Selection of Yeast Expression Vectors

I. What are some common yeast expression vectors?

Answer: Due to differences in post-translational modification mechanisms—such as glycosylation and acetylation—between prokaryotes and eukaryotes, it is difficult to obtain active proteins when expressing eukaryotic genes in prokaryotes; therefore, yeast has become the preferred system for eukaryotic expression. Before proceeding with yeast expression, we first need to understand yeast expression vectors.

    Most yeast expression vectors are derived from the pUC57 basic backbone. Common examples include pPIC9K, pPIC3.5K, as well as pGAPZ A, pGAPZ B, and pGAPZ C. All of these use the AOX1 promoter and are secretory expression vectors for Pichia pastoris. pPIC9K contains an additional alpha-secretory signal peptide compared to pPIC3.5K, resulting in a larger vector size. It is important to note that the gene inserted into pPIC3.5K must include the start codon ATG.

    Most yeast expression vectors are shuttle vectors, meaning they can replicate in E. coli to yield sufficient vector DNA for in vitro manipulation, and they can also replicate, express, and be selected in yeast. Their host strains are: GS115 (defective) and X33 (wild-type). The host strain for the pPIC9K and pPIC3.5K vectors is GS115, while the host strain for the pGAP series is wild-type X33.

II. What are the corresponding yeast expression systems for yeast expression vectors? 

III. How to Choose the Right Yeast Expression Vector?

    Yeast expression products can be expressed either intracellularly or secreted extracellularly, depending on the choice of expression vector and whether the construct contains a signal peptide. The appropriate yeast protein expression vector can be selected based on the purpose of gene expression.

    For intracellular expression, common vectors include the pGAP series and pPIC3.5K. The advantage of intracellular expression vectors is that the target gene is expressed within the cell, thereby avoiding yeast glycosylation. They are suitable for proteins that are typically expressed in the cytoplasm or for non-glycosylated proteins lacking disulfide bonds. Compared to extracellular secretion, intracellular expression offers higher expression levels but is relatively more complex to purify.

    For extracellular expression vectors, the pPIC9K vector is commonly used. The advantage of extracellular expression vectors is that they facilitate secretion of the protein outside the cell. The benefit of selecting this type of vector is that yeast naturally secretes very few exogenous proteins; secreting the exogenous protein outside the cell facilitates the purification and accumulation of the target protein.

    If the protein is inherently secreted, it can be expressed directly in Pichia pastoris. In this case, the exogenous gene is located on the yeast chromosome. By linearizing the constructed plasmid or vector (primarily through enzymatic digestion), it is integrated downstream of the AOX promoter via homologous recombination, typically inserted into the single-cloning site between the 5’ AOX promoter and the transcription termination signal.