Sample Answer
Overexpression, Isolation and Functional Verification of the Q-Protein Using a T7-Based Expression System
Introduction
Recombinant protein expression is a core technique in molecular biology and biotechnology, particularly for producing sufficient quantities of protein for biochemical characterisation. In this coursework, the Q gene has been cloned into the pETSHU plasmid under the control of a T7 promoter. The translated Q-protein is 183 amino acids long, with a molecular weight of approximately 21.5 kDa and a theoretical isoelectric point close to 6.06. These characteristics strongly influence the design of the expression, lysis, purification and functional validation strategy. This section outlines how expression of the Q-protein will be induced, how the cytosolic fraction will be obtained, how the protein will be purified to a yield greater than 80 percent and more than 30 mg, and finally how biological activity will be confirmed.
Induction of Q-Protein Expression in the T7 System
The pETSHU plasmid relies on a T7 promoter, meaning expression will be carried out in an Escherichia coli strain that encodes T7 RNA polymerase, such as BL21(DE3). In this system, the gene for T7 RNA polymerase is integrated into the bacterial genome under the control of the lacUV5 promoter.
Protein expression will be induced by the addition of isopropyl β-D-1-thiogalactopyranoside (IPTG) to a mid-log phase culture, typically at an optical density at 600 nm of around 0.6. IPTG acts as a lactose analogue and binds to the lac repressor, relieving repression of the lacUV5 promoter. This allows transcription of the T7 RNA polymerase gene. Once produced, T7 RNA polymerase specifically recognises the T7 promoter on the pETSHU plasmid and drives high-level transcription of the Q gene.
To minimise the risk of inclusion body formation and to promote correct folding of the Q-protein, induction would be performed at a reduced temperature, such as 20 to 25 °C, with a moderate IPTG concentration around 0.1 to 0.5 mM. Cells would then be harvested by centrifugation after several hours of induction.
Lysis and Collection of the Cytosolic Fraction
Following expression, the next step is to lyse the bacterial cells while preserving the solubility and integrity of the Q-protein. Since the target protein is expected to be cytosolic, the goal is to gently disrupt the cells and remove insoluble debris.
Cell pellets will be resuspended in a carefully designed lysis buffer. A suitable buffer would contain 50 mM Tris-HCl at pH 7.5 to provide stable buffering above the protein’s pI, 150 mM NaCl to maintain ionic strength and protein solubility, and 1 mM EDTA to chelate divalent metal ions that could activate proteases. A reducing agent such as 1 mM dithiothreitol would be included to prevent disulfide bond formation that could lead to aggregation. A commercial protease inhibitor cocktail would also be added to limit proteolytic degradation during lysis.
Cell disruption would be achieved using sonication on ice, applying short pulses to prevent excessive heating. Alternatively, a French press could be used for larger-scale preparations. The lysate would then be centrifuged at high speed to pellet cell debris, membranes and any insoluble material. The supernatant obtained after centrifugation represents the cytosolic fraction and contains the soluble Q-protein ready for purification.
Purification Strategy for the Q-Protein
The Q-protein has a molecular weight of approximately 21.5 kDa and a theoretical pI of 6.06. Purification will be carried out at pH 7.5, meaning the protein will carry an overall negative charge. Information from the simulated two-dimensional gel indicates that the protein is well resolved from many contaminants, allowing a rational multi-step purification strategy. In accordance with good purification practice, no single method will be repeated.
At pH 7.5, the Q-protein will be negatively charged.
Stage 1
Method
Anion exchange chromatography using a DEAE or Q-type resin.
Mechanism
Anion exchange chromatography separates proteins based on their net negative charge. The positively charged resin binds negatively charged proteins through electrostatic interactions. Bound proteins are then eluted by increasing the salt concentration, which shields these electrostatic interactions and causes proteins to dissociate from the resin at different ionic strengths depending on their charge density.
Rationale
Since the Q-protein has a pI of approximately 6.06, it will be negatively charged at pH 7.5 and will bind effectively to an anion exchange column. Many contaminating proteins with higher pI values will either not bind or elute at lower salt concentrations. This step provides strong initial enrichment and removes a large proportion of host cell proteins while maintaining high yield.
Stage 2
Method
Size exclusion chromatography.
Mechanism
Size exclusion chromatography separates proteins based on their hydrodynamic radius rather than charge or binding affinity. The column matrix contains pores of defined sizes. Larger proteins are excluded from the pores and elute earlier, while smaller proteins enter the pores and elute later.
Rationale
With a molecular weight of around 21.5 kDa, the Q-protein will elute at a predictable position distinct from both smaller peptides and larger contaminating proteins. This step also acts as a polishing step that removes aggregates and ensures the protein is monodisperse, which is important for downstream activity assays. Importantly, this method does not rely on binding, reducing stress on the protein and helping to preserve activity.
