This set of Molecular Endocrinology Multiple Choice Questions & Answers (MCQs) focuses on “Mechanisms of Disease and Receptor Serine Kinases – Set 2”.
1. Why do the activated receptor complex Cytokines such as TGF-β exist as heterotetramers?
a) Due to their existence as dimers
b) Due to their binding affinity
c) Due to the specificity
d) Due to their reliability
View Answer
Explanation: Cytokines such as TGF-β exist as dimers, allowing them to bind to two RII molecules simultaneously. Therefore, the activated receptor complex potentially occurs in the form of (RI)2(RII)2 as a heterotetramer.
2. Which is the peptide that inhibits activin signaling by binding to the receptor?
a) Follistatin
b) Inhibin
c) Leptin
d) Pectin
View Answer
Explanation: Inhibin is a peptide which inhibits the signaling of activin. This is achieved without activating the phosphorylation of RI by binding to the receptor. Inhibin, a hormone secreted by the granulosa cells in women’s ovaries serves primarily to suppress the anterior pituitary gland’s secretion of follicle-stimulating hormone.
3. Which is the membrane anchored protein that functions as a coreceptor for inhibin?
a) Follistatin
b) Leptin
c) Betaglycan
d) Pectin
View Answer
Explanation: Betaglycan is a protein anchored to the membrane, that acts by facilitating inhibin binding to the activin receptor, as a coreceptor for inhibin. Betaglycan does not bind activin to itself. Betaglycan was originally designated as a Transforming Growth Factor-β (TGFβ) and it is a non-signalling co-receptor.
4. Follistatin is a “ligand trap”.
a) True
b) False
View Answer
Explanation: There are many examples of activin-related biology and its function. Follistatin is a “ligand trap,” which is a soluble protein that binds activin and thereby blocks RI and RII entry. Follistatin is a naturally occurring protein that functions as an extracellular trap to regulate the TGFβ superfamily ligands’ biological activity.
5. The SMAD proteins are the mammalian homologs of the Drosophila mad-encoded proteins.
a) True
b) False
View Answer
Explanation: SMAD proteins are the mammalian homologs of the proteins Drosophila mad (mothers against decapentaplegic) gene and the Caenorhabditis elegans Sma genes. Five human R-SMAD proteins are present in total.
6. Which among the following proteins, functions as the downstream effector of receptor serine kinases?
a) Betaglycan
b) R-SMADs
c) Follistatin
d) Leptin
View Answer
Explanation: Receptor serine kinases are able to phosphorylate other protein substrates once activated. Receptor-regulated SMAD proteins (R-SMADs) act as receptor serine kinases immediate downstream effectors.
7. Which among the following R-SMAD proteins mediate the actions of the activin/TGF-β family of cytokines?
a) SMAD 2 and SMAD 3
b) SMAD 2 and SMAD 5
c) SMAD 5 and SMAD 7
d) SMAD 1 AND SMAD 8
View Answer
Explanation: Five human R-SMAD proteins are present in the human body. The activities of the activin/TGF-β family of cytokines are carried out by SMAD2 and SMAD3. Smad2 and Smad3 are structurally similar and in mediating TGFβ signals, they are mostly considered to be equally significant.
8. Where should SMAD2 bind in order to carry out its receptor mediated phosphorylation?
a) SMAD anchor for ligand deactivation (SALD)
b) SMAD anchor for ligand activation (SALA)
c) SMAD anchor for receptor deactivation (SARD)
d) SMAD anchor for receptor activation (SARA)
View Answer
Explanation: SMAD2 receptor-mediated phosphorylation takes place when SMAD2 is bound to SMAD anchor for receptor activation (SARA). This can be observed in early endosomes, especially in the mitochondria.
9. Which among the following is the co-mediator of SMAD 2?
a) SMAD 4
b) SMAD 5
c) SMAD 6
d) SMAD 8
View Answer
Explanation: The co-mediator of SMAD 2 is SMAD 4 signal transducers. Phosphorylation facilitates SMAD2 dissociation from SARA and SMAD2 binding to the co-mediator (CoSMAD), SMAD4. The major role of these SMAD proteins is the regulation of transcription and many signaling pathways including cAMP pathway.
10. Which among the following is an inhibitory SMAD?
a) SMAD 1
b) SMAD 7
c) SMAD 8
d) SMAD 5
View Answer
Explanation: SMAD7 is an inhibitor of SMAD signal transducers. It binds, in competition with R-SMADs, to active receptors. The binding of SMAD7 facilitates ubiquitination and degradation of receptors. Inhibitory Smads (I-Smads) are members of the Smad family with preserved carboxy-terminal MH2 domains that interact with activated type I receptors and R-Smads to inhibit intracellular signalling.
11. In order to mediate the translocation, to which protein does the lysine rich nuclear localization signals bind?
a) Importin
b) Transportin
c) Leptin
d) Promptin
View Answer
Explanation: Some R-SMADs contain nuclear localization (KKLKK) lysine-rich signals that bind to importin. Translocation into the nucleus is mediated by this process which requires energy derived from ATP. Importin is a type of karyopherin that carries molecules of protein from the cytoplasm of the cell to the nucleus.
12. Which among the following is the minimum sequence for SMAD binding?
a) 5′-AGAC-3′
b) 5′-AGTC-3′
c) 5′-TGTC-3′
d) 5′-ACAG-3′
View Answer
Explanation: A 4-base-pair chain, 5′-AGAC-3′, is the minimum sequenced SMAD binding element. This sequence is very limited and a high degree of precision is not required for this when compared to other sequences.
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