Forms passive diffusion pores that allow small molecular weight hydrophilic materials across the outer membrane. Monomer. 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 (By similarity). The pore formed by Omp2a is larger than the one formed by Omp2b. Omp2b pores have optimal permeability to allow growth and protection against harmful compounds. The larger pore formed by Omp2a may be advantageous for intracellular growth, when the bacterium is competing with the host cell for nutrients whose concentration is particularly low within the phagosome. Belongs to the alphaproteobacteria porin family.