The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 17

1.4.1 Protein Translocation
As evident from earlier discussions, proteins and enzymes are essential to every met-
abolic, synthetic, structural, or regulatory functioning in the cell. They are synthe-
sized on the ribosomes but are present at all nooks and corners of the cell. This is
possible because of the existence of a system for protein translocation. Protein tar-
geting to various cellular destinations is made possible by the presence of a short
sequence of amino acids called a signal sequence. The signal sequence may or may
not be cleaved, and if cleaved, it is cleaved either during transport or after reaching
the target organelles/location. The proteins that need to be translocated to ER,
mitochondria, or chloroplasts have an amino terminus-attached signal sequence.
The signal sequences are 1-6 amino acids long, with around half of the amino
acids in the chain being hydrophobic. The protein synthesis initiates on the free
cytosolic ribosome, and the signal sequence is synthesized initially. The large sig-
nal recognition particle (SRP) binds with the ribosome and directs the ribosome
bound to mRNA to the SRP receptor on the ER. The length of the polypeptide
is around 0 amino acids at this stage. The polypeptides interact with the pep-
tide translocation complex at ER and release the SRP. The polypeptide synthesis
resumes again, and the growing chain is directed inside the ER lumen until com-
plete synthesis. The ribosome becomes dissociated, the signal sequence attached to
the polypeptide is cleaved, and the nascent protein achieves its three-dimensional
conformation. The protein so formed may be further modified by glycosylation and
post-translational modifications and is packaged into transport vesicles and moved
to the Golgi complex, where it undergoes further sorting. Similarly, the proteins
intended for mitochondria are also targeted using specific amino acid sequences.
But the process differs because the complete synthesis of protein takes place on
the ribosomal assembly, and once released, they become associated with chaperone
proteins that deliver the protein to the mitochondrial membrane. The proteins are
then internalized using specific mechanisms not yet fully understood, and subse-
quently the signal sequences are removed. The nuclear targeting of proteins fol-
lows yet another process in which the nuclear localization sequence (NLS) targets
the protein to nucleus. The peculiarity of NLS is that it may be present anywhere in
the polypeptide sequence and is not cleaved at destination or post-polypeptide syn-
thesis. The process is mediated by proteins called importins and a GTPase known
as Ran. Similar to eukaryotes, bacteria also target their proteins to either the cell
membrane or extracellular matrix through signal sequences at the amino end. In
eukaryotes, apart from intracellular localization of synthesized proteins sometimes
the proteins are imported from extracellular media. These proteins bind to special
areas called coated pits. The coated pits are the areas where there is a high pres-
ence of endocytic receptors. The cytosolic side of the pits is lined with clathrin
protein, which helps the membrane to invaginate, and an endocytic vesicle con-
taining proteins is formed. The clathrin then becomes dissociated from the vesicle,
and the endocytic vesicle fuses with endosome to ultimately release the imported
protein [51] .