We described the amphiphilic character of glycogenin (Gn), which forms by itself Gibbs and Langmuir monolayers at the air-buffer interface and associates to phospholipids (FEBS Lett., 509, 323-26, 2001). Now we compare the surface activities and phospholipid interactions of non-glucosylated apoglycogenin (a-GN) and slightly-glucosylated glycogenin (sl-GN). The sl-GN contained up to 8 (sl-8-GN) or 13 (sl-13-GN) linked glucoses. Injected into the subfase, a-GN and sl-13-GN induced a rapid increase of the surface pressure, reaching 14 mN/m in 6 min. They spread as stable monolayers from aqueous solutions; the surface pressure- and surface potential-molecular area isotherms of compression and expansion were remarkably different, the sl-13-GN occupying a higher area than a-GN at a given surface potential. When sl-8-GN, labeled by autoglucosylation from UDP-[14C]glucose, was mixed and over night incubated with pre-formed large unilamelar vesicles of palmitoyl-oleoyl-glycerophosphocholine, and vesicles separated from the labeled GN by exclusion on a Sephacryl column, about 9-24 % of the labeled protein coeluted with the vesicles, indicating interaction of the sl-8-GN with phospholipid. These results further sustain our hypothesis that GN, before or after autoglucosylation, can associate to ER membranes, were the biosynthesis of proteoglycogen might initiate, followed by dissociation when reaching a higher glucosylation degree.
