STRUCTURE AND MEMBRANE ACTIONS OF A MARINE WORM PROTEIN CYTOLYSIN, CEREBRATULUS TOXIN-A-III

Four homologous Cerebratulus lacteus A toxins are the first and as yet only protein cytolysins to be isolated from an ancient phylum of mari ne worms, the nemertines. The most abundant and toxic variant, toxin A -III, has been sequenced and its mechanisms of action studied in the m ost detail. It consists of a single basic polypeptide chain of 95 amin o acid residues cross-linked by three disulfide bonds, and possesses a predominantly alpha-helical secondary structure. The C-terminal third of the toxin sequence is postulated to be a helical 'hairpin' structu re involved in pore formation. Toxin A-III permeabilizes a variety of cells as well as liposomes made from a variety of phospholipids; appar ently large pores are formed, as large proteins are released almost as rapidly as small organic molecules and inorganic ions. At sublytic co ncentrations, the toxin also inhibits protein kinase C and endogenous voltage-gated cation selective (sodium, calcium) channels occurring in the nervous and cardiovascular systems. A curious observation, also r eported for colicins and some other protein cytolysins, was the conser vation of toxin secondary structure upon insertion into phospholipid l iposomes, despite the strong likelihood that significant changes in te rtiary structure occur to provide a hydrophobic surface for interactio n with membrane lipids. Because of its small size and presumed single helical hairpin secondary structure, Cl toxin A-III is an excellent mo lecular subject for investigating protein insertion into biological me mbranes and mechanisms of pore formation.