Procathepsins L and D are membrane-bound in acidic microsomal vesicles

Area of Science:

• Cell Biology
• Biochemistry
• Molecular Biology

Background:

• Lysosomal proteases, procathepsins L and D, are crucial for cellular degradation.
• Understanding their trafficking and sorting mechanisms is essential for lysosomal function.

Purpose of the Study:

• To investigate the mechanism of lysosomal proenzyme binding to microsomal membranes.
• To identify potential receptors involved in lysosomal sorting.

Main Methods:

• Incubation of mouse fibroblast microsomes with procathepsins L and D at acidic pH.
• Elution studies varying pH and ionic strength.
• Competition assays with mannose and mannose 6-phosphate.
• Protease treatment (trypsin) of microsomes.

Main Results:

• Procathepsins L and D bind peripherally to membranes at acidic pH via ionic interactions.
• The propeptide of procathepsin L is essential for membrane binding.
• Binding is protein-mediated, saturable, and not dependent on carbohydrate moieties.
• Trypsin treatment significantly reduces procathepsin L binding.

Conclusions:

• A novel, pH-dependent, protein-mediated binding mechanism for lysosomal proenzymes to membranes exists.
• This interaction may involve specific proenzyme receptors distinct from the mannose 6-phosphate system.
• Such receptors could play a complementary role in lysosomal proenzyme sorting.