An Engineered SARS-CoV-2 S1 Glycoprotein Produced in Pichia pastoris as a Candidate Vaccine Antigen.

Since December 2019, the SARS-CoV-2 virus has caused the global COVID-19 pandemic. Antiviral and anti-inflammatory treatments have had limited success, positioning vaccine development as a key strategy for public health. This study constructed a chimeric S1 protein fused to a human Fc domain using the Pichia pastoris expression system. Yeast expression system was selected for its low-cost and relatively easier process comparing mammalian and insect. In addition, two human commercial vaccines including human Hepatitis B virus and human papilloma virus are produced currently in yeast system. The chimeric protein named S1Fc was codon-optimized and expressed via the pPICZaA vector as pPICZaA-S1Fc construct. This construct consists of 918 amino acids: 673 amino acids of the S1 protein (N-terminal) linked to 227 amino acids from the human IgG1 Fc region (C-terminal) via 18 amino acids linker. Two yeast strains, a standard glycosylating strain and a mammalian-like GlycoSwitch strain, were selected for expression. SDS-PAGE and western blot analyses indicated successful S1Fc expression in both strains, with a molecular weight of approximately 130 kDa. The GlycoSwitch strain demonstrated enhanced antigenicity in ELISA, indicating a glycosylation pattern more similar to the native viral S1 protein. Purification was achieved using a protein G chromatography column, yielding 14.6 µg/ml in the GlycoSwitch strain and 18.9 µg/ml in the standard strain. These findings highlight the Pichia pastoris expression system as a cost-effective platform for S1Fc protein production, meriting further study as a potential vaccine antigen.