1. What is Recombinant Protein Expression?

The exploration of life activities can be conducted at different levels, such as genes and proteins. With the continuous advancement of science and technology, protein research has evolved from simple protein extraction to recombinant protein expression. Currently, we can utilize various recombinant protein expression systems—including prokaryotic, yeast, insect, mammalian, and plant expression systems—to achieve recombinant expression and purification of target proteins. This further facilitates the study of protein functions, providing a convenient approach for scientific research.

2. Introduction to Several Types of Recombinant Expression

Based on the secretion location of protein expression, protein expression forms can be categorized into intracellular expression and secretory expression, with intracellular expression being the predominant form. Depending on the product form, protein expression can be classified as soluble expression or insoluble expression (inclusion body expression). Additionally, based on the expression method, target protein expression can be divided into fusion expression and non-fusion expression.

（1） Fusion Expression

Fusion expression refers to a method in which the gene sequence of the target protein is constructed together with another protein tag sequence in an expression vector, allowing them to be co-expressed under the control of the same regulatory elements. This approach has the following characteristics: Fusion expression can enhance the expression efficiency of the target protein, facilitate proper protein folding, promote soluble expression, and improve protein stability. Additionally, protein tags such as His-tags simplify purification, and the recombinant protein can be cleaved to remove the tag in vitro after purification.

（2） Non-Fusion Expression

In contrast to fusion expression, non-fusion expression means that the target protein is expressed without any fusion tags. However, proteins expressed this way are often degraded by host proteases and may fail to fold correctly, resulting in loss of biological activity.

（3） Intracellular Expression

Most target proteins are primarily expressed through intracellular expression. However, this expression method can sometimes affect protein structure. For example, in prokaryotic expression systems, intracellularly expressed proteins often form inclusion bodies due to excessively rapid expression rates that prevent proper folding and post-translational modifications, resulting in loss of protein activity. In yeast expression systems, intracellularly expressed proteins typically show lower expression levels and are prone to hyperglycosylation. Therefore, during protein expression, we can utilize target protein sequences or secretion signal peptides to facilitate secretory expression of recombinant proteins.

（4） Secretory Expression

The expression form where recombinant proteins are secreted outside the cells is called secretory expression. This expression method significantly simplifies protein purification and produces more stable proteins that are less susceptible to degradation.

（5） Soluble Expression

Since only a minority of proteins can be expressed through secretory pathways while most rely on intracellular expression, several approaches can be employed to achieve soluble protein expression: fusion expression strategies, inclusion body refolding techniques, and reduction of protein expression rates.

（6）Insoluble Expression (Inclusion Body Expression)

Insoluble expression refers to the phenomenon where proteins form solid granular precipitates, known as inclusion bodies, due to their inability to fold correctly during high-level expression in the host. Therefore, insoluble expression is also termed inclusion body expression. Inclusion bodies typically lack biological activity because they do not adopt the correct structure, though their amino acid sequence remains accurate. The formation of inclusion bodies is primarily influenced by the properties of the protein itself, culture conditions, and molecular chaperones.

To address inclusion body formation, common strategies include adding protein tags for fusion/secretory expression, optimizing induction conditions (e.g., lowering culture temperature, selecting appropriate promoters or inducers), reducing protein expression rates, and inclusion body refolding.

KMD Bioscience has long been dedicated to recombinant protein expression and purification. To meet diverse protein expression needs, we have established multiple expression platforms, including prokaryotic, yeast, insect, mammalian, and plant expression systems. For protein expression and purification, we offer the following services and solutions.

1. Provide different expression systems based on the properties of the protein.

2. Soluble Protein Expression

Based on the analysis of the protein's properties, we can recommend an optimal expression strategy and system.  We offer various protein tags, such as GST, His, SUMO, etc., to facilitate soluble expression.  Additionally, we have independently developed and optimized a variety of vectors to enable rapid protein expression.  Our company also has a mature protein purification platform capable of purifying proteins through multiple methods, as well as removing protein tags from recombinant proteins in vitro.  Furthermore, we possess unique inclusion body refolding technology to achieve successful renaturation of inclusion bodies.

3. Multiple Protein Expression Systems,We provide protein expression systems in various specifications to meet the diverse needs of both research and industrial users.

4. Strict Quality Control Standards：Purified proteins undergo rigorous quality testing, including SDS-PAGE and Western blot (WB) analysis.

This article is for reference by scientific research enthusiasts. It cannot replace professional knowledge or practical experimental procedures that require more detailed and specialized information. If there is any infringement of content, please contact the author immediately to remove the disputed material.