Question

During non-sense mediate decay, how does the cell physically detect early stop codons.

a-Translational Release Factors

b-PolyA binding protein

c-Exon Junction Complex

d-CP80 Cap binding protein.

Answer 1

Correct ans is C. Exon junction complex

c. Basically, this pathway occurs in eukaryotes which functions to remove the mRNA transcrips that has premature stop codons. It plays a role in controlling gene expression and thus its regulation. This pathway contains 3 genes-UPF1, UPF2 and UPF3 all of which code for a protein. UPF1 is phosphorylated by proteins SMS-1, SMG-5, SMG-6 and SMG-7. This triggers NMD pathway caused by transphosphorylation of UPF-1 by SMG-1. This SMG-1 which is a part of SMUF complex (complex comprise of UPF-1, SMG-1, SMG-8 and SMG-9) recognizes the early stop codons. Through C terminal region of UPF-3, SMG-1 bounds to this exon junction which inturn creates a complex called DECID which the connecting bridge between SURF and EJC.

UPF2 and UPF3 take part in exon junction complex, whic tend to bound mRNA after splicing with other proteins. After splicing, ribosome removes this juction from mRNA. After this, NMD tends to get activated if any f the proteins still remains attached to mRNA.

In this way, Exon junction complex detects the early stop codons and eliminate them.

b.PolyA binding RNA protein basically binds to polyadenosine RNA. Nuclear PABPN 1 stabilizes the poly A tail length whereas PABPC stabilizes poly A RNA in the cytoplasm. So these polyA binding protein helps to enhance translation.

d.CP80 Cap binding protein works in association with UPF-1 to inhibit NMD pathway. Basically, CP80 Cap binding protein binds UPF-1 at the 5'cap of newly synthesized mRNA. It also enhances efficiency of NMD pathway by its interaction with UPF-1 and UPF-2 gene.