Characterization of the Neisseria Igaβ-core: The Essential Unit for Outer Membrane Targeting and Extracellular Protein Secretion

Abstract

Extracellular transport of Neisseria IgA proteases across the bacterial outer membrane is accomplished by the translocation function contained within the C-terminal Igaβ domain of IgA protease precursor proteins. Recently, we reported that Igaβ from N. gonorrhoeae MS11 (Val1097 to Phe1505), fused, to a periplasmic passenger protein, facilitated its transport across the outer membrane, leading to surface exposure of the passenger. In the present work we show, by systematic N-terminal truncation of Igaβ, that the functional and structural unit, termed Igaβ-core, corresponds to the C-terminal approximately 274 amino acid residues (Ser1231 to Phe1505). This minimal region retains all the essential features necessary for the translocation of an N-terminally attached passenger across the outer membrane of Escherichia coli, and for its own correct integration into the outer membrane, even in the absence of a passenger protein. The membrane-integrated Igaβ-core constitutes a conserved entity found in the C-terminal regions of Igaβ domains of different N. gonorrhoeae, N. meningitidis and Haemophilus influenzae strains. In contrast, the surface-exposed N termini of the Igaβ domains vary in size and sequence. Based on secondary structure predictions, the key structural feature of the core is a β-barrel (amphipathic, antiparallel transmembrane β-strands, interspersed by hairpin turns and loops) which is common to many integral outer membrane proteins of Gram-negative bacteria. We propose that the core has been conserved in evolution, to provide a selective outer membrane export channel for covalently attached polypeptides.