DIFFERENTIAL SORTING OF LYSOSOMAL-ENZYMES IN MANNOSE 6-PHOSPHATE RECEPTOR-DEFICIENT FIBROBLASTS

In higher eukaryotes, the transport of soluble lysosomal enzymes invol ves the recognition of their mannose 6-phosphate signal by two recepto rs: the cation-independent mannose 6-phosphate/insulin-like growth fac tor II receptor (CI-MPR) and the cation-dependent mannose 6-phosphate receptor (CD-MPR). It is not known why these two different proteins ar e present in most cell types. To investigate their relative function i n lysosomal enzyme targeting, we created cell lines that lack either o r both MPRs. This was accomplished by mating CD-MPR-deficient mice wit h T-hp mice that carry a CI-MPR deleted allele. Fibroblasts prepared f rom embryos that lack the two receptors exhibit a massive missorting o f multiple lysosomal enzymes and accumulate undigested material in the ir endocytic compartments. Fibroblasts that lack the CI-MPR, like thos e lacking the CD-MPR, exhibit a milder phenotype and are only partiall y impaired in sorting. This demonstrates that both receptors are requi red for efficient intracellular targeting of lysosomal enzymes. More i mportantly, comparison of the phosphorylated proteins secreted by the different cell types indicates that the two receptors may interact in vivo with different subgroups of hydrolases. This observation may prov ide a rational explanation for the existence of two distinct mannose 6 -phosphate binding proteins in mammalian cells.