Question

(1) In an experiment, you purify a cysteine-charged tRNA and chemically alter the amino acid attached to it by converting it to alanine. You place this charged tRNA in a cell-free protein-synthesizing system with all of the other charged tRNAs, mRNA and other substances needed for the synthesis of proteins. Why might this have an effect on the final protein produced (be specific)?

(2) _____What would happen to a “housekeeping” receptor that is endocytosed from the plasma membrane? It would be brought to ((please explain why))

A. the early endosome, then late endosome, then moved back to plasma membrane.

B. the early endosome and immediately recycled back to the surface.

C. the early endosome, then late endosome, then lysosome to be degraded.

D. the lysosome directly to be degraded.

(3) _____Synthesis of which protein would be completed in the cytosol? (please explain why)

A. Transported in to nucleus, mitochondria, chloroplast or peroxisome

B. Secreted

C. Integral membrane

D. Endomembrane resident

Answer 1

1. The tRNA for cysteine is converted so that the amino acid attached is now alanine. Hence, wherever, cysteine codons are present on mRNA, the tRNA will bind. However, instead of inserting cysteine, the amino acid inserted would be alanine. Thus, there will be a replacement of cysteine with alanine in the final protein. Cysteine is required for disulphide bond formation in the tertiary structure of the protein. These disulphide bonds will no longer be formed as an alanine is present instead. Thus, tertiary structure is affected in the mature protein. Cysteine bonds may also be involved in catalysis as well in case of enzyme. Thus, enzymatic function may be affected.

2. The receptor is a housekeeping receptor. Hence, its levels do not alter in the cell. Hence, these receptors are not degraded in the cells. Once the receptor completes its action on the plasma membrane, it will be carried to the early endosome. In these endosome, processing of the receptor occurs, and the receptors gain its active configuration. The receptor is then recycled back to plasma membrane either directly from early endosomes or indirectly via recycling endosomes. VLDL receptors are housekeeping receptors. Other receptors/proteins that are not recycled will be sent to lysosome from late endosome (via transport vesicles) and degraded.

Right choice: B. the early endosome and immediately recycled back to the surface.

3. Proteins that are synthesized in cytoplasm are synthesized by free ribosomes. Soluble proteins are synthesized in cytoplasm. These proteins lack ER signal peptide and hence are synthesized in cytosol. Proteins that are targeted to nucleus, mitochondria, chloroplast or peroxisome lack ER N-terminal signal peptides and are hence are synthesized in cytosol. Proteins targeted to endosome, endomembrane, lysosomes, ER, Golgi, plasma membrane (integral membrane protein) and secreted proteins are synthesized in ER bound ribosomes. All these protein have ER signal peptide.

Right choice: A. Transported in to nucleus, mitochondria, chloroplast or peroxisome