Downstream sequences control the processing of the pestivirus Erns-E1 precursor
Like other enveloped viruses pestiviruses employ cellular proteases for processing of their structural proteins. While typical signal peptidase cleavage motifs are present at the carboxyterminus of the signal sequence preceding Erns, and the E1/E2 and E2/P7 sites, the Erns-E1 precursor is cleaved by signal peptidase at a highly unusual structure, in which the transmembrane sequence upstream of the cleavage site is replaced by an amphipathic helix. As shown before, the integrity of the amphipathic helix is crucial for efficient processing. The data presented here demonstrate that also the E1 sequence downstream of this cleavage site is important for the cleavage. Carboxyterminal truncation of the E1 moiety as well as internal deletions in E1 reduced the cleavage efficiency to less than 30% of the wt level. Moreover, the C-terminal truncation by more than 30 amino acids resulted in strong secretion of the uncleaved fusion proteins. The reduced processing and increased secretion was even observed when 10 to 5 aminoterminal residues or E1 were left whereas extensions by 1 or 3 E1 residues resulted in reduced processing but no significantly increased secretion. In contrast to the E1 sequences, a 10 amino acid c-myc tag fused to the Erns C-terminus had only marginal effect on secretion but was also not processed efficiently. Mutation of the von Heijne sequence upstream of E2 not only blocked the cleavage between E1 and E2 but also prevented the processing between Erns and E2. Thus, processing at the ErnsE1 site is a highly regulated process. IMPORTANCE Cellular signal peptidase (SPase) cleavage represents an important step in maturation of viral envelope proteins. Fine tuning of this system allows for establishment of concerted folding and processing processes in different enveloped viruses. We report here on SPase processing of the Erns-E1-E2 glycoprotein precursor of pestiviruses. Erns-E1 cleavage is delayed and only executed efficiently when the complete E1 sequence is present. C-terminal truncation of the Erns-E1 precursor impairs processing and leads to significant secretion of the protein. The latter is not detected when internal deletions preserving the E1 carboxyterminus are introduced, but also these constructs show impaired processing. Moreover, Erns-E1 is only processed after cleavage at the E1/E2 site. Thus, processing of the pestiviral glycoprotein precursor by SPase is done in an ordered way and depends on the integrity of the proteins for efficient cleavage. The functional importance of this processing scheme is discussed in the paper.