Before we see how some protein’s synthesis is directed to the ER, let’s look at an
overview of protein targeting.
Protein targeting refers to getting a protein where it needs to be in the cell.
The synthesis of all nuclear- encoded proteins START on free ribosomes in the
cytoplasm. Figure 12.15, page 272.
Once translation begins the amino acids of the protein that is being made begin to emerge
from the ribosome. Now a decision as to be made about where to COMPLETE the
synthesis of proteins.
There are two categories of proteins. Some proteins are completed on free ribosomes, and
(2) some proteins have their protein synthesis completed on ribosomes attached to the
How is the decision made? The information is given at the beginning of the newly
synthesized polypeptide, and is based on the amino acid sequence.
Proteins without a signal peptide complete translation on free ribosomes.
Figure 12.15, page 272, left side.
The type of proteins that do not have signal peptides are proteins that are destined for the
double membrane organelles (nucleus, chloroplast, or mitochondria) or proteins of the
cytosol, or to an organelle known as a peroxisome. All of these types of proteins have
their translation begun and completed on free ribosomes (ribosomes in the cytosol).
Once such polypeptides are created, then the sequence of certain amino acids within the
polypeptide directs them to their proper organelle and they move there.
For example nuclear proteins have a nuclear target sequence that allows them to get
through the nuclear pores, and into the nucleus.
Mitochondrial proteins have a localization signal that gets them into the mitochondria,
and plastid proteins have a localization signal that gets them into the plastid.
If a polypeptide is made on a free ribosome and has neither a nuclear, mitochondrial,
peroxisomal or plastid localization signal it will end up in the liquid portion of the
cytoplasm (the liquid part of the cytoplasm is known as the cytosol).
Now let’s look at proteins that complete translation on ribosomes attached to the
Many proteins have their translation START on free ribosomes but their translation will
be COMPLETED on ribosomes that are attached to the endoplasmic reticulum.
These are proteins that have a sequence near the amino terminus of the polypeptide
known as the signal peptide. Figure 12.15, page 272 right side.