Complete amino acid sequence of luffin-b, a ribosome-inactivating protein from sponge gourd (Luffa cylindrica) seeds.

The complete amino acid sequence of luffin-b has been determined. All the twenty-seven tryptic peptides were isolated by reverse-phase HPLC from the tryptic digests of intact luffin-b and one of its CNBr fragments (CB4), and sequenced using the DABITC/PITC double coupling method. The overlap of these peptides was achieved by analyzing the CNBr fragments and their chymotryptic peptides. Luffin-b consists of 250 amino acid residues with a relative molecular mass of 27,275 Da. Investigation for glycosylation sites indicated that Asn at positions 2, 78, and 85 might carry sugars. Sequence comparison with luffin-a showed that amino acid substitution occurred in 55 positions. Luffin-b contains three glycosylation sites instead of the six sites in luffin-a, of which two were found to be conserved.