A MODEL OF MALTODEXTRIN TRANSPORT THROUGH THE SUGAR-SPECIFIC PORIN, LAMB, BASED ON DELETION ANALYSIS

LamB facilitates the uptake of maltose and maltodextrins across the ba cterial outer membrane and acts as a general porin for small molecules . Using directed deletion mutagenesis we removed several regions of th e LamB polypeptide and identified a polypeptide loop that both constri cts the maltoporin channel and binds maltodextrins. In conjunction wit h a second sugar binding site that we identified at the rim of the cha nnel, these data clarify, for the first time, the mechanism of transpo rt through a substrate-specific porin. Furthermore, unlike the transve rse loops of general porins, which originate from a central location i n their primary structure, the loop that regulates LamB permeability o riginates from a C-terminal site. Thus LamB represents a second distin ct class of porins in the bacterial outer membrane that is differently organized and separately evolved from OmpF-type, general porins.