1.B.36 The Borrelia Porin p13 (BP-p13) Family
A single functionally characterized protein, the p13 gene product of Borrelia burgdorferi serves as the prototype for the BP-p13 family. This protein is a 13 kDa integral outer membrane protein. Channel-forming activity has been demonstrated both in vivo and in vitro. The protein has been purified, and channel-forming activity was reconstituted in a lipid bilayer. The single channel conductance in 1 M KCl of 3.5 nS was the same as the porin activity lost in a p13 null mutant. The channel is cation-selective but voltage-independent. Surprisingly, this protein exhibits three peaks of hydrophobicity with a breath of about 20 residues in addition to the N-terminal leader sequence. Only short regions are predicted to be of β-structure. Therefore, it is not clear that it is a β-structured porin. However, its presence in the outer membrane has been documented (Ostberg et al., 2002). Eight paralogues are found in B. burgdorferi, and other Borrelia species have close homologues (Pinne et al., 2004).
The Borrelia burgdorferi genome exhibits redundancy, with many plasmid-carried genes belonging to paralogous gene families. The chromosomally located p13 gene which codes for a channel-forming protein belongs to paralog family 48, which consists of eight additional genes. Of the paralogous genes from family 48, the BBA01 gene has the highest similarity to p13. The BBA01 protein, like P13, is posttranslationally processed at its C terminus. BBA01 is a channel-forming protein with properties similar to those of P13. The single-channel conductance of about 3.5 nS, formed by BBA01, is comparable to that of P13, which together with the high degree of sequence similarity suggests that the two proteins have similar and interchangeable functions (Pinne et al., 2006).
P13 shows a general (non-specific) channel-forming activity of 0.6 nanosiemens in 1 m KCl, has no preference for either cations or anions and shows no voltage-gating up to ±100 mV. The native P13 protein complex has a high molecular mass of about 300 kDa and is composed only of P13 monomers. The channel diameter is about 1.4 nm with a 400-Da molecular mass cut-off (Bárcena-Uribarri et al. 2014).
The reaction catalyzed by the p13 protein is:
Small molecules (out) ⇌ small molecules (periplasm)