| Abstract: | We have previously reported that concanavalin A (ConA) is precipitated by a high mannose type glycopeptide (Brewer, C. F. (1979) Biochem. Biophys. Res. Commun. 90, 117-122; Bhattacharyya, L., and Brewer, C. F. (1986) Biochem. Biophys. Res. Commun. 137, 670-674). In the present study, we have investigated the ability of a series of high mannose and bisected hybrid type glycopeptides to bind and precipitate the lectin. The modes of binding of the glycopeptides were studied by nuclear magnetic relaxation dispersion (NMRD) techniques, and their affinities were determined by hemagglutination inhibition measurements. The stoichiometries of the precipitation reactions were investigated by quantitative precipitation analysis. The equivalence zones (regions of maximum precipitation) of the precipitin curves indicate that certain high mannose and bisected hybrid type glycopeptides are bivalent for lectin binding. From the NMRD and precipitation data, we have identified two protein binding sites on each glycopeptide: one site on the alpha(1-6) arm of the core beta-mannose residue involving a trimannosyl moiety which binds with high affinity (primary site); and the other site on the alpha(1-3) arm of the core beta-mannose residue involving an alpha-mannose residue(s), which binds with lower affinity (secondary site). These two types of sites bind to ConA by different mechanisms. Certain bisected hybrid type glycopeptides were found to possess only the primary ConA binding sites, but not the secondary sites, and hence were able to bind but not precipitate the lectin. Other related glycopeptides have only the secondary type sites and thus exhibit low affinity and are unable to precipitate the protein. The results are related to the possible structure-function properties of cell-surface glycopeptides. |
