Microcin E492 (MccE492) is an antibacterial protein whose activity on target cells requires ManYZ, the inner membrane component of the mannose permease. We show here that MceA, the polypeptide core of MccE492, stably associates with ManYZ both in the presence and in the absence of MceB, the MccE492 immunity protein. The two known physiological activities of the mannose permease were assayed in cells co-expressing MceA and MceB. Under these conditions, growth on mannose as the sole carbon source is prevented; this was not observed in cells expressing only MceB. In contrast, susceptibility to bacteriophage λ infection was not affected.