Omp2a porin of 386 aas. The 3-d structure reveals a 16-stranded β-barrel with an α-helix on the third loop folding inside the barrel and forming the constriction zone of the channel, a typical feature of general porins (Lopes-Rodrigues et al. 2019). The preferential diffusion of cations over anions has been experimentally observed.  Transports maltotetraose (Lopes-Rodrigues et al. 2019).