Lysosomal enzyme binding receptor protein from monkey brain and its phosphorylation

Area of Science:

• Neuroscience
• Biochemistry
• Cell Biology

Background:

• Lysosomal enzymes are crucial for cellular waste degradation.
• Receptor-mediated endocytosis facilitates lysosomal enzyme uptake.
• The specific mechanisms regulating lysosomal enzyme receptor function are not fully understood.

Purpose of the Study:

• To investigate the phosphorylation of the lysosomal enzyme binding receptor protein from monkey brain.
• To determine the effect of phosphorylation on receptor function and lysosomal enzyme binding.

Main Methods:

• Isolation of the receptor protein using phosphomannan-Sepharose affinity chromatography.
• Phosphorylation assays using [gamma-32P] ATP and associated protein kinases.
• Analysis of phosphorylated proteins via SDS-gel electrophoresis, autoradiography, and phosphoamino acid analysis.
• Cross-linking studies with 125I-labelled lysosomal enzymes and disuccinimidyl suberate.

Main Results:

• A >200 kDa protein was phosphorylated on serine and tyrosine residues; a ~45 kDa protein was phosphorylated on serine residues.
• Phosphorylation was inhibited by quercetin, polylysine, and polymyxin B.
• The phosphorylated receptor showed reduced efficiency in binding lysosomal enzymes compared to the unphosphorylated form.

Conclusions:

• The lysosomal enzyme binding receptor protein is subject to phosphorylation on both serine and tyrosine residues.
• Phosphorylation significantly impairs the receptor's ability to bind lysosomal enzymes.
• These findings suggest a regulatory role for phosphorylation in lysosomal enzyme uptake.