Mannose 6-Phosphate Receptors

Abstract

Themannose 6-phosphate receptors (MPRs) play a critical role in the biogenesis of lysosomes by targeting newly synthesized, hydrolytic enzymes to the lysosome. The high mannose-type N-glycans on acid hydrolases acquire a unique tag, mannose 6-phosphate (Man-6-P), which is recognized by two distinct MPRs, a 300 kDa cation-independent MPR (CI-MPR) and a 46 kDa cation-dependent MPR (CD-MPR). The large extracytoplasmic region of the CI-MPR contains 15 contiguous repeating units, termed mannose 6-phosphate receptor homology (MRH) domains, whereas the much smaller CD-MPR contains a single MRH domain. Biochemical and structural studies have shown that the CI-MPR contains multiple Man-6-P binding sites, allowing it to be a more efficient carrier of hydrolytic enzymes than the CD-MPR. Furthermore, the ability of CI-MPR to internalize ligands at the cell surface is exploited in enzyme replacement therapy (ERT) for the treatment of several lysosomal storage diseases.