4 Signals for compartmentalisation
We have noted how proteins for different destinations are packaged in transport vesicles, a process that depends on signal sequences in the proteins. In this section we shall look in a little more detail at the nature of the signal sequences. Except for the few proteins synthesised in the mitochondria or chloroplasts, cellular proteins are encoded by nuclear genes and synthesised on ribosomes in the cytosol or at the ER. Consequently, these proteins, if destined for organelles, must be sorted to the correct target membrane and translocated to the organelle. Sorting of proteins for their ultimate destinations occurs in conjunction with a variety of post-translational modifications. These modifications represent ‘recognition signatures’ for a variety of biological processes, including organelle targeting, subcellular anchoring and the formation of macromolecular complexes.
In the absence of a signal sequence, or if the signal sequence is removed by genetic modification, where is the protein likely to end up?
At the plasma membrane. Constitutive secretion is the default pathway (Section 2.4) and specific signals are needed to retain proteins in the ER and the Golgi or to direct them to the secretory pathways or lysosomes.