MspA is a membrane porin produced by Mycobacteria, allowing hydrophilic nutrients to enter the bacterium. The protein forms a tightly interconnected octamer with eightfold rotation symmetry that resembles a goblet and contains a central channel. Each subunit fold contains a beta-sandwich of Ig-like topology and a beta-ribbon arm that forms an oligomeric transmembrane barrel [1].

1: The structure of a mycobacterial outer-membrane channel. Faller M, Niederweis M, Schulz GE; Science. 2004;303:1189-1192. PMID:14976314