Question

hich of the following organelles are the destination for proteins synthetized by cytosolic ribosomes Peroxisomes, endoplasmic reticulum, and the Golgi apparatus The Golgi apparatus, lysosomes, and the nucleus Peroxisomes, mitochondria, and lysosomes Peroxisomes, mitochondria, and the nucleus Mitochondria, lysosomes, and the nucleus

Answer 1

1. Peroxisomes, mitochondria and nucleus

The proteins produced by translation of cytosolic mRNA has signal sequences which directs them to the particular organelles. The proteins to nucleus has soert stretch of lysine and arginine rich amioacids called nuclear localisatiion sequence which takes them through the nuclear pore . The proteins to mitochondria whicha are needed for electron transport chain and matrix have characteristic signal sequence . Proteins to peroxisomes have signal sequence called peroxisome target signal.

2.It is cleaved and released into the membrane.

The N-terminal polypeptide chain directed to the ER( endoplasmic reticulum) has a signal sequence of 20 hydrophobic amino acids in its N terminal, which are recognised by the SRP (signal recognition proteins) near the ER membrane. They bind with this signal sequence and attaches to the SRP receptor. The end of signal sequence move to the translocon channel of ER by the hydrolysis of signal sequence-SRP proteins complex and further translation of the polypeptide chain by Ribosomes continues in traslocon. The signal peptide will interact with the hydrophobic aminoacids in the side of translocon and the signal peptidases in he ER cleves it away fron the rest of polypeptide chain. the result id the signal peptide remain in ER membrane and the newly synthesised polypeptide chain moves into the ER lumen.

3. the proteins are delivered directly from cytosol to the nucleus

The polypeptides formed by the translation by the ribosomes in the cytosol have special sequence of 7–41 amino acids of which the basic amino acids lysine and arginine called nuclear localization sequence. They will be targeted to the nuclear pore where they are recognised by nuclear transport receptors and enter into the nucleus by active transport in their folded conformation.