Rapid equilibrium between monomeric, dimeric and tetrameric forms of the 46‐kDa mannose 6‐phosphate receptor at 37°C

Abstract

At 4°C, detergent-solubilized 46-kDa mannose 6-phosphate receptor (MPR 46) exists as a mixture of dimeric and tetrameric forms [A. Waheed, A. Hille, U. Junghans & K. von Figura (1990) Biochemistry 29, 2449–2455]. At 37°C, MPR 46 exists as a mixture of monomeric, dimeric and tetrameric forms, which can be separated by sucrose density centrifugation or chromatography on a mannose-6-phosphate affinity matrix. Monomeric MPR 46 did not bind to the affinity matrix, while dimeric and tetrameric receptors were eluted with increasing concentrations of mannose 6-phosphate. Depending on the incubation temperature, dimeric MPR 46 preferentially associates to tetramers (≤16°C) or dissociates to monomers (≥25°C). The presence of mannose 6-phosphate shifts the equilibrium to higher quaternary structure, with a maximal effect at 37°C. Incubation of dimeric or tetrameric receptor at pH 4.0 induces a rapid dissociation of about a third of the receptor with t_1/2 of < 2 min, followed by a phase of slow dissociation. Readjusting the pH to 7.5 induces a rapid formation of dimers and tetramers, which is promoted by the presence of mannose 6-phosphate or an increase of the receptor concentration. These observations may indicate that the recycling of the receptor between the Golgi apparatus, where it binds ligands at near-neutral pH, and the acidic prelysosomes, where it releases the ligands, is associated with a change of its quaternary structure, which in turn may affect the recycling kinetics. In keeping with this assumption, we observed in baby hamster kidney cells over-expressing MPR 46 and in membrane prepared from these cells monomeric, dimeric and tetrameric forms of the receptor.