Loop X/XI, the largest cytoplasmic loop in the membrane-bound melibiose carrier of Escherichia coli, is a functional re-entrant loop
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- 作者：Ding ; Ping Z.
- 关键词：Melibiose carrier ; Cytoplasmic loop X/XI ; Re-entrant loop ; Membrane transport protein ; PCMBS ; p-chloromercuribenzene sulfonic acid ; BM ; (+)-biotinyl-3-maleimidopropionamidyl-3 ; 6-dioxaoctanediamine ; MTSEA ; (2-aminoethyl) methanethiosulfonate ; MTSES ; sodiu
- 刊名：Biochimica et Biophysica Acta (BBA)/Biomembranes
- 出版年：2004
- 出版时间：January 28, 2004
- 年：2004
- 卷：1660
- 期：1-2
- 页码：106-117
- 全文大小：404 K
文摘
The melibiose carrier of Escherichia coli is a membrane-bound sugar-cation cotransporter consisting of 12 transmembrane helices connected by cytoplasmic and periplasmic loops, with both N- and C-terminus on the cytoplasmic side. Using a functional cysteine-less carrier, cysteine was substituted individually for residues 347–378 that comprise the largest cytoplasmic loop X/XI. The majority of the cysteine mutants have good protein expression levels. The cysteine mutants were studied for their transport activities, and the inhibitory effects of two sulfhydryl reagents, PCMBS (7-Å long) and BM (29-Å long). Cysteine substitution resulted in substantial loss of transport in 12 mutants. While PCMBS caused significant inhibition in only two mutants, T373C and V376C, from the periplasmic side (in a substrate-protective manner), more extensive inhibition pattern was observed from the cytoplasmic side, in seven mutants: V353C, Y358C, V371C, Q372C, T373C, V376C and G378C, suggesting that these residues are along the sugar pathway in the aqueous channel, close to the cytoplasmic side. Furthermore, the inhibitory effect of BM on the inside-out vesicles of the above mutants was clearly less than that of PCMBS, suggesting channel space limitation to large molecules, consistent with those residues being inside the channel. Three second-site revertants (A350C/F268L, A350C/I22S, and A350C/I22N) were selected. They may suggest proximities between loop X/XI and helices I and VIII, in agreement with a re-entrant loop structure. Self thiol cross-linkings of the cysteine mutants on loop X/XI failed to form dimmers, suggesting that most of the loop is not surface-exposed from cytoplasmic side. Together, these results strongly indicated a functional re-entrant loop mechanistically important in Na+-coupled transporters.