Heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1)
Heterogeneous nuclear ribonucleoprotein (hnRNP) is a general term designated for the members of the RNA binding protein family, which consists of at least twenty members, referred as to from A to U, with molecular weight ranging from 34kDa to 120kDa. HnRNPA1, one of the most abundant members of this family (CK et al., 2017), can be divided into two different parts: the N-terminal functional region with two closely linked RNA recognition motifs mainly responsible for binding to RNA, and the C-terminal glycine enrichment region with RNA binding sites and localization sequence M9 principally involved in RNA binding, cell localization and protein-protein interaction (U & H, 