In expression studies, SARS-CoV-1 nonstructural protein (nsp1) induces degradation of host mRNAs and suppresses host translation (Kamitani et al, 2006). Host translational suppression, often detected in virus-infected cells, is an effective viral defensive strategy for suppressing host innate immune responses and freeing host translational machinery for viral-specific gene expression. Nsp1 binds tightly to 40S ribosomal subunits and inactivates them, preventing formation of the 80S complex and efficiently suppressing translation (Kamitani et al, 2009; Narayanan et al, 2015).
Although nsp1 induces endonucleolytic cleavage near the 5′UTR of the capped mRNA, SARS-CoV-1 mRNA is resistant to cleavage by nsp1 (Huang et al, 2011). Specifically, interaction of nsp1 with the 5′UTR, the leader sequence located in the 5′ ends of all of the viral mRNAs of SARS-CoV-1, confers resistance to the nsp1-mediated translational shutoff and enhances viral RNA replication (Tanaka et al, 2012). However, nsp1 does not possess any intrinsic nuclease activity and possibly, recruits a cellular endonuclease for inducing mRNA cleavage (Huang et al, 2011, Kamitani et al, 2009). The identity of this putative cellular endonuclease is still unknown (Narayanan et al, 2015). The nsp1-mediated cleavage of host mRNAs has not yet been well-established and therefore is not annotated here.
