Extended sugar slide function for the periplasmic coiled coil domain of ScrY

Several bacterial outer membrane proteins have a periplasmic extension whos e structure and function remain elusive. Here, the structure/function relat ionship of the N-terminal periplasmic domain of the sucrose-specific outer membrane channel ScrY was investigated. Circular dichroism and analytical c entrifugation demonstrated that the N-terminal domain formed a parallel, th ree-stranded coiled coil. When this domain was fused to the maltose-specifi c channel LamB, permeation of maltooligosaccharides in liposomes increased with increasing sugar chain length whereas wild-type LamB showed the opposi te effect. Current fluctuation analysis demonstrated increased off-rates fo r sugar transport through the fusion protein. Moreover, equilibrium dialysi s showed an affinity of sucrose for the isolated N-terminal peptide. Togeth er these results demonstrate a novel function for coiled coil domains, oper ating as an extended sugar slide. (C) 2000 Academic Press.