Lysosomal enzyme binding to mouse P388D1 macrophage membranes lacking the 215-kDa mannose 6-phosphate receptor: evidence for the existence of a second mannose 6-phosphate receptor.

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Abstract
Mouse P388D1 macrophages target newly synthesized acid hydrolases to lysosomes in spite of their lack of the 215-kDa [kilodalton] mannose 6-phosphate (Man-6-P) receptor. These cells contain a membrane-associated Man-6-P receptor that is distinct from the previously described receptor. The new receptor binds lysosomal enzymes containing phosphomannosyl residues. This binding is inhibited by Man-6-P or by pretreatment of the lysosomal enzymes with alkaline phosphatase. Lysosomal enzyme binding occurs at neutral pH and dissociation of the bound ligand occurs at low pH values comparable to those found within endosomes or lysosomes. The new receptor differs from the 215-kDa Man-6-P receptor in 2 ways. It has an absolute requirement for divalent cations and is unable to bind Dictyostelium discoideum lysosomal enymes, which contain methylphosphomannosyl residues rather than the usual phosphomannosyl monoesters. Based on the difference in cation requirement, the 2154-kDa receptor might be referred to as Man-6-P receptor CI (cation independent) and the new receptor as Man-6-P receptor CD (cation dependent). The Man-6-P receptor CD functions in the targeting of newly synthesized acid hydrolases to lysosomes in P388D1 macrophages.

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