Characterization of Vector Elements and Soluble Expression of <i>Pcv3</i> Cap Protein in <i>Kluyveromyces Marxianus</i>

The yeast Kluyveromyces marxianus (K. marxianus), characterized by its thermotolerance and rapid growth, is emerging as a promising new platform organism for the production of recombinant proteins. In this study, we constructed an expression vector designed for the efficient expression of exogenous proteins in K. marxianus. Initially, qPCR was employed to assess the expression efficiency of endogenous promoters within the yeast. The PDC1 promoter was selected, and its ability to drive the expression of EGFP was validated. The constructed vector exhibited high stability, maintaining approximately 5.2-fold higher copy numbers than the K. marxianus genome after 72 hours of cultivation without hygromycin selection. Notably, the fluorescence signal intensity of K. marxianus harboring the vector was approximately 15.6-fold higher than that of the wild-type strain at 72 h. Subsequently, the cap gene of porcine circovirus type 3 (PCV3) was integrated into the vector, resulting in the production of soluble PCV3 cap protein. Electron microscopy analysis revealed that the PCV3 cap protein self-assembled into virus-like particles (VLPs). This study successfully established the expression vector and characterized its key elements in K. marxianus, which will facilitate further research on the expression of exogenous proteins in this yeast species. Moreover, the soluble expression of the PCV3 cap protein and its formation of VLPs provide a solid foundation for the future development of PCV3 vaccines.