1.B.7 The Rhodobacter PorCa Porin (RPP) Family
The Rhodobacter PorCa Protein, the only well characterized member of the RPP family, was the first porin to yield its three-dimensional structure by X-ray crystallography. It has a 16-stranded β-barrel structure similar to that of the members of the GBP (TC #1.B.1) family. Paupit et al. (1991) presented crystal structures of phosphoporin (PhoE; TC# 1.B.1.1.2), maltoporin (1.B.3.1.1; LamB) and Matrixporin (OmpF), all of E. coli, and found these have 3-d folds similar to that of the Rhodobacter porin, PorCa. It was not determined if this similarity is due to divergent evolution from a common ancestor or covergent evolution from dissimilar precursors. However, we have been able to demonstrate homology between members of families 1.B.1 and 1.B.7 using statistical means (M. Saier, unpublished results).