Glycosylation patterns of human alpha2-macroglobulin: Analysis of lectin binding by electron microscopy

Abstract

Human alpha2-macroglobulin (α2M) is a 720kDa glycoprotein that presents two ultrastructural conformations: slow (S-α2M) and fast (F-α2M). α2M acts mainly as a proteinase scavenger, but an immunomodulatory role was also proposed. This work studies the effect of desialylation and deglycosylation on the structure patterns of α2M by ultrastructural analysis of lectin-induced aggregates, which represents a new approach that had never been previously used. Transmission electron microscopy (TEM) analysis showed the loss of S-α2M conformation after deglycosylation, indicating that glycosidic side-chains contribute to the molecular stability of S-α2M. TEM proved to be an important tool to analyze the effect of biochemical changes on α2M, yielding an objective qualitative control of its morphological state. Certain carbohydrate residues did not vary between the α2M conformations, since both bound similarly ConA and WGA lectins. However, the binding of PNA and BSI-B4 was slightly lower in F-α2M than in S-α2M. Among the neuraminidases used to desialylate both conformations of α2M that from Arthrobacter ureafaciens was the most effective. Incubation with the lectins ConA or SNA, respectively specific for mannosyl and sialyl residues, led to dose-dependent patterns of aggregation of α2M molecules, mediated by lectin binding and clearly visualized by TEM.