The best known and studied bacterial toxin is the diphtheria toxin,produced by . Diphtheria toxin is abacterialexotoxin of the A/B prototype. It is produced as single polypeptidechainwith a molecular weight of 60,000 daltons. The function of the proteinis distinguishable into two parts: subunit A, with a m.w. of 21,000daltons,contains the enzymatic activity for inhibition of elongation factor-2involvedin host protein synthesis; subunit B, with a m.w. of 39,000 daltons, isresponsible for binding to the membrane of a susceptible host cell. TheB subunit possesses a region T (translocation) domain which insertsinto the endosome membrane thus securing the release of the enzymaticcomponent into the cytoplasm.
A summary of bacterial protein toxins and their activities is givenin Tables 4. Details of the mechanisms of action of these toxins andtheirinvolvementin the pathogenesis of disease is discussed in chapters with thespecificbacterial pathogens.
Bacterial Protein Translocation
In both cases above,a large protein molecule must insert into and cross a membrane lipidbilayer, either the cell membrane or the endosome membrane. Thisactivity is reflectedin the ability of most A+B or A/B toxins, or their B components, toinsertinto artificial lipid bilayers, creating ion permeable pathways. If theB subunit contains a hydrophobic region (of amino acids)that insert into the membrane (as in the case of the diphtheria toxin),it may be referred to as theT(translocation) domain of the toxin.