MISTLETOE LECTIN-I FORMS A DOUBLE TREFOIL STRUCTURE

The quaternary structure of mistletoe lectin I (MLI), a type II riboso me inactivating protein, has been determined by X-ray crystallography. A definitive molecular replacement solution was determined for MLI us ing the co-ordinates of the homologue ricin as a search model. MLI exi sts as an [AB](2) dimer with internal crystallographic two-fold symmet ry. Domain I of the B chains is non-covalently associated through inte ractions involving three looped chains (alpha, beta, gamma) in each mo lecule of the dimer, forming a double trefoil structure, The ricin mol ecule which shares 52% sequence homology with MLI has a disulphide bri dge between Cys(20) and Cys(39) in the a loop. An evolutionary mutatio n has replaced Cys(39) with serine in MLI. This mutation appears to al low the a loop the flexibility required to take up its place at the di mer interface, and also suggests a rationale for why ricin does not fo rm dimers, Measurement of retention times using FPLC gel filtration co nfirms that dimerisation also occurs in solution between MLI B chains with an association constant K-a = 10(6) M. (C) 1998 Federation of Eur opean Biochemical Societies.