46-kDa mannose 6-phosphate-specific receptor: biosynthesis, processing, subcellular location and topology

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The cation-dependent mannose 6-phosphate receptor plays a vital role in targeting lysosomal enzymes.
• Understanding its synthesis and localization is key to deciphering intracellular trafficking pathways.

Purpose of the Study:

• To investigate the synthesis, maturation, and cellular localization of the cation-dependent mannose 6-phosphate receptor.
• To characterize the structural properties of the receptor in different cell types.

Main Methods:

• Synthesis and maturation tracking in human (fibroblasts, Hep G2, U937, macrophages) and rat (Morris hepatoma) cells.
• Analysis of molecular size, N-linked oligosaccharide modification (endo H resistance), and membrane partitioning (Triton X-114).
• Cell surface and internal membrane localization studies, including endocytic pathway and lysosome association.

Main Results:

• Mature receptor size varied (46 kDa generally, 43 kDa in fibroblasts).
• Receptor exhibited significant N-linked glycosylation, with rapid conversion to endo H-resistant forms.
• Predominantly localized to internal membranes, with a small fraction at the cell surface; absent in dense lysosomes.
• Hydrophobic transmembrane protein, with distinct cytosolic (5 kDa) and luminal (≥37.5 kDa) domains.
• Cross-linking suggested the mature receptor exists as a dimer.

Conclusions:

• The cation-dependent mannose 6-phosphate receptor undergoes complex post-translational modifications and exhibits specific cellular compartmentalization.
• Its transmembrane and dimeric nature are important for its function in intracellular transport.