Gene 1 of the murine coronavirus, MHV-A59, encodes approximately 800 kDa of protein products within two overlapping open reading frames (ORFs 1a and 1b). The gene is expressed as a polyprotein that is processed into individual proteins, presumably by virus-encoded proteinases. ORF 1a has been predicted to encode proteins with similarity to viral and cellular proteinases, such as papain, and to the 3C proteinases of the picornaviruses (A. E.

The coronavirus mouse hepatitis virus-A59 (MHV-A59) encodes a serine-like proteinase (3C-like proteinase or 3CLpro) in ORF 1a of gene 1 between nucleotides 10,209 and 11,114. We previously have demonstrated that proteins expressed in vitro from a cDNA clone of the 3CLpro region possess proteinase activity, and that the proteinase is able to cleave substrate in trans. We sought to determine if the 27-kDa in vitro cleavage product (p27) was an active form of the 3CLpro and whether this was consistent with the 3CLpro expressed in virus-infected cells. Antibodies directed against the 3CLpro domain detected 27-kDa MHV proteins in vitro and in MHV-A59-infected cells. The 27-kDa proteins were able to cleave substrate in trans without other protein cofactors or supplemental membranes, and the p27 proteinase activity was retained after purification by immunoprecipitation and gel electrophoresis. When p27 was expressed in vitro with portions of the amino-and carboxy-terminal flanking domains (MP1 and MP2), p27 was not liberated by cls cleavage. The proteolytic activity of the 27-kDa proteins was inhibited by a variety of cysteine and serine proteinase inhibitors, and was eliminated by the cysteine proteinase inhibitor E64d. These results indicate that the 27-kDa protein is a mature proteinase in MHV-A59-infected cells, and that appropriate processing of this molecule occurs in vitro.

Signal peptidase removes amino-terminal signal peptides from precursor proteins during or immediately following their translocation to the lumen of the endoplasmic reticulum (ER) and may participate in ER degradation, a poorly defined process whereby abnormal proteins are rapidly degraded early in the secretory pathway. Here, the involvement of signal peptidase in ER degradation is examined through the use of two chimeric membrane proteins that lack amino-terminal signal peptides: A189invHD, which contains sequences derived from arginine permease and histidinol dehydrogenase, and AHDK2, containing the ER-resident protein Kar2p fused to the carboxyl terminus of A189invHD. Degradation of approximately 95% of A189invHD is observed in yeast cells expressing enzymatically active signal peptidase, whereas only 60% undergoes rapid degradation in a sec11 mutant bearing a temperature-sensitive mutation in the gene encoding the 18-kDa subunit (Sec11p) of the signal peptidase complex. AHDK2 is proteolyzed in a reaction yielding at least two fragments in wild-type cells and in the sec11 mutant containing a plasmid bearing the SEC11 gene. The proteolytic reaction is catalyzed in a temperature-dependent manner in the sec11 mutant, with AHDK2 remaining stable at the nonpermissive temperature. Using conditional mutants defective in protein translocation into and out of the ER and in vitro protease protection studies, the site of degradation for AHDK2 is localized to the ER lumen. The data therefore indicate (i) A189invHD is degraded through both signal peptidase-dependent and independent processes; (ii) signal peptidase, specifically the Sec11p subunit, is required for the proteolysis of AHDK2; and (iii) the Kar2 fragment at the carboxyl terminus of AHDK2 permits detection of proteolytic intermediates.

The coronavirus, mouse hepatitis virus strain A59 (MHV), expresses a chymotrypsin-like cysteine proteinase (3CLpro) within the gene 1 polyprotein. The MHV 3CLpro is similar to the picornavirus 3C proteinases in the relative location of confirmed catalytic histidine and cysteine residues and in the predicted use of Q/(S, A, G) dipeptide cleavage sites. However, less is known concerning the participation of aspartic acid or glutamic acid residues in catalysis by the coronavirus 3C-like proteinases or of the precise coding sequence of 3CLpro within the gene 1 polyprotein. In this study, aspartic acid residues in MHV 3CLpro were mutated and the mutant proteinases were tested for activity in an in vitro trans cleavage assay. MHV 3CLpro was not inactivated by substitutions at Asp3386 (D53) or Asp3398 (D65), demonstrating that they were not catalytic residues. MHV 3CLpro was able to cleave at a glutamine-glycine (QG3607-8) dipeptide within the 3CLpro domain upstream from the predicted carboxy-terminal QS3636-6 cleavage site of 3CLpro. The predicted full-length 3CLpro (S3334 to Q3635) had an apparent mass of 27 kDa, identical to the p27 3CLpro in cells, whereas the truncated proteinase (S3334 to Q3607) had an apparent mass of 24 kDa. This 28-amino-acid carboxy-terminal truncation of 3CLpro rendered it inactive in a trans cleavage assay. Thus, MHV 3CLpro was able to cleave at a site within the putative full-length proteinase, but the entire predicted 3CLpro domain was required for activity. These studies suggest that the coronavirus 3CL-proteinases may have a substantially different structure and catalytic mechanism that other 3C-like proteinases.

Inflammatory bowel disease (IBD) has long been a worldwide health care problem with a persistently increasing incidence. Although its clinical features have been well described, its etiology and pathogenesis remain unclear. IL-16 is a chemoattractant cytokine with various effects on cellular activities and diseases. However, the involvement of IL-16 in IBD remains poorly understood. In this study, to our knowledge we report for the first time the mechanism by which IL-16 induces intestinal inflammation by upregulating the expression of oligopeptide transporter member 1 (PepT1) in a Tetraodon nigroviridis fish model. The dextran sodium sulfate–induced colitis model in this species revealed that IL-16 levels significantly increase accompanied by elevations in PepT1 in the colon. Moreover, the signs of colitis were dramatically attenuated by IL-16 depletion using anti–IL-16 Abs. In vivo IL-16 administration induced remarkable intestinal inflammation with typical ulcerative colitis–like features, including histologic damage, inflammatory cell infiltration, increased myeloperoxidase activity, and proinflammatory cytokines expression, which corresponded with significant PepT1 upregulation in the colon. The IL-16–induced PepT1 expression and its upregulated fMLF transport were also demonstrated in vitro. To our knowledge, our study provides the first evidence of the connection between IL-16 and PepT1, which provides new insights into the molecular mechanism underlying IBD development. Additionally, this study suggests that fish species are an attractive model for studying IBD. By providing a better understanding of IL-16 biology from fish to mammals, this study should aid the development of IL-16–based therapies for IBD.