapamycin proteins (TORs) are Ser/Thr kinases serving a central role in cell growth control. TORs function in two conserved multiprotein complexes, TOR complex 1 (TORC1) and TORC2; the mechanisms underlying their actions and regulation are not fully elucidated. Saccharomyces TORC2, containing Tor2p, Avo1p, Avo2p, Avo3p/Tsc11p, Bit61p, and Lst8p, regulates cell integrity and actin organization. Two classes of avo3 temperature-sensitive (avo3ts) mutants that we previously identified display cell integrity and actin defects, yet one is suppressed by AVO1 while the other is suppressed by AVO2 or SLM1, defining two TORC2 downstream signaling mechanisms, one mediated by Avo1p and the other by Avo2p/Slm1p. Employing these mutants, we explored Avo3p functions in TORC2 structure and signaling. By observing binary protein interactions using coimmunoprecipitation, we discovered that the composition of TORC2 and its recruitment of the downstream effectors Slm1p and Slm2p were differentially affected in different avo3ts mutants. These molecular defects can be corrected only by expressing AVO3, not by expressing suppressors, highlighting the role of Avo3p as a structural and signaling scaffold for TORC2. Phenotypic modifications of avo3ts mutants by deletion of individual Rho1p-GTPase-activating proteins indicate that two TORC2 downstream signaling branches converge on Rho1p activation. Our results also suggest that Avo2p/Slm1p-mediated signaling, but not Avo1p-mediated signaling, links to Rho1p activation specifically through the Rho1p-guanine nucleotide exchange factor Tus1p.

* Corresponding author. Mailing address: Institute of Biochemistry and Molecular Biology, School of Life Sciences, National Yang-Ming University, 155, Sec. 2, Li-Nong St., Taipei 11221, Taiwan. Phone: 886-2-2826-7269. Fax: 886-2-2826-4843. E-mail: meychen{at}ym.edu.tw

{triangledown} Published ahead of print on 13 June 2008.

{dagger} Supplemental material for this article may be found at http://ec.asm.org/.

Eukaryotic Cell, August 2008, p. 1328-1343, Vol. 7, No. 8
1535-9778/08/$08.00+0     doi:10.1128/EC.00065-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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