A THREEFOLD SYMMETRIC ß-PRISM FOLD WITHOUT INTERNAL SEQUENCE HOMOLOGY IN THE STRUCTURE OF THE TWO-CHAIN LECTIN, JACALIN. M. Vijayan, K. Sekar, R. Banerjee, S.K. Mahanta, A. Surolia and R. Sankaranarayanan, Molecular Biophysics Unit, Indian Institute of Science, Bangalore - 560012, INDIA.

Jacalin, a tetrameric lectin from the seeds of jackfruit (Artocarpus integrifolia) specific to the tumor associated T-antigenic disaccharide, contains a 133 residue long a-chain and a 20 residue long b-chain in the subunit. The protein does not exhibit sequence homology with any other protein of known structure and hence its structure with bound methyl-a-galactose was determined by the multiple isomorphous replacement method and refined to R=17.5% for 20,822 reflections at 2.43Å resolution. The crystal asymmetric unit contains two half-tetramers, each with 222 symmetry. Each subunit essentially consists of two Greek keys and one Greek key-like motif, arranged in a threefold symmetric fashion. The b-chain forms part of the Greek key-like motif and plays an important role in subunit association. Unlike in the case of vitelline membrane outer layer protein-I, the threefold symmetry is not reflected in the sequence. Furthermore, the three Greek keys in the former are connected by disulfide bridges while in Jacalin the threefold symmetric structure is stabilized entirely by non-covalent interactions. The structure of Jacalin presents the first observation of a b-prism fold in a lectin. Two of the loops at one end of the prism constitute the carbohydrate binding site which shows novel features including the involvement of a N-terminal amino group generated by post-translational modification.