Practice question

Detailed topology of glycophorin A major human erythrocyte sialoglycoprotein clarifies single-pass characteristics. Gene GYPA on chromosome four encodes one hundred fifty residues including cleavable signal peptide. Mature protein seventy kilodalton apparent due to extensive glycosylation: seventy residue extracellular domain heavily O-glycosylated fifteen O-linked tetrasaccharides NeuAc alpha2-3 Gal beta1-3 GalNAc plus one N-linked complex chain adding sialic acid dense negative charge defining MNS blood group M and N antigens and preventing rouleaux formation. Hydrophobic anchor residues seventy three to ninety five forms nineteen residue alpha helix containing GXXXG dimerization motif facilitating high affinity dimerization free energy minus twelve kilocalories measured by analytical ultracentrifugation, widely used model for helix-helix interactions. Cytosolic tail thirty six residues acidic interacts with protein 4.1R FERM domain linking to spectrin actin junctional complex near actin protofilament, regulating lateral mobility and mechanosensing. Unlike fourteen-pass Band 3 anion transporter, glycophorin does not transport chloride nor span multiple times nor bind actin directly via actin binding domain, therefore single-pass nature best describes architecture and enables studies of membrane protein folding energetics and Plasmodium invasion receptor function.