Question

Which of the following statements describing the import of proteins into the nucleus is correct? Answers A-D A Transport into the nucleus requires the activity of a G protein. B The receptor for nuclear proteins is found in the cytosol and travels into the nucleus with the protein being transported o С Proteins destined for the nucleus are transported in a fully-folded conformation. D These statements are all correct. o

Answer 1

Nuclear transport: D. All of the statements are correct. Nuclear proteins have a target sequence called Nuclear Localization Signal (NLS) which is bound by certain cytoplasmic proteins called importins. Importin bound with target protein crosses the nuclear membrane and binds to a G protein called Ran inside the nucleus. In the nucleus the Ran is bound to GTP, where binding of importin causes hydrolysis of GTP to GDP. The protein, already in its fully folded conformation, gets transported to the nucleus in this way.

ER lumen protein False statement: C. the protein contains a hydrophobic stop-transfer sequence in addition to its N-terminal signal sequence. Such a stop transfer sequence is found in transmembrane proteins whereas the given protein resides in the ER lumen. So it cannot contain a stop-transfer sequence. The other statements are true as translation begins by a ribosome in the cytoplasm after which the ER signal sequence is generated. This sequence in bound by SRP that brings the ribosome-peptide complex to the ER membrane and begis transfer of the nascent peptide. Once inside the ER, the N-terminal signal sequence is cleaved from the peptide.

Electrochemical gradient force: 1. Chemical Force (difference of concentration of the ion across the membrane)

2. Electrical force (the effect of the membrane potential on the ions' movement)