Midnolin inhibits coronavirus proliferation by degrading viral proteins

ABSTRACT Cells utilize proteasomes and selective autophagy to degrade ubiquitin-labeled viral proteins and inhibit viral proliferation. Midnolin, a midbrain nucleolar protein, is reported to use ubiquitin-independent proteasomal degradation to degrade various nuclear proteins including transcription factors encoded by the immediate-early genes. However, it remains unclear whether midnolin can degrade viral protein to hinder viral replication. In this study, midnolin degraded porcine epidemic diarrhea virus (PEDV) S1/S2/M/E proteins to suppress PEDV proliferation. Midnolin has two essential structural domains: the Catch domain and the ubiquitin-like domain. We found that the Catch domain and the ubiquitin-like domain of midnolin concerted to target and degrade PEDV S1/S2/M/E proteins by both the proteasome and selective autophagy. Furthermore, this study indicated that the individual Catch domain of midnolin degraded PEDV S1/S2/M/E proteins through the autophagy pathway, which recruited the E3 ubiquitinating enzyme MARCH8 to ubiquitinate S1/S2/M/E proteins, and that the ubiquitinated proteins were recognized by Tollip and transported to the lysosome for degradation. Collectively, midnolin degraded viral proteins not only through the proteasomal pathway but also revealed a new mechanism of midnolin-mediated virus restriction, where the Catch domain degraded viral proteins using the midnolin (Catch)-MARCH8-Tollip-autophagosome pathway. IMPORTANCE Proteasomes and selective autophagy are two ways that inhibit viral proliferation in cells. Midnolin can degrade nuclear proteins. However, whether midnolin can degrade viral protein is unknown. In this study, we found that midnolin degraded porcine epidemic diarrhea virus (PEDV) S1/S2/M/E proteins to suppress PEDV proliferation. During the degradation, two domains of midnolin exerted a vital role. The Catch domain and the ubiquitin-like domain concerted to interact and degrade PEDV S1/S2/M/E proteins through the proteasome pathway. In addition, the individual Catch domain of midnolin degraded PEDV S1/S2/M/E proteins through the autophagy pathway using the midnolin (Catch)-MARCH8-Tollip-autophagosome pathway. Overall, we have discovered a new mechanism of midnolin which acts as a host factor for antiviral function.