2. Inclusion bodies are nuclear or cytoplasmic aggregates of stainable
substances, usually proteins.
They typically represent sites of viral multiplication in a bacterium or
a eukaryotic cell and usually consist of viral capsid proteins.
Inclusion bodies can also be hallmarks of genetic diseases, as in the
case of Neuronal Inclusion bodies in disorders like Frontotemporal
dementia and Parkinson's disease
Inclusion bodies
3. Inclusion bodies contain very little host protein,
ribosomal components or DNA/RNA fragments.
They often almost exclusively contain the over
expressed protein and aggregation in inclusion
bodies has been reported to be reversible.
It has been suggested that inclusion bodies are
dynamic structures formed by an unbalanced
equilibrium between aggregated and soluble
proteins
It is generally assumed that high level expression
of non-native
Protein (higher than 2% of cellular protein) and
highly hydrophobic protein is more prone to lead
to accumulation as inclusion bodies
Inclusions in neonatal hepatitis
4. Inclusion bodies have a non-unit lipid
membrane. Protein inclusion bodies are
classically thought to contain misfolded
protein.
However, this has recently been contested,
as green fluorescent protein will
sometimes fluoresce in inclusion bodies,
which indicates some resemblance of the
native structure and researchers have
recovered folded protein from inclusion
bodies.
Composition
5. When genes from one organism are expressed in another the
resulting protein sometimes forms inclusion bodies.
This is often true when it happens with in large evolutionary
distances
A cDNA isolated from Eukarya for example, risks the formation of the
inactive aggregates of protein known as inclusion bodies in bacteria.
The internal microenvironment of a prokaryotic cell (pH, osmolarity)
may differ from that of the original source of the gene. Mechanisms
for folding a protein may also be absent, and hydrophobic residues
that normally would remain buried may be exposed and available for
interaction with similar exposed sites on other ectopic proteins.
Processing systems for the cleavage and removal of internal abnormal
peptides from the cell may not properly works .
Mechanism of formation
6. Over expression is more likely to cause inclusion bodies
formation
The amino acid composition of recombinant proteins: the
more with sulfur amino acid, the easy to form
The expression environment of recombinant protein: over
high fermentation temperature
In eukaryotes, the enzymes of post-translational
modification are lacked, which results in the accumulation of
intermediates. So IBs are easy to form in cells
Formation of inclusion body
7. Dialysis, dilution and ultrafiltration - These three methods are
traditional and generally used in the protein refolding. The recovery
rate is very low and it is difficult to be separated from hybrid
proteins.
Dilution method is time-consuming and easy to form aggregates of
inactive proteins, not suitable for industrial production.
Gel filtration chromatography - Gel filtration chromatography is used
for refolding high concentration of protein. The recovery rate is
higher than dilution method and this method can also be applied to
purification of protein.
Add accelerant - The function of accelerant can stabilize the natural
structure of folding protein, increase the solubility of refolding
intermediate or increase the unfolded protein solubility.
Refolding Methods
8. LC (liquid chromatography) - Liquid chromatography is one of the effective
methods for purifying proteins, which becomes an essential tool
for recombinant protein purification, like ion exchange chromatography
(IEC), gel exclusion chromatography (SEC), affinity chromatography (AFC),
etc. Compared with the traditional dilution and dialysis method, liquid
chromatography has the below advantages:
Denaturing agent can be quickly removed.
The adsorption of denatured protein are obviously reduced, therefore
to avoid the generation of precipitation.
It can separate the protein of interest from hybrid proteins, and make
renaturation and purification at the same time.
9. The human genome:
25% of the human genes
encode for membrane
proteins
75%
25%
membrane proteins
soluble proteins
15. Fusion proteins (tagged proteins)
Translation fusion of sequences coding a recombinant protein and
a) short peptides [ex. (His)n, (Asp)n, (Arg)n ... ].
b) protein domains, entire proteins [ex. MBP, GST, thioredoxin …].
Engineering a tagged protein requires adding
the DNA encoding the tag to either the 5’ or 3’
end of the gene encoding the protein of interest
to generate a single, recombinant protein with a
tag at the N- or C-terminus. The stretch of
amino acids containing a target cleavage
sequence (CS) is included to allow selective
removal of the tag.
Expression plasmids containing various tags are commercially available.
