Enhancing Yeast Surface Display: UPR, ERAD, and ER Dynamics in Recombinant Protein Production

Over the past two decades, the display of various recombinant proteins on the surfaces of microorganisms, particularly yeast, has garnered significant research attention. This method is rapid, simple and cost-effective, combining the biosynthesis and secretion of recombinant proteins with their immobilization on the host cell surface. Proteins synthesized using this technique are transported to the cell surface and incorporated into the cell wall through mild, native processes, avoiding aggressive chemical immobilization methods that often lead to a loss of physiological activity. Surface-displayed proteins are generally more stable and resistant to environmental changes than those in a solution. Depending on the promoter used, cells can continuously renew the recombinant protein on their surface or express it only under certain conditions. Additionally, cells carrying surface-displayed enzymes can be easily separated from the reaction mixture and reused multiple times. These enzymes can also catalyze reactions with substrates that cannot enter the cells, facilitating extracellular synthesis and simplifying product purification. However, the main obstacle to the industrial application of this method is often low efficiency, resulting in limited amounts of displayed protein. The efficiency depends on the processes that the protein undergoes on its way to the cell surface, following the same pathway as native secretory proteins: synthesis in the endoplasmic reticulum (ER), transport to the Golgi, and delivery to the cell surface via transport vesicles. Large amounts of secretory proteins can overload the ER, triggering the unfolded protein response (UPR) and endoplasmic reticulum-associated degradation (ERAD). Despite significant improvements for some proteins, a universal system for all recombinant proteins has yet to be developed. However, the complexity of protein processing and secretion pathways suggests that a single system improving productivity for all recombinant proteins is unlikely. Instead, several optimized systems tailored to specific protein structures may be necessary. This article provides an overview of the processes that recombinant proteins intended for surface display undergo on their way to the cell surface in the endoplasmic reticulum and represent a crucial bottleneck for the successful immobilization of recombinant proteins at the cell surface.