PFA, a novel mollusk agglutinin, is structurally related to the ribosome-inactivating protein superfamily

The structural organization of PFA, a novel beta -galactose-specific agglut inin from the snail Pomacea flagellata, was partially characterized. Using mass spectrometry, the molecular weight of this glycoprotein was determined as 32,444 Da (7.4% carbohydrate). The agglutinin was found to form very la rge aggregates in solution, which were dissociated to monodisperse native-l ike dimers upon addition of polyethyleneglycol. The identity of the first 3 8 and the last 11 residues of the polypeptide chain was determined. It was found that PFA and the N-glycosidase subunit of ricin, a heterodimeric cyto toxin isolated from castor bean seeds, are homologous to each other in the N-terminal region. Furthermore, the far-UV circular dichroism. spectra of t hese proteins were found to be nearly superimposable, evidencing that they share common general features in their secondary and tertiary structures. O n the basis of these similarities, it can be concluded that PFA is structur ally related to the ribosome-inactivating protein superfamily. (C) 2001 Aca demic Press.