Bacteriophage XReceptor Site ontheEscherichia coli K-12 LamBProtein

Wehaveanalyzed eight newphage-resistant missense mutations inlamB.These mutations identify five new aminoacidresidues essential forphageAadsorption. Twomutations atpositions 245and382affect residues whichwerepreviously identified, butleadtodifferent aminoacidchanges. Threemutations atresidues 163, 164, and250enlarge andconfirm previously proposed phage receptor sites. Twodifferent mutations atresidue 259andoneat18alter residues previously suggested asfacing theperiplasmic face. Themutation atresidue 18implicates forthefirst time theamino-terminal region oftheLamBprotein inphage adsorption. Theresults arediscussed interms ofthetopology oftheLamBprotein. TheLamBprotein isanintegral outer membrane protein ofEscherichia coli that functions asaspecific poreforthe diffusion ofmaltose andmaltodextrins across theouter membrane(reviews inreferences 9 and18).LamB also serves asareceptor forphageAh+,its hostrange derivatives Xh°andXhh*, andseveral other phages (6,12,20,21,24). We haveundertaken toidentify portions oftheLamB protein specifically involved inphageadsorption. Forthis purpose, we haveisolated mutations inlamBleading to phage resistance that retain atleast someoftheactivities of LamBsuchassensitivity toXhh*.We believe thatsuch mutations donotalter severely theoverall structure ofthe protein. By further assuming thatthemutated residues belong toorareclose totheprimary site ofinteraction with thephage, i.e., the"receptor site," wehavedevised workingmodels ofthefolding oftheLamBprotein intheouter membrane (5)(Fig. 1). We havealready reported theidentification of10such residues inLamB(5,7,22). Theseresidues areclustered in fourdifferent regions oftheLamBprotein andbelong to portions that arehydrophilic andmostly predicted as,-turns ofthemolecule. Inthepresent study, toidentify moreresidues belonging totheXreceptor site, weselected newresistant mutations by using three different mutagenic treatments andscreened the resistant clones withabattery ofLamB-specific phages for phenotypes distinct fromprevious mutations. Theanalysis oftheeight newmutants extends theconclusions ofthe previous studies andsuggests that other parts ofthemoleculemaybeexposed atthecell surface.