Isolation and characterization of cDNA clones encoding jacalin isolectins.

Four jacalin cDNA clones (pSKcJA1, pSKcJA3, pSKcJA15, and pSKcJA17) have been obtained from an Artocarpus integrifolia (jackfruit) seed cDNA library. These clones share over 94% sequence homology, and their deduced polypeptide sequences confirm the existence of multiple jacalin isolectins in jackfruit seeds. The deduced amino acid sequences show that jacalin appears to be initially synthesized as a prepropeptide with the following structure: N-signal (21 residues)-->propeptide (39 residues)-->beta-peptide (20 residues)-->linker region (4 residues)-->alpha-peptide (133 residues). These observations are supported by Western blot analysis of jackfruit seed extract and by immunoprecipitation of in vitro translated products of both pSKcJA3 transcript and jackfruit seed poly(A)+ RNA. Sequence analysis of the 39-residue propeptide reveals that it has the potential to facilitate proper folding of jacalin protein. The unusual primary structure of jacalin prepropeptide suggests a quite interesting processing of this lectin precursor into mature alpha- and beta-subunits.