Below is a snippet of a study guide. Glycophorin\'s carboxy terminus does not re

Question

Below is a snippet of a study guide. Glycophorin's carboxy terminus does not react with membrane-impermeable reagents, while its amino terminus does (react with trypsin).

In that case, why is glycophorin a transmembrane protein? What is the role of its carboxy terminus inside the membrane? (I basically want to know if glycophorin could work as a monotopic integral membrane protein OR if the transmembrane structure of glycophorin is necessary for its function)

Asymmetric arrangement of membrane proteins

If a membrane protein interacts with a membrane-impermeant reagent, that protein must have at least one domain exposed on the outer face of the membrane

For glycoproteins, glycosylated domains are generally on the outer leaflet

Trypsin cleaves carbohydrates on glycophorin’s amino-terminal, but cannot cross the membrane

Glycophorin’s carboxy terminus does NOT react with impermeant reagents, like trypsin

Asymmetric arrangement of membrane proteins

If a membrane protein interacts with a membrane-impermeant reagent, that protein must have at least one domain exposed on the outer face of the membrane

For glycoproteins, glycosylated domains are generally on the outer leaflet

Ex) orientation of glycophorin is asymmetric: outside=amino-terminal; inside=carboxy-terminal

Trypsin cleaves carbohydrates on glycophorin’s amino-terminal, but cannot cross the membrane

Glycophorin’s carboxy terminus does NOT react with impermeant reagents, like trypsin

Aja Amino terminus Thr Arg Lys His Thr Asp Asn Thr Gin Ser Ser ThPro Arg Ala His Glu Val Ser Glu 60 4 Arg Glu Outside Thr Glu Glu Thr Gln lle Phe Gly Pro Val) -Glu Met Phe Ala Gly Val lle Gly Thr lle Leu lle Gly 95 Inside Lys Arg Arg Pro Ser val La Pro Ser Po Asp Thr Ap Ve P Le Sers Carboxyl Gin Asp Ser hr Glu terminus Glu Pro Asn 131 11-7 Transbilayer disposition of glycophorin in an erythro- ophilic domain, containing all the sugar residues, is on the ace, and another hydrophilic domain protrudes from the inner membrane. Each red hexagon represents a tetrasaccharide ning two Neu5Ac (sialic acid), Gal, and GalNAc) O-linked to a Ser ue the blue hexagon represents an oligosaccharide N-linked to ue. The relative size of the oligosaccharide units is larger than ere. A segment of 19 hydrophobic residues (residues 75 to 93) elix that traverses the membrane bilayer (see Fig. 11-10a) from residues 64 to 74 has some hydrophobic residues and rates the outer face of the lipid bilayer, as shown. (Source he Contain or Thr resi an Asn ms an a h tion from V. Marc I. Marchesi et al., Annu. Rev. Biochem, 45:667, 1976]

Explanation / Answer

This is not a monotopic intergal membrane protein because in monotopic there is no N-terminal in out side of the cell, the N-terminal present in the lipid bilayer itself.

The transmembrane proteins are capable of initiating signals in the N-terminal that are responsive to the external environment of the cell. The C-terminal present in inside of the cell pass the signals through G-proteins to activate the genes.

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