IDENTIFICATION OF A NEW PORIN, RAFY, ENCODED BY RAFFINOSE PLASMID PRSD2 OF ESCHERICHIA-COLI

The conjugative plasmid pRSD2 carries a raf operon that encodes a peri pheral raffinose metabolic pathway in enterobacteria. In addition to t he previously known raf genes, we identified another gene, rafY, which in Escherichia coli codes for an outer membrane protein (molecular ma ss, 53 kDa) similar in function to the known glycoporins LamB (maltopo rin) and ScrY (sucrose porin), Sequence comparisons with LamB and ScrY revealed no significant similarities; however, both lamB and scrY mut ants are functionally complemented by RafY, Expressed from the tac pro moter, RafY significantly increases the uptake rates for maltose, sucr ose, and raffinose at low substrate concentrations; in particular it s hifts the apparent K-m for raffinose transport from 2 mM to 130 mu M. Moreover, RafY permits diffusion of the tetrasaccharide stachyose and of maltodextrins up to maltoheptaose through the outer membrane of E. coli, A comparison of all three glycoporins in regard to their substra te selectivity revealed that both ScrY and RafY have a broad substrate range which includes alpha-galactosides while LamB seems to be restri cted to malto-oligosaccharides. It supports growth only on maltodextri ns but not, like the others, on raffinose and stachyose.