2) Proteins imported to which of the following organelles do NOT have their signal sequence cleaved?...

2) Proteins imported to which of the following organelles do NOT have their signal sequence cleaved?

A. Peroxisomes

B. Mitochondria

C. Endoplasmic reticulum

D. Proteins targeted to all of these organelles have their signal sequences cleaved

Expert Solution

Ans2) The correct option is "D".

Option "A" is incorrect because The peroximal targeting sequence (PTS1) has C-terminal SKL consensus sequence which when transported in the peroxisome lumen, it is still not cleaved by any signal peptidase.But, peroxisome has one more Peroxisome targeting sequence (PTS2) which has N-terminal 9 amino acid sequence as follows:

i.e R-(L/V/I/Q)-X-X-(L/V/I/H)-(L/S/G/A)-X-(H/Q)-(L/A) which is cleaved by TYSND1 peroxisomal protease .

Option "B" is incorrect because the N-terminal Arginine and leucine rich signal sequence of mitochondria is cleaved by MPP (Mitochondrial processing peptidase). (Only, if the protein is multiple bypass then it is never cleaved by signal peptidase).

Option "C" is incorrect because signal sequence of the Endoplasmic reticulum is cleaved by the signal peptidase.It recognizes the C-terminal side of the signal sequence and cleaves the signal sequence and the polypeptide chain is released in the ER lumen.

gladiator answered 1 year ago

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