Final answer:
Proteins are sorted to specific organelles in the cell based on signal sequences. Our predictions include retention in the cytosol for a mutated nuclear signal, ER for a protein with nuclear and ER retention signals, complex shuttling for coexisting mitochondrial import and nuclear export signals, and ER targeting for a protein with both an N-terminal ER signal and a mitochondrial signal.
Step-by-step explanation:
The transport and final destination of proteins within the cell depend on signal sequences that act as 'train tickets', directing proteins to specific compartments. The nature and combination of these signals determine the routing of each protein. Here are predictions for each of the engineered proteins mentioned:
Nuclear localization signal with three lysines changed to leucines: Mutations in the localization signal typically prevent the protein from entering the nucleus, so this protein may remain in the cytosol.
Nuclear localization signal and an ER retention signal (KDEL) on its C-terminus: The presence of both signals could lead to an intracellular conflict, with the nuclear signal directing the protein to the nucleus and the KDEL motif causing retention in the endoplasmic reticulum. It's likely that the ER retention signal will dominate, sequestering the protein in the ER.
A protein containing both a mitochondrial import signal and a nuclear export signal: The presence of opposing signals suggests this protein may shuttle between the mitochondria and the cytosol, but it is difficult to predict without further information on the relative strengths of these signals or the context of their action.
A protein with a mitochondrial signal sequence with an added N-terminal ER signal sequence: The N-terminal ER signal sequence should direct the protein to the endoplasmic reticulum, potentially overriding the mitochondrial signal.