An intrinsic membrane glycoprotein with cytosolically oriented n-linked sugars

Area of Science:

• Biochemistry
• Cell Biology
• Membrane Protein Research

Background:

• The Na(+)-pump (sodium-potassium adenosine triphosphatase) is crucial for cellular ion transport.
• Understanding the post-translational modifications of membrane proteins like the Na(+)-pump alpha-subunit is essential for elucidating their function.
• Glycosylation patterns can significantly impact protein structure, stability, and localization.

Purpose of the Study:

• To investigate whether the Na(+)-pump alpha-subunit polypeptide undergoes glycosylation.
• To determine the location and type of glycosylation on the Na(+)-pump alpha-subunit.
• To characterize the protein-carbohydrate linkage and orientation of the oligosaccharide moieties.

Main Methods:

• Enzymatic assays using bovine milk galactosyltransferase to assess glycosylation.
• Preparation and permeabilization of right-side-out vesicles from kidney outer medulla.
• Hydrolysis of protein-carbohydrate linkages using peptide-N glycosidase F.

Main Results:

• The Na(+)-pump alpha-subunit polypeptide is glycosylated by galactosyltransferase, indicating the addition of galactose residues.
• Glycosylation occurs only after vesicle permeabilization, suggesting oligosaccharide moieties face the cytoplasm.
• The protein-carbohydrate linkage is N-linked, specifically bound to asparagine residues, confirmed by peptide-N glycosidase F activity.

Conclusions:

• The Na(+)-pump alpha subunit is a glycoprotein with N-linked oligosaccharides.
• These oligosaccharide moieties are located on the cytosolic face of the cell membrane.
• This represents a novel finding for intrinsic membrane glycoproteins regarding their oligosaccharide linkage and orientation.