Forms passive diffusion pores that allow small molecular weight hydrophilic materials across the outer membrane. Homotrimer. Consists of 16-stranded beta-barrel sheets, with large surface-exposed loops, that form a transmembrane pore at the center of each barrel. The pore is partially ocluded by a peptide loop that folds into the pore lumen. Belongs to the alphaproteobacteria porin family.