Question

After being exported to the cytoplasm, the 7-methyl guanosine cap was inadvertently (by mistake) cleaved off of a eukaryotic mRNA. As a consequence, which of the following will probably happen?

	a.	The ribosomal SSU initiation complex will fail to assemble correctly
	b.	The introns will not be spliced in their proper location
	c.	The resultant protein cannot be exported out of the cell
	d.	The protein will fail to terminate in the correct location
	e.	The message will be read, but in the wrong reading frame

Answer 1

The 7 methylguanosine cap which is known as the 5’ cap is attached to the mRNA at the 5’ end by triphosphate linkage when transcription initiates. Its major role is in protecting the mRNA from degradation. Further it is involved in initiation of translation, elongation of transcription, RNA splicing, addition of the poly A tail etc.

Its major role in cytoplasm is in initiation of translation by the ribosome. The small subunit of the ribosome will bind to the 5’ cap to initiate translation (Ribosomal SSU is ribosomal small subunit). During translation initiation, 43 pre-initiation complex (43PIC) is formed in eukaryotes. For formation of PIC, initially, a ternary complex is formed, this ternary complex contains eIF2 (elongation initiation factor), and Met-tRNAi (initiator tRNA form methionine and is the one that binds the small ribosomal subunit (40S). Other initiation elongation factors eIF1, eIF1A, eIF3 are required for this step. During translation initiation, 5’ cap is first bound by the eIF4E, a protein in the eIF4 F complex. The preinitiation complex then binds to the 5’ cap. This forms the initiation complex and initiates the later events in translation. Thus, 43PIC complex first binds to the mRNA at the 5’ cap to form the initiation complex. When the 5’ cap is absent in the mRNA, the initiation complex will not form.

Intron splicing occurs in nucleus of the cell. Exporting the protein out of the cell occurs via ER, Golgi, a step after protein has been translated. This process occurs in ER and not in cytoplasm. 5’ cap is not involved in termination of protein synthesis. The poly A tail at 3’ end is required for protein synthesis termination. The message is read in wrong direction only if the mRNA is produced in 3’ to 5’ direction, which does not occur in transcription.

Right choice:

a. The ribosomal SSU initiation complex will fail to assemble correctly.