Calpains mediate the proteolytic modification of human cytomegalovirus UL112-113 proteins

The human cytomegalovirus (HCMV) UL112-113 gene is implicated in lytic viral replication. The UL112-113 proteins p34, p43, p50 and p84 are expressed via alternative splicing. However, the mechanism for the generation of three additional virus-associated proteins (p20, p26 and p28), which share the UL112 reading frame, remains unknown. Bioinformatic analyses indicated that p34, p43, p50 and p84 contain potential PEST-like degradation motifs. In this study, inhibitors of calpains, lysosomes and proteasomes reduced p20, p26 and p28 levels in virus-infected cells, suggesting the involvement of proteolytic modification. Moreover, maitotoxin, which increases intracellular calcium levels and activates calpain activity, induced the intracellular proteolysis of p34 into p20, p26 and p28 and the cleavage of p43, p50 and p84 into p38 and a novel protein, p34c. Proteolytic assays further indicated that p34, p43, p50 and p84 were substrates of calpain-1 and calpain-2 and that they generated proteolytic products that corresponded to those detected during the HCMV infectious period. Furthermore, substitution mutations in the putative calpain cleavage sites of p34 reduced accumulation of proteolytic products. The knockdown of endogenous calpain-1 and calpain-2 by RNA interference reduced accumulation of p20, p26 and p28 and concurrently increased levels of nascent p43, p50 and p84 during the infectious cycle. Intriguingly, calpain depletion enhanced viral genome synthesis. Moreover, HCMV-permissive cells that stably expressed p20, p26 or p28 exhibited reduced viral genome synthesis and mature virus production. Our findings suggest that cognate UL112-113 proteins derived from calpain-catalysed proteolysis are involved in the HCMV replication process.