Structural Insights into Complexes of Glucose-Regulated Protein94 (Grp94) with Human Immunoglobulin G. Relevance for Grp94-IgG Complexes that Form In Vivo in Pathological Conditions

While the mechanism by which Grp94 displays its chaperone function with client peptides in the cell has been elucidated extensively, much less is known about the nature and properties of how Grp94 can engage binding to proteins once it is exposed on the cell surface or liberated in the extra-cellular milieu, as occurs in pathological conditions. In this work, we wanted to investigate the molecular aspects and structural characteristics of complexes that Grp94 forms with human IgG, posing the attention on the influence that glycosylation of Grp94 might have on the binding capacity to IgG, and on the identification of sites involved in the binding. To this aim, we employed both native, fully glycosylated and partially glycosylated Grp94, and recombinant, non-glycosylated Grp94, as well as IgG subunits, in different experimental conditions, including the physiological setting of human plasma. Regardless of the species and type, Grp94 engages a similar, highly specific and stable binding with IgG that involves sites located in the N-terminal domain of Grp94 and the hinge region of whole IgG. Grp94 does not form stable complex with Fab, F(ab)2 or Fc. Glycosylation turns out to be an obstacle to the Grp94 binding to IgG, although this negative effect can be counteracted by ATP and spontaneously also disappears in time in a physiological setting of incubation. ATP does not affect at all the binding capacity of non-glycosylated Grp94. However, complexes that native, partially glycosylated Grp94 forms with IgG in the presence of ATP show strikingly different characteristics with respect to those formed in absence of ATP. Results have relevance for the mechanism regulating the formation of stable Grp94-IgG complexes in vivo, in the pathological conditions associated with the extra-cellular location of Grp94.