A purified recombinant human rhinovirus-14 3C protease preparation contained only ϳ50% active enzyme as titrated using specifically designed irreversible 3C protease inhibitors. Analysis of the purified 3C protein by isoelectric focusing showed differently charged 3C isoforms that had isoelectric points (pI) of 8.3 (55%) and 9.0 (45%), with the latter one being consistent with the predicted pI of the human rhinovirus-14 3C protein. Further analysis indicated that the pI 8.3 protein was the deamidated form of 3C, and it displayed ϳ10-fold reduced cleavage activity relative to the original 3C protease sample. Peptide mapping followed by sequence analysis revealed that a single asparagine, Asn-164, was deamidated to aspartic acid in the pI 8.3 isoform. Converting Asn-164 to Asp by site-directed mutagenesis resulted in a mutated 3C protease with extremely low activity, as seen with the pI 8.3 isoform, indicating a role of Asn-164 in substrate recognition and binding. In addition, the deamidated 3C protease was found to be present in vivo, and its abundance was related to the viral replication cycle. Moreover, mutant virus carrying Asp-164 showed reduced viability in infected cells. Taken together, our data suggest that 3C protein deamidation plays a role in the regulation of its enzymatic activity.
Human rhinovirus (HRV)1 infections are considered to be the most frequent causative agents of the common cold and various other upper respiratory tract infections (1). Rhinoviruses are members of the picornavirus family, which also includes the apthoviruses (foot-and-mouth disease virus), cardioviruses (encephalomyocarditis virus), enteroviruses (poliovirus and Coxsackie virus), and hepatitis A virus. All picornaviruses have a positive-sense, single-stranded RNA genome that is translated into a single polyprotein precursor. In the case of HRVs, the viral polyprotein is mainly processed by the viral proteases 2A and 3C to generate functional proteins and enzymes (2, 3). The 2A protease catalyzes the first cleavage between the structural and nonstructural proteins, whereas the 3C protease catalyzes most of the subsequent internal cleavages (2, 3).The availability of active recombinant 3C protease greatly facilitated its biochemical characterization. Purified recombinant viral 3C protease was able to cleave different proteins and peptides at the bond formed between glutamine and glycine (4, 5). It has been found that 3C protease could regulate host cell function by cleaving important cellular proteins during infection (6, 7). In addition to its proteolytic activity, viral 3C protease has been shown to be a RNA-binding protein and may be involved in formation of the viral replication complex (8). As illustrated by its crystal structure, HRV 3C protease represents a novel class of cysteine protease that contains a cysteine as the active site nucleophile but is structurally like a serine protease (9 -11). It has been considered to be an ideal target for antiviral intervention due to its essential role in viral replication and i...