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 bacterial outer membrane and acts as a general porin for small molecules. Using directed deletion mutagenesis we removed several regions of the LamB polypeptide and identified a polypeptide loop that both constricts the maltoporin channel and binds maltodextrins. In conjunction with a second sugar binding site that we identified at the rim of the channel, these data clarify, for the first time, the mechanism of transport through a substrate‐specific porin. Furthermore, unlike the transverse loops of general porins, which originate from a central location in their primary structure, the loop that regulates LamB permeability originates from a C‐terminal site. Thus LamB represents a second distinct class of porins in the bacterial outer membrane that is differently organized and separately evolved from OmpF‐type, general porins.