AMINO-ACID-SEQUENCE, S-S BRIDGE ARRANGEMENT AND DISTRIBUTION IN PLANT-TISSUES OF THIONINS FROM VISCUM-ALBUM

The complete primary structure of a cytotoxic 5 kDa polypeptide, visco toxin A1, isolated from Viscum album L., has been determined by combin ing classical Edman degradation methodology with advanced mass spectro metric procedures. The same integrated approach allowed correction of the sequence of viscotoxin A2 and definition of the pattern of the dis ulfide bridges. The arrangement of the cysteine pairing was determined as Cys3-Cys40, Cys4-Cys32 and Cys16-Cys26. The primary structure of V iscotoxin Al shares a high degree of similarity with the known viscoto xins and more generally with the plant alpha- and beta-thionins. The p attern of S-S bridges determined for viscotoxin A2 and A1 is similar t o that inferred by X-ray and NMR analysis in crambin and related to th at present in alpha-purothionin and beta-hordothionin, thus indicating a highly conserved organization of the S-S pairings within the entire family. This arrangement of S-S bridges describes a peculiar structur al motif, indicated as 'concentric motif', which is suggested to stabi lize a common structure occurring in various small proteins able to in teract with cell membranes, The distribution of the new Variant toxin in different mistletoe subspecies was investigated. Viscotoxin Al is a bundant in the seeds of the three European subspecies of V. album wher eas it represents a minor component in the shoots.