1.B.41 The Corynebacterial Porin B (PorB) Family
Corynebacterium glutamicum and Corynebacterium efficiens (but no other sequenced bacteria as of 1/04) have two homologous porin-encoding genes designated porB and porC. PorB is 126 aas long in C. glutamicum and 157 aas long in C. efficiens. They have hydrophobic signal sequences of 27 aas. PorC is of either 142 aas or 123 aas in C. glutamicum and 151 aas in C. efficiens. PorB has been characterized in C. glutamicum (Costa-Riu et al., 2003) and forms small anion-selective channels with a diameter of about 1.4 nm. It can be blocked by citrate. In C. glutamicum, PorB and PorC are encoded within a single operon.
PorB has been purified, crystallized and analyzed by X-ray diffraction, yielding 16 independent molecular structures in four different crystal forms at resolutions up to 1.8 A. All 16 molecules have the same globular core, which consists of 70 residues forming four alpha-helices tied together by a disulfide bridge. The 16 structures vary greatly with respect to the 29 residues in the N- and C-terminal extensions. The native porin has to be oligomeric, and the reported structure is one of the subunits. An alpha-helical porin in a bacterial outer envelope is surprising because all presently known structures of such porins consist of beta-barrels. Since none of the four crystal packing arrangements was compatible with an oligomeric membrane channel, Ziegler et al (2008) constructed a model of such an oligomer that was consistent with all available data for native PorB. The proposed model is based on the required polar interior and nonpolar exterior of the porin, on a recurring crystal packing contact around a 2-fold axis, on the assumption of a simple C(n) symmetry (a symmetric arrangement around an n-fold axis), on the experimentally established electric conductivity and anion selectivity and on the generally observed shape of porin channels.
The transport reaction catalyzed by PorB is:
small hydrophilic anionic solutes (out) ⇌ small hydrophilic anionic solutes (in).