This set of Protein Engineering Questions and Answers for Entrance exams focuses on “Structure and Function of Transmembrane Proteins – 2”.
1. Which of the following integral membrane proteins mediate calcium-dependent cell-cell adhesion?
c) IgSF members
Explanation: The integral membrane proteins that mediate calcium-dependent cell-cell adhesion are cadherins. Selectins, Integrins, and IgSF members are integral membrane proteins but do not mediate calcium-dependent cell-cell adhesion.
2. Which of the following transmembrane proteins form tight junctions?
a) Selectins and IgSF
b) Cadherins and selectins
c) Integrins and selectins
d) Occludin and Claudins
Explanation: Occludin and Claudins are the transmembrane proteins that form tight junctions. Selectins, IgSF, Cadherins, and Integrins do not form tight junctions. Occludin and Claudins span the membrane four times.
3. Which of the following transmembrane protein is present in gap junctions?
Explanation: The transmembrane protein that is present in gap junctions is connexins. Six connexin molecules combine to form a quaternary structure called as connexon. Selectins, Integrins, and Cadherins are not present in gap junctions.
4. Intermediate filaments are linked to the cytoplasmic domain of which transmembrane protein?
c) Members of IgSF
d) Desmosomal cadherins
Explanation: Intermediate filaments are linked to the cytoplasmic domain of a transmembrane protein called desmosomal cadherins. Catenin family of proteins tether cadherins to the cytoskeleton and transmit signals to the cytoplasm and nucleus.
5. All membrane receptors are transmembrane proteins.
Explanation: The above statement is true. All membrane receptors are transmembrane proteins. They bind to their ligand on the extracellular side and transmit the signal to the interior of the cell. They are of immense importance to a cell and the organism as a whole.
6. Find the odd one out.
c) JAK-STAT receptors
d) G protein
Explanation: G protein is not a transmembrane protein receptor. It is a component of GPCR, and is present on the cytosolic leaflet of the plasma membrane. GPCRs, RTKs, and JAK-STAT receptors are transmembrane protein receptors. Hence, the G protein is the odd one out.
7. Which of the following is not a transmembrane protein?
a) CD4 receptor
b) T cell receptor
c) B cell receptor
Explanation: JAKs (Janus activated kinases) are not transmembrane proteins. CD4 receptor, T cell receptor, and B cell receptor are transmembrane proteins. JAKs are peripheral membrane proteins. They are involved in the JAK-STAT signaling pathway.
8. Which of the following proteins are involved in the transport of certain substances across lipid bilayer?
a) Extracellular matrix proteins
b) Peripheral proteins
c) Lipid-anchored proteins
d) Transmembrane proteins
Explanation: Transmembrane proteins are involved in the transport of certain substances across the lipid bilayer. This is possible due to the presence of hydrophilic channels in them. E.g. aquaporins transport water, glycerol, and other hydrophilic molecules across membranes.
9. Which of the following is not true for Na+/K+ – ATPase?
a) 3 sodium ions are exported out
b) 2 potassium ions are imported
c) It uses ATP to function
d) It is found in specific animal cells
Explanation: Na+/K+ – ATPase is an active pump, hence, it uses ATP to carry out its function. It exports 3 sodium ions out of the cell per molecule of ATP hydrolyzed. Also, it imports 2 potassium ions inside the cell per APT molecule hydrolyzed. It is found in all animal cells. Hence, the statement “It is found in specific animal cells” is not true for Na+/K+ – ATPase.
10. Which of the following is not a transporter protein?
a) Na+/K+ – ATPase
b) GLUT 1
c) Sodium/glucose co-transporter
d) T cell receptor
Explanation: T cell receptor is indeed a transmembrane protein but is not a transporter protein. It is involved in immune function. Na+/K+ – ATPase, GLUT 1, and sodium/glucose co-transporters are transporter proteins. They transport substances in and out of the cell.
11. Find the odd one out.
d) Na+/K+ – ATPase
Explanation: Cadherins, Selectins, Integrins, and Na+/K+ – ATPase are all transmembrane proteins. But Na+/K+ – ATPase is an active transport protein. Whereas, Cadherins, Selectins, and Integrins are proteins involved in cell-cell communication and signal transduction.
12. Which of the following is true for sodium/glucose co-transporter?
a) They transport glucose which has sodium ion attached
b) It is an antiport protein
c) It is a uniport protein
d) It is a symport protein
Explanation: Sodium/glucose co-transporter is a symport protein. It is also known as sodium-dependent glucose co-transporter. It uses the energy from the downhill sodium ion gradient created by the ATPase pump to transport glucose across the membrane, against an uphill glucose gradient.
13. Who discovered Na+/K+ – ATPase?
a) Christian de Duve
c) Robert Hooke
d) Jens Christian Skou
Explanation: Jens Christian Skou (a Danish scientist) discovered Na+/K+ – ATPase in 1957. He was awarded a Nobel prize for his work in 1997. Robert Hooke discovered the cell. Landshulz discovered the leucine zipper. Christian de Duve discovered lysosomes.
14. Transmembrane proteins cannot have which of the following role?
a) Structural role
b) Catalytic role
c) Transport role
d) Replicative role
Explanation: Transmembrane proteins cannot have a replicative role. Replication is mediated by severalfactors and polymerases none of which are transmembrane proteins. Transmembrane proteins can have Structural, Transport, and Catalytic role.
15. Which of the following receptors allow ions such as Na+, K+, and Ca+2, etc. to pass through the membrane in response to the binding of a chemical messenger?
a) Sodium/glucose co-transporter
b) GLUT 1
c) Na+/K+ – ATPase
d) Ionotropic receptors
Explanation: Ionotropic receptors allow ions such as Na+, K+, and Ca+2, etc. to pass through the membrane in response to the binding of a chemical messenger. The e.g. of messenger chemical is a neurotransmitter. Ionotropic receptors are also referred to as ligand-gated ion channels.
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