Chaperone-like functions of high-mannose type and complex-type N-glycans and their molecular basis.

It has recently become apparent that high-mannose type N-glycans directly promote protein folding, whereas complex-type ones play a crucial role in the stabilization of protein functional conformations through hydrophobic interactions with the hydrophobic protein surfaces. Here an attempt was made to understand more deeply the molecular basis of these chaperone-like functions with the aid of information obtained from spacefill models of N-glycans. The promotion of protein folding by high-mannose N-glycans seemed to be based on their unique structure, which includes a hydrophobic region similar to the cyclodextrin cavity. The promotive features of high-mannose N-glycans newly observed under various conditions furnished strong support for the view that both intra- and extramolecular high-mannose N-glycans are directly involved in the promotion of protein folding in the endoplasmic reticulum. Further, it was revealed that the N-acetyllactosamine units in complex-type N-glycans have an amphiphilic structure and greatly contribute to the formation of extensive hydrophobic surfaces and, consequently, to the N-glycan-protein hydrophobic interactions. The processing of high-mannose type N-glycans to complex-type ones seems to be an ingenious device to enable the N-glycans to perform these two chaperone-like functions.

N-glycans stabilize human erythropoietin through hydrophobic interactions with the hydrophobic protein surface: studies by surface plasmon resonance analysis.

Human erythropoietin (EPO) produced in Chinese hamster ovary cells is a hydrophobic protein highly stabilized by multibranched complex-type N-glycans. To reveal the molecular basis of the interaction between the N-glycans and the EPO protein, complex-type N-glycans of different structures were analyzed as to their binding affinity for Escherichia coli-expressed EPO by means of the surface plasmon resonance technique. It appears well established that complex-type N-glycans, particularly multibranched ones, have hydrophobic regions that extensively stretch across the plane holding acetylamino groups and that N-glycan-protein hydrophobic interactions characterized by a slow rate of dissociation stabilize the protein conformation.