ACYLATION OF ESCHERICHIA-COLI HEMOLYSIN - A UNIQUE PROTEIN LIPIDATIONMECHANISM UNDERLYING TOXIN FUNCTION

The pore-forming hemolysin (HlyA) of Escherichia coli represents a uni que class of bacterial toxins that require a posttranslational modific ation for activity. The inactive protoxin pro-HlyA is activated intrac ellularly by amide linkage of fatty acids to two internal lysine resid ues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action disting uishes HlyC from all bacterial acyltransferases such as the lipid A, l ux-specific, and nodulation acyltransferases, and from eukaryotic tran sferases such as N-myristoyl transferases, prenyltransferases, and thi oester palmitoyltransferases. Most lipids directly attached to protein s may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylati on of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflamm atory cytokines and mitogenic and cholinergic receptors. While the loc ation and structure of lipid moieties an proteins vary, there are comm on effects on membrane affinity and/or protein-protein interactions De spite being acylated at two residues, HlyA does not possess a ''double -anchor'' motif and does not have art electrostatic switch, although i ts dependence on calcium binding for activity suggests that the calciu m-myristoyl switch may have relevance. The acyl chains on HlyA may pro vide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein inter-actions either between HlyA and components of a host signal transduction pathway to influenc e cytokine production or between HlyA monomers to bring about oligomer ization during pore formation.