Expression and characterization of infectious bursal disease virus polyprotein in yeast.

Various expression vectors containing a cDNA fragment encoding all but the first five amino acids (aa) of the large polyprotein (N-VP2-VP4-VP3-C) of infectious bursal disease virus were transformed into yeasts. In both Saccharomyces cerevisiae and Schizosaccharomyces pombe, co- or post-translational processing of the unfused large polyprotein occurred, generating a stable C-terminal product (VP3) or correct size, but without any detectable N-terminal product (VP2). Furthermore, when the processing of the polyprotein was interrupted, because of an engineered in-frame site-specific insertion of 4 aa, even VP3 (as part of the unprocessed polyprotein) was undetected. VP2 was detected in S. cerevisiae only when fused to yeast pre-sequences at the N terminus, suggesting that in yeast, VP2 or the unprocessed polyprotein, in the absence of its native N terminus or proper protection of its N-terminal aa residues is susceptible to proteolytic degradation. The first 8 aa of a modified pre-sequence of the CUP1 gene product and the pre-pro sequence of MF alpha 1 gene product have been used for stable intra- and extra-cellular production of VP2, respectively.