| Co-Investigator(Kenkyū-buntansha) |
MORI Hiroyuki Kyoto Univ., Inst. Virus Res., Instructor, ウイルス研究所, 助手 (10243271)
AKIYAMA Yoshinori Kyoto Univ., Inst. Virus Res., Associate Professor, ウイルス研究所, 助教授 (10192460)
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| Research Abstract |
We studied protein interactions in the cellular machinery that mediates protein translocation across the E. coli cytoplasmic membrane. We found that in the membrane-embedded SecY-SecE-SecG complex of protein translocase, each of SecE and SecG interacts independently with SecY, the central submit. The residue 240 of SecY is particularly important for the SecY's interaction with SecE. We established a system, using a SecY-interaction protein, Syd, and a secY mutation that impairs SecY-SecE interaction, in which we can manipulate the SecA-binding ability of the SecYEG channel complex. SecA, the preprotein-driving ATPase, inserts into the membrane in response to ATP and a preprotein and deinserts from the membrane upon hydrolysis of ATP, thereby facilitating the preprotein movement into the membrane, We identified a secY mutation(SecY205)that impairs the SecA insertion reaction, and suggested that it alters the translocation channel such that the altered channel cannot accept a SecA molecu
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le that is in complex with the preprotein. We isolated suppressor mutations in SecA that overcame the SecY205 defect, and characterized the mutant proteins biochemically. These studies suggested that SecY interacts specifically with SecA to allow the insertion of preprotein-SecA complex, and the SecA insertion reactions is indeed important for protein translocation both in vivo and in vitro. Our mutant analyses suggest that the regions around the two Walker A motifs in SecA are important for the functional SecA interaction, while the secondary ATP-binding region is important for the regulation of the SecA functions. Some SecA mutations suppressed the SecG deletion mutation and our analyses suggest that SecG has a role of assisting SecA and its effective utilization of ATP for protein translocation. We have shown that SecA is activated by its interaction with SecY, and that the fifth cytoplasmic domain of SecY is important for the SecA activation. We identhified particular residues in this domain of SecY, which is crucial for the SecA activation. Less
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