1.B.59 The Outer Membrane Porin, PorH (PorH) Family
Corynebacterium callunae and Corynebacterium efficiens are close relatives of the glutamate-producing mycolata species Corynebacterium glutamicum. The properties of the pore-forming proteins, extracted by organic solvents, were studied by Hünten et al. (2005a; 2005b). The cell extracts contained channel-forming proteins that formed ion-permeable channels with a single-channel conductance of about 2 to 3 nS in 1 M KCl in a lipid bilayer assay. The corresponding proteins from the first two of these corynebacteria were purified to homogeneity and were named PorH(C.call) and PorH(C.eff). Electrophysiological studies of the channels suggested that they are wide and water-filled. Channels formed by PorH(C.call) are cation-selective, whereas PorH(C.eff) forms slightly anion-selective channels. Purified PorH of C. glutamicum formed cation-selective pores with an average conductance of about 2.5 nS in 1 M KCl when incorporated into an artificial lipid bilayer. All three proteins were partially sequenced and proved to be homologous. PorH(C.call) shows high homology to PorH(C.eff). PorH(C.eff) is encoded in the bacterial chromosome by a gene that is localized within the vicinity of the porA gene of C. efficiens. The porH gene in C. glutamicum is present in a 13 gene operon near porA. These proteins have no signal sequence at their N termini, which suggests they are not exported by the Sec-secretion pathway. The structures of homologues of PorH in the cell wall of corynebacteria have been considered (Hünten et al., 2005a, 2005b).
More recent studies revealed that PorA (1.B.34) and PorH form heterooligomeric pores, and in spite of the earlier reports, they are both required for pore formation (Barth et al. 2010). They comprise the major outer membrane porin in these Corynebacteria, and no other protein is required. However, they are apparently post-translationally modified by mycolic acids (Rath et al. 2011). Although the molecular weight of each subunit is about 5 KDa, the complex has an apparent molecular mass of about 66 KDa, suggesting that it contains about 12 subunits (Schiffler et al. 2007).
PorA and PorH are of about the same size and topology, show substantial sequence similarity with each other, and probably are homologous (T. Su and MH Saier, unpublished observations). In contrast to most outer membrance proteins in Corynebacteria which have signal sequences and beta-barrel structures resembling those of Gram-negative bacteria (Marchand et al. 2012), these small proteins may have strongly amphipathic transmembrane alpha-helical structures (T. Su and MH Saier, unpublished results).