1 A) As part of an early experiment on the secretory system, an in vitro transla

Question

1 A) As part of an early experiment on the secretory system, an in vitro translation system consisting only of mRNA and ribosomes resulted in secretory proteins that were larger than the identical protein when translated in a cell. Why?

(a)Alternative splicing of secretory protein mRNAs requires additional proteins not included in the in vitro system.

(B)Cotranslational protein synthesis and ER import lead to preferential utilization of a distinct translation stop site, changing the resulting protein size.

(C)Secretory proteins translated in the cytosol are post-translationally modified by phosphorylation.

(D)Signal sequence cleavage requires ER entry and action of ER-resident proteins.

1 B) What is the source of energy needed for unidirectional translocation across the membrane in (a) cotranslational translocation into the endoplasmic reticulum (ER); (b) post-translational translocation into the ER; and (c) translocation into the mitochondrial matrix.

(a) ATP hydrolysis; (b) GTP hydrolysis; (c) ATP hydrolysis

(a) GTP hydrolysis; (b) ATP hydrolysis; (c) ATP hydrolysis

ATP hydrolysis in all cases

(a) GTP hydrolysis; (b) ATP hydrolysis; (c) GTP hydrolysis

Explanation / Answer

1) option D. signal sequence cleavage requires ER entry and action of ER- resident proteins.

in an in-vitro system signal sequence cleavage is not posible while in cell system it is possible.

2) GTP hydrolysis in cotranslational translocation into the ER.

ATP hydrolysis in post-translational translocation into the ER

and ATP hydrolysis in translocation into the mitochondrial matrix.

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