Question

During exercise, protein synthesis in muscle cells stops. After exercise, protein synthesis starts again and contributes to the growth of muscle caused by exercise. A study by Dreyer et al found that the protein 4E-BP1 is normally phosphorylated in muscle cells, but becomes dephosphorylated during exercise and phosphorylated again after exercise. The dephosphorylated form of 4E-BP1 can bind to elF4E to block its normal function. How might 4E-BP1 block protein synthesis during exercise?

A. 4E-BP1 binding to elF4E prevents release factors from binding to the A site of the ribosome.
B. 4E-BP1 binding to elF4E blocks the function of EF-Tu.
C. 4E-BP1 binding to elF4E makes the ribosome too big to translocate along the mRNA
D. 4E-BP1 binding to elF4E prevents loading of the mRNA onto the ribosome.

Answer 1

D. 4E-BP1 binding to elF4E prevents loading of the mRNA onto the ribosome.

Step-by-step explanation

In eukaryotic organisms, the eIF4E translation initiation factor functions by directing the ribosomes to the 5'-terminal cap structure of the messenger RNA (mRNA) in order to start the translation. Moreover, phosphorylation is a posttranslational modification of specific amino acids on proteins that play diverse cellular functions by altering protein stability, location, and/or enzymatic activity. It has been shown that elF4E phosphorylation is increased in response to cellular stimuli that induce translation in the ribosomes (e.g., growth factors, hormones, etc). The eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) is a repressor of mRNA translation which is phosphorylated and inactivated by growth factors and hormones, thereby inhibiting 4E-BP1 binding to elF4E and consequently activating translation.