1.B.34 The Corynebacterial Porin A (PorA) Family
The mycolata are a group of mycolic acid-containing bacteria which include the genera Rhodococcus, Gordona, Dietzia, Tsukamurella, Corynebacterium, Mycobacterium and Norcadia. They have an outer membrane in which the constituent mycolic acids are linked via ester bonds to the arabinogalactan that is attached to the murein of the cell wall. The mycolic acids are 2-branched, 3-hydroxylated fatty acids of varying lengths depending on the organism (i.e., 60-90 carbon atoms for the Mycobacteria, 46-58 carbon atoms for the Norcadia and 22-38 carbon atoms for the Corynebacteria). Porins allow permeation of small hydrophilic molecules across the outer membrane permeability barrier (see the MBP family, TC# 1.B.24).
PorA is a small (45 aa) porin of Corynebacterium glutamicum with an excess of four negative charges in agreement with its cation selectivity. It forms wide, oligomeric water-filled pores. It is encoded by a 138 bp gene, porA. Deletion of this gene from C. glutamicum results in slow growth and lower antibiotic sensitivities. PorA is therefore believed to be a major hydrophilic pathway through the cell wall (outer membrane) of some Corynebacteria (Costa-Riu et al., 2003).
PorA does not have an N-terminal leader sequence suggesting that its export across the cytoplasmic membrane does not occur via the Sec pathway. Only Corynebacteria possess homologues of PorA. C. glutamicum has only one gene coding for a PorA homologue. C. efficiens does not have a porA gene in its genome, so another outer membrane porin must serve the equivalent of the PorA function. Both C. glutamicum and C. efficiens have two homologous anion-selective outer membrane porins called PorB and PorC. They are in the PorB family (TC #1.B.41) and provide outer membrane, anion selective pore functions (Costa-Riu et al., 2003).
C. diphtheriae does have a 43aa PorA. It lacks a signal sequence but shows 40% identity and 62% similarity (with no gaps) in a stretch of 26 compared residues (15-41 in the C. glutamicum PorA). The C. diphtheriae PorA had an apparent molecular mass of about 66 kDa as determined on Tricine-containing sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and consisted of subunits having a molecular mass of about 5 kDa. Single-channel experiments with the purified protein suggested that the protein formed channels with a single-channel conductance of 2.25 nS in 1 M KCl. Further single-channel analysis suggested that the cell wall channel is wide and water filled because it has only slight selectivity for cations over anions and its conductance followed the mobility sequence of cations and anions in the aqueous phase (Schiffler et al., 2007).
More recent studies revealed that PorA and PorH (1.B.59) 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). Porin A-Porin H ion channels have been reconstituted in liposomes. With properly mycoloylated proteins, it manifests the typical voltage dependent ion channel properties of an outer membrane porin (Hünten et al. 2005; Rath et al. 2013).
PorA and PorH are of about the same size and topology, show substantial sequence similarity with each other, and 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 seem to lack signal sequences, and may have strongly amphipathic transmembrane alpha-helical structures (T. Su and MH Saier, unpublished results).
The generalized transport reaction catalyzed by PorA is:
small molecules (out) → small molecules (in).