16. Fusion partner (tag) Size Tag placement Uses
His-tag 6, 8, or 10 aa N- or C-terminus Purification, detection
Thioredoxin 109 aa (11.7 kDa) N- or C-terminus Purification, solubility enhancement
Calmodulin-binding domain
(CBD)
26 aa N- or C-terminus Purification
Avidin/streptavidin Strep-tag 8 aa N- or C-terminus Purification, secretion
Glutathione S-transferase (GST) 26 kDa N-terminus Purification, solubility enhancement
Maltose binding protein (MBP) 396 aa (40 kDa) N- or C-terminus Purification, solubility enhancement
Green fluorescent protein (GFP) 220 aa (27 kDa) N- or C-terminus Localization, detection, purification
Poly-Arg 5-16 aa N- or C-terminus Purification, solubility enhancement
N-utilization substance A (NusA) 495 aa (54.8 kDa) N-terminus Solubility enhancement
17. Increasing the yield of recombinant proteins – Fusion of the N-terminus of the target protein
to the C-terminus of a highly expressed fusion partner results in high level expression of the
target protein.
Enhancing the solubility of recombinant proteins – Fusion of the N-terminus of the target
protein to the C-terminus of a soluble fusion partner often improves the solubility of the target
protein.
Improving detection – Fusion of the target protein to either terminus of a short peptide
(epitope tag) or protein which is recognized by an antibody (Western blot analysis) or by
biophysical methods (e.g. GFP by fluorescence) facilitates the detection of the resulting protein
during expression or purification.
Localization – A tag, usually located on the N-terminus of the target protein, which acts as an
address for sending a protein to a specific cellular compartment.
Facilitating the purification of recombinant proteins – Simple purification schemes have
been developed for proteins used at either terminus which bind specifically to affinity resins.
Purposes of fusion tags
No single tag is ideally suited for all of these purposes.
20. Removal of fusion tags- the Achilles' heel of the fusion approach
All tags, whether small or large, have the potential to interfere with the biological
activity of a protein, impede its crystallization (presumably due to the conformational
heterogeneity allowed by the flexible linker region), be too large for NMR analysis,
cause a therapeutic protein to become immunogenic or otherwise influence the target
protein`s behavior.
The fusion tags can be removed by:
Chemical cleavage
Self - cleavage
Enzymatic cleavage
21. Rarely used.
Cyanogen bromide Met/X
Hydroxylamine Asn-Gly
Removal of fusion tags – chemical cleavage
Amino – acid sequence of the P. falciparum C-terminal segment of CSP
(PfCSP C-ter) fused to a purification tag (Rais-Beghdadi et al., 1998).
M28V
M105V
M12 M15
Chemical cleavage is a harsh method, efficient, but rather non-specific and may
lead to unnecesary denaturation or modification of the target protein.
22. SUMO gene fusion system
SUMO protease recognizes the tertiary structure of SUMO rather than an amino acid
sequence.
As a result, SUMO protease never cleaves within the fused protein of interest!
SUMO protease recognizes the tertiary
sequence of SUMO. As a result, SUMO
protease never cleaves within the fused
protein of interest.
SUMO protease recognizes the tertiary
sequence of SUMO. As a result, SUMO
protease never cleaves within the fused
protein of interest.
SUMO protease recognizes the tertiary
sequence of SUMO. As a result, SUMO
protease never cleaves within the fused
protein of interest.
SUMO protease recognizes the tertiary
sequence of SUMO. As a result, SUMO
protease never cleaves within the fused
protein of interest.
23. Optimization of protein cleavage conditions (mainly enzyme-to-substrate ratio, temperature, pH, salt
concentration, length of exposure).
Cleavage efficiency (Optimization is needed. The efficiency varies with each fusion protein in an unpredictable
manner, probably due to aggregation or steric issues; the problem can be solved by introducing short linkers
between the protease site and the fusion tag).
Unspecific cleavage (SOLUTION: optimization of protein cleavage conditions or using re-engineered proteases
with increased specificity such as ProTEV and AcTEV proteases).
Precipitation of the target protein when the fusion partner is removed (so-called soluble aggregates;
SOLUTION: another approach for protein solubilization has to be found).
Failure to recover active or structurally intact protein
Target protein modification (some proteases like thrombin, TEV, Precision leave one or two amino-acids on
the target protein near the cleavage site).
Re-purification step is needed to separate the protease from target protein.
High cost of proteases
The alternative is to leave the tag in place for structural analysis:
The small tags are a better choice in structural and
functional analysis of proteins.
Removal of fusion tags - enzymatic cleavage
24. Affinity tags and affinity purification
A tag is fused to the N- or C-terminus of the protein of interest to facilitate purification,
which relies on a specific interaction between the affinity tag and its immobilized binding
partner. Genetically engineered fusion tags allow the purification of virtually any protein
without any prior knowledge of its biochemical properties.
