Structural and functional characterization of OmpF porin mutants selected for larger pore size .1. Crystallographic analysis

OmpF porin is a nonspecific pore protein from the outer membrane of Escherichia coli. Previously, a set of mutants was selected that allow the passage of long maltodextrins that do not translocate through the wild-type pore. Here, we describe the crystal structures of four point mutants and one deletion mutant from this set; their functional characterization is reported in the accompanying paper (Saint, N,, Leu, K.-L., Widmer, C,, Luckey, M., Schirmer, T,, Rosenbusch, J, P, (1996) J, Biol. Chem. 271, 20676-20680), All mutations have a local effect on the structure of the pore constriction and result in a larger pore cross-section, Substitution of each of the three closely packed arginine residues at the pore constriction (Arg-42, Arg-82, and Arg-132) by shorter uncharged residues causes rearrangement of the adjacent basic residues, This demonstrates mutual stabilization of these residues in the wild-type porin, Deletion of six residues from the internal loop (Delta 109-114) results in disorder of seven adjacent residues but does not alter the structure of the beta-barrel framework, Thus, the large hollow beta-barrel motif can be regarded as an autonomous structure.