Question

7. In addition to nuclear import, where else are signal sequences utilized? Are the signal sequences on proteins targeted to these different compartments the same?

Answer 1

Signal peptides are short sequences present at the N-terminus end of many newly synthesized proteins and are designated to move towards specific organelle. They can be utilised for cell membrane channels, ER and other organelles as well. Thus, protein trafficking through signal sequences could be directed to other organelles besides nucleus.

Sequences targetted to other compartmens vary structurally and in use of energy. Nuclear Localization Signal has five basic positively charged amino acids including lysine and arginine.

Whereas, those targeted for mitochondria are usually unfolded and energy is required for transport.  However, some characteristics are common. In chlorplasts, the sequence containing proteins use membrane electric potential for transfer. For ER, ER retention signal is utlised which is present at C terminal. It has sequence as Lys-Asp-Glu-Leu.