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Subcellular distribution of the mannan-binding protein and its endogenous inhibitors in rat liver
Abstract:
The subcellular distribution of the mannan-binding protein from rat liver, a lectin specific for mannose and N-acetylglucosamine, was studied. Approximately 75% of the binding activity of the homogenate was recovered in microsomes, approximately 76% of which was accounted for by rough microsomes. Rough microsomes had the highest specific activity of binding, followed by the Golgi apparatus and smooth microsomes, whereas plasma membranes, lysosomes, mitochondria, and the soluble fraction had little or no binding activity. A topographical survey indicated that the binding protein was localized exclusively on the cisternal surface of microsomal vesicles. Thus, the binding protein of microsomal vesicles was protected from protease digestion and was released from the vesicles by mild detergent treatment. Competitive inhibitors, which presumably represent endogenous ligands of the binding protein, were found among subcellular fractions. More than 50% of the inhibitory activity of the homogenate was recovered in rough microsomes, while the highest specific activity of inhibition was found in lysosomes. The Ki values estimated for rough microsomes and lysosomes were 25.9 and 8.67 micrograms/ml, respectively. The distribution profiles of inhibitors were correlated roughly with those of the binding protein, resulting in masking of the binding activity in organelles up to the level of 86%. On the basis of the known localization and topology of the binding protein and endogenous inhibitors (ligands), possible physiological functions of the binding protein relevant to the transport of biosynthetic intermediates of glycoproteins from the rough endoplasmic reticulum to the Golgi apparatus and from the Golgi apparatus to lysosomes were discussed.
Insights
Rat liver mannan-binding protein (MBP), a key lectin, is primarily located on rough microsomes. Endogenous inhibitors suggest MBP plays a role in glycoprotein transport within the cell.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Biology
Background:
- Mannan-binding protein (MBP) is a lectin involved in innate immunity and glycoprotein processing.
- Its subcellular localization and function in rat liver require detailed investigation.
Purpose of the Study:
- To determine the subcellular distribution and topographical orientation of rat liver MBP.
- To identify endogenous ligands and explore the physiological role of MBP in glycoprotein transport.
Main Methods:
- Subcellular fractionation of rat liver homogenates.
- Assays for MBP binding activity and inhibitory activity.
- Protease protection assays and mild detergent extraction.
- Determination of inhibition constants (Ki).
Main Results:
- MBP was predominantly found in microsomes (75%), particularly rough microsomes (76%), with highest specific binding activity.
- MBP was localized to the cisternal surface of microsomal vesicles, protected from proteases.
- Endogenous inhibitors of MBP were concentrated in rough microsomes and lysosomes.
- Inhibitor distribution correlated with MBP, suggesting ligand-mediated masking of binding activity.
Conclusions:
- Rat liver MBP is primarily located on the luminal side of the rough endoplasmic reticulum and Golgi apparatus.
- MBP and its endogenous ligands are implicated in the transport of glycoprotein intermediates.
- MBP may function in the transport of glycoproteins from the ER to the Golgi and subsequently to lysosomes.