The prototype of this family is the rafY porin of the conjugative plasmid pRSD2 which enables Escherichia coli to grow on raffinose. RafY is part of an operon that includes also a permease and the enzymes required for raffinose utilization. RafY forms an ion-permeable single channel, which is approximately twice as that of the general diffusion pores OmpF and OmpC. Since rafY is able to accommodate the diffusion of other oligo-saccharides, it seems that it is a general diffusion pore with a diameter larger than that of the general diffusion porins, allowing the diffusion of high-molecular-mass carbohydrates through the outer membrane. Homologs are found in other Escherichia coli strains, in Salmonella typhi and Photorhabdus luminescens.