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A new binding protein which recognizes the phosphomannosyl recognition marker of lysosomal enzymes has been isolated from P388D1 macrophages which lack the 215-kDa Man-6-P receptor. This receptor was detergent-solubilized from total membranes and purified by chromatography on a lysosomal enzyme affinity column. An identical protein was purified from bovine liver using a two-step procedure which is based on the difference in ion requirement and ligand specificity of the new Man-6-P binding protein and the previously characterized 215-kDa Man-6-P receptor. After detergent solubilization of membranes, both receptors were co-purified on a phosphomannan-Sepharose affinity column run in the presence of MnCl2 and then were separated using a Dictyostelium discoideum lysosomal enzyme-Affi-Gel column which, in the absence of MnCl2, retains only the 215-kDa Man-6-P receptor. The new Man-6-P receptor binds efficiently to phosphomannosyl monoester-containing ligands when MnCl2 is present. This binding is specifically inhibited by Man-6-P or by low pH. The cation-dependent Man-6-P receptor has a subunit molecular size of 46 kDa and appears to be an oligomer composed of three subunits. The receptor contains both high mannose (or hybrid)- and complex-type oligosaccharide units on the basis of sensitivity to digestion with endo-beta-N-acetylglucosaminidase H and endo-beta-N-acetylglucosaminidase F. Radioimmunoassays carried out with two different antibodies demonstrate that the 46-kDa cation-dependent Man-6-P receptor and the 215-kDa cation-independent Man-6-P receptor not only differ in their properties but are also immunologically distinct.
Hoflack et al. (Tue,) studied this question.