Identification of the largest non-essential regions of the C-terminal portion in 3A protein of foot-and-mouth disease virus for replication in cell culture

Background Recent study has shown that the C-terminal portion of 3A (amino acids (aa) 81–153) is not essential for foot-and-mouth disease virus replication in cell culture, however, the complete C-terminal portion (aa 77–153) of 3A is highly variable and prone to occur deletions and mutations, therefore, we presume that this region plays a very limited role and probablely is completely nonessential for virus viability. Methods In this study, to identify the largest non-essential region of the C-terminal portion in 3A for FMDV viability, several deletions containing aa 80–153, 77–153 and 76–153 of 3A protein were introduced into an FMDV full-length infectious cDNA clone pOFS by the overlapping extension PCR. Additionally, to explore the importance of the highly conserved residue 76 L of 3A for the FMDV of Cathay topotype, two mutants containing 3A L76I and 3A L76V were generated based on the 3A deletion mutant by point mutation. We also introduced the enhanced green fluorescent protein (eGFP) into one of the 3A deletion mutants by the extension PCR to investigate the genetic flexibility of 3A to express foreign genes. All linearized full plasmids were transfected into BSR/T7 cells to rescue infectious foot-and-mouth disease viruses. The rescused viruses were analyzed by RT-PCR, nucleotide sequencing, immunofluorescence assay and western blot and were characterized by plaque assays and one-step growth kinetics. Results The results demonstrated that the deletion of aa 80–153 and aa 77–153 and the substitutions of 3A L76I and 3A L76V did not affect the production of infectious virus, while the fusion of the eGFP gene to the C-terminus of 3A resulted in nonviable FMDV. Conclusions Our results firstly reported that the aa 77–153 rather than aa 81–153 of 3A protein was dispensable for FMDV replication in cell culture. This study is of great significance for development of FMD marker vaccine and foreign gene expression in the future.