1.B.92. The Proteobacterial Outer Membrane Porin, NilB (NilB), Family
The γ-proteobacterium Xenorhabdus nematophila is a mutualistic symbiont that colonizes the intestine of the nematode Steinernema carpocapsae. NilB (nematode intestine localization) is essential for X. nematophila colonization of nematodes and is predicted to encode an integral outer membrane beta-barrel protein. When expressed with two other factors encoded by nilA and nilC, it confers upon non-cognate Xenorhabdus spp. the ability to colonize S. carpocapsae nematodes. Bhasin et al. 2012 presented evidence that NilB is a surface-exposed outer membrane β-barrel protein whose expression is repressed by NilR when grown in nutrient rich medium. Bioinformatic analyses reveal that NilB is the only functionally characterized member of a family of proteins distinguished by N-terminal region tetratricopeptide repeats (TPR) and a conserved C-terminal domain of unknown function (DUF560). Members of this family occur in diverse bacteria, and are prevalent in the genomes of mucosal pathogens. Insertion and deletion mutational analyses support a beta-barrel structural model with an N-terminal globular domain, fourteen transmembrane strands, and seven extracellular surface loops. The globular domain and surface loop 6 play a role in nematode colonization. NilB is necessary at early stages of colonization (Bhasin et al. 2012).