1.C.58 The Microcin E492/C24 (Microcin E492) Family
Two microcins (C24, 90 aas and E492, 103 aas) of E. coli and K. pneumoniae, respectively, comprise the well characterized members of the MCC24 family although there are many homologs in the γ-proteobacteria. Microcin E492 forms ion channels in lipid bilayers (Lagos et al., 1993), and the gene sequence has been established (Lagos et al., 1999). Its bacterocidal and anti-tumor actions involve the loss of the transmembrane potential due to pore formation (Lagos et al. 2009). The pores are cation-selective with the permeability order: NH4 > K =Rb =Cs > Na =Li > Tris . It is active against enterobacteria.
Microcin E492 is a channel-forming bacteriocin that is found in two forms: post-translationally modified by the covalent linkage of salmochelin-like molecules to serine 84, and in an unmodified form (Mercado et al., 2008). The production of modified microcin E492 requires the synthesis of enterochelin, which is subsequently glycosylated by MceC and converted into salmochelin. Mutants in mceC produced inactive microcin E492, and this phenotype was reversed either by complementation with iroB from Salmonella enterica or by the addition of exogenous salmochelin. Cyclic salmochelin uptake by E. coli occurred mainly through the outer membrane catecholate siderophore receptor, Fiu. The production of inactive microcin E492 by mutants in entB and entC was reverted by the addition of the end product of the respective mutated pathway (2,3-dihydroxybenzoic acid, enterochelin/salmochelin), while mutants in entF did not produce active microcin E492 in the presence of enterochelin or salmochelin. The EntF adenylation domain was the only domain required for this microcin E492 maturation step. Inactivation of the enzymatic activity of this domain by site-directed mutagenesis did not prevent the synthesis of active microcin E492 in the presence of salmochelin, indicating that the adenylation activity is not essential for the function of EntF at this stage of microcin E492 maturation (Mercado et al., 2008).
Microcin E492 (MccE492) is active against various species of Enterobacteriaceae. Interaction of MccE492 with target cells leads to the depolarization and permeabilization of their inner membranes, and it forms cation-selective channels in lipid bilayers. However, several MccE492-specific proteins are required for the maturation and secretion of active MccE492. Surprisingly, the production of only MceA, the polypeptide backbone of MccE492, is toxic by itself. It is called ''endogenous MceA'' bactericidal activity to differentiate it from the action of extracellularly secreted MccE492. The toxicity of endogenous MceA is enhanced by efficient targeting to the inner membrane. However, a periplasmic intermediate state is not required for MceA toxicity. Indeed, endogenous MceA remains fully active when it is fused to thioredoxin-1, a fast-folding protein that promotes retention of the C-terminus of MceA in the cytoplasm (Bieler et al., 2006). The C-terminal domain of MccE492 is required only for delivery from the extracellular environment to the periplasm, and it is not required for inner membrane damage. A common component is absolutely essential for the bactericidal activity of both endogenous MceA and extracellular MccE492. Indeed, toxicity is strictly dependent on the presence of ManYZ, the inner membrane protein complex involved in mannose uptake (TC# 4.A.6.1.1). Based on these findings, Bieler et al. (2006) proposed a new model for cell entry, inner membrane insertion, and toxic activity of MccE492.
The generalized transport reaction catalyzed by microcin E492 is:
Cation (in) Cation (out).