Maltoporin (LamB protein) forms a trimeric structure which facilitates the diffusion of maltodextrins across the outer membrane of Gram-negative bacteria. The membrane channel is formed by an antiparallel beta-barrel [1].

1: Structural basis for sugar translocation through maltoporin channels at 3.1 A resolution [see comments] Schirmer T, Keller TA, Wang YF, Rosenbusch JP; Science 1995;267:512-514. PMID:7824948