Seed proteins are often made out of components cleaved from a precursor molecule.
But biologists now suggest a particularly puzzling assembly plan for the jackbean’s seed protein, concanavalin A (con A). They propose that the protein is produced by rejoining, in transposed order, two major segments of its precursor. British scientist D.M.
Carrington of the University of Cambridge in England and colleagues isolated a DNA squence that they argue represents the complete con A precursor. From its DNA, this precursor appears to have two segments arranged in the opposite order from their positions in the mature con A molecule. Their order in the jackbean con A precursor is the same as in the mature form of a related protein, lectin, in pea seeds, the scientists report in the Jan. 3 NATURE. To produce con A from the proposed precursor requires several cleavages and then ligation of transposed pieces. “But the postulated final ligation step is unprecedented and should be treated with caution,” warn John A.
Gatehouse and Donald boutler of Durham University in England. “One suspects this system still has a few tricks up its sleeve.”