Major outer membrane proteins of E. coli K12 serve as receptors for the phages T2 (protein Ia) and 434 (protein Ib).

Mutants of E. coli resistant to bacteriophage T2 have lowered amounts of protein Ia in their outer membrane. Bacteriophage T2 was inactivated by a mixture of protein Ia-lipopolysaccharide. Protein Ia or lipopolysaccharide alone had no neutralizing activity. However, only protein Ia was required to inactivate a T2 host range mutant. In the presence of polymyxin B T2 receptor activity of protein Ia--lipopolysaccharide mixtures could not be restored. E. coli strains missing protein Ib were resistant against the lambdoid phage 434. Purified protein Ib inactivated 434 and lambdavirh434. Addition of lipopolysaccharide did not enhance the neutralizing activity of protein Ib, indicating that lipopolysaccharide may not be necessary for the inactivation of the phage.