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J. Biol. Chem., Vol. 277, Issue 50, 48550-48557, December 13, 2002

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Superactive SecY Variants That Fulfill the Essential Translocation Function with a Reduced Cellular Quantity^*

Hiroyuki Mori, Yuhsuke Shimizu, and Koreaki Ito

From the Institute for Virus Research, Kyoto University, Sakyo-ku, Kyoto 606-8507, Japan

The fifth and the sixth cytoplasmic regions (C5 and C6) of SecY are important for the SecA-driven preprotein translocation reaction. A cold-sensitive mutation, secY205 (Tyr-429  Asp), in C6 impairs the ATP- and precursor-dependent SecA insertion into the membrane. We now identified second site mutations that suppressed the defect. Cis-placement of these mutations proved to suppress mutations at another essential residue (Arg-357) of SecY as well. Thus, they tolerate the otherwise defective SecY alterations in the same molecule. Two alterations (Ile-195 to Ser in TM5 region and Ile-408 to Leu in TM10 region) were found to make the translocation channel more active, because it enabled cells to survive with reduced content of the SecYE complex. These mutations only very weakly suppressed a signal sequence defect of the  receptor protein. The mutant SecYEG translocase exhibited higher than normal activity in vitro, being accompanied by striking independence of the proton motive force as well as by stabilization of a bound and active SecA species against urea treatment. These results have been interpreted in terms of balance shifts between channel closing and channel opening alterations in the SecYEG translocase.

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				H. Mori and K. Ito Different modes of SecY-SecA interactions revealed by site-directed in vivo photo-cross-linking PNAS, October 31, 2006; 103(44): 16159 - 16164. [Abstract] [Full Text] [PDF]