1.B.15 The Raffinose Porin (RafY) Family
The RafY family consists of a single porin protein, RafY, encoded within the E. coli plasmid pRSD2. pRSD2 also encodes the permease and enzymes required for raffinose utilization. The gene order is: rafR (regulatory protein), rafA (α-galactosidase), rafB (permease), rafD (invertase) and rafY (porin). The RafY porin in the outer membrane of E. coli can accommodate many oligosaccharides including several disaccharides, the trisaccharide raffinose, the tetrasaccharides maltotetraose and stachyose, the hexasaccharide maltohexaose and the heptasaccharide maltoheptaose. Thus, RafY is a general porin capable of transporting many oligosaccharides and other molecules across the outer membrane of Gram-negative bacteria. In this respect it resembles ScrY (TC #1.B.3.1.2), the sucrose porin, which also exhibits broad specificity, but not MalL (LamB) (TC #1.B.3.1.1), maltoporin which is largely specific for maltooligosaccharides. However, it has been shown to transport ions with slight selectivity for cations over anions. It appears to have a relatively large inner pore diameter and to lack a carbohydrate binding site.
The generalized transport reaction catalyzed by RafY is:
Solute (out) Solute (periplasm).