Read this article to learn about the protein targeting in bacteria and eukaryotes.
Proteins that must be targeted within the cell must be intercepted early during synthesis so that this can happen correctly. As a protein is being synthesized, decisions must be taken about sending it to the correct location in the cell where it will be required.
The information for doing this resides in the nascent protein sequence itself. Once the protein has reached its final destination, this information may be removed by proteolytic processing.
Targeting in Bacteria:
In bacterial cells, the targeting decision is relatively straightforward is the protein destined to be an intracellular protein or an extracellular one? However, even here gradations are possible. In a gram negative bacterium, such as E. coli, there must be some way of knowing whether a protein is destined to go to the cell membrane, the periplasmic space, or the outer membrane.
Secreted proteins contain a signal sequence. This is a short (6 – 30) stretch of hydrophobic amino acids, flanked on the N-terminal side by one or more positively charged amino acids such as lysine or arginine, and containing neutral amino acids with short side-chains (such as glycine or alanine) at the cleavage site.
As proteins with signal sequences are synthesized, they are bound by the SecB protein. This prevents the protein from folding. SecB delivers the protein to the cell membrane where is is secreted through a pore formed by the SecE and SecY proteins. Secretion is driven by the SecA ATPase. After the protein has been secreted, the signal sequence is removed by a membrane bound leader peptidase.
Targeting in Eukaryotes:
In eukaryotic cells, the situation is more complex. Extracellular proteins can be targeted for secretion or to the cell membrane, or to one of the many internal organelles such as the lysosome. Intracellular proteins can be targeted for the cyoplasm, to the nucleus or to special organelles such as the mitochondrion or the chloroplast. The Signal Sequence hypothesis was first enunciated by Gunther Blobel who was awarded the Nobel Prize in Medicine in 1999 for his work.
The following diagram summarizes the choices/fates available to newly synthesized proteins in a eukaryotic cell. Protein secretion in eukaryotic cells also involves a signal sequence. As in bacteria, the signal sequence is a short (6 – 30) stretch of hydrophobic amino acids, flanked on the N-terminal side by one or more positively charged amino acids such as lysine or arginine, and containing neutral amino acids with short side-chains (such as glycine or alanine) at the cleavage site.
Signal sequences are recognized by Signal Recognition Particles (SRPs) which are ribonucleoprotein particles containing a stable 305 nt 7S RNA and 6 polypeptides (Fig. 8.9). The 7S RNA has 2 domains the Alu domain (so called because it is related to the Alu family of repeating sequences) and the S domain. The 7S RNA has 3 activities – a signal sequence recognition activity, an SRP receptor binding activity, and an elongation arrest activity.
As proteins are being synthesized, the N-terminal signal sequence is bound by the SRP particle. Protein synthesis is temporarily halted (elongation arrest) until the particle with the nascent polypeptide and the ribosome -attaches to an SRP receptor complex in the endoplasmic reticulum membrane. The SRP receptor is a heterodimer formed by a larger 68 kD alpha subunit and a smaller 30 kD subunit.
The subunit is a GTP-binding protein. Attachment of the ribosome triggers the release of GDP and binding of GTP. The GTP form of the SRP receptor binds the ribosome very tightly. After the transfer takes place, GTP is hydrolyzed and the SRP receptor is free to interact with another SRP particle-ribosome complex. Protein synthesis then resumes but now the nascent polypeptide is secreted into the lumen of the ER as it is being synthesized.
Shortly after the polypeptide enters the ER, signal peptidase cleaves the signal peptide. The finished polypeptide is then targeted through the ER and Golgi apparatus to the cell surface or to the lysosome. During this process, the proteins are carried in coated vesicles which are enclosed either by a coat protein skeleton (ER→ Golgi) or by a clathrin skeleton (Golgi → exterior).
(i) Proteins that are targeted for organelles have their own N-terminal uptake-targeting sequence(s) that determines whether the protein must cross one or two membranes. In the former case, proteins destined for the inter-membrane space of the chloroplast or mitochondrion are first transported into the stroma (chloroplast) or matrix (mitochondrion) and then re-transported back through the inner chloroplast or mitochondrial membrane to the inter-membrane space.
(ii) Proteins that must be targeted to the nucleus have a nuclear localization signal (NLS). One common type of signal is a series of five or so closely spaced positively charged amino acids.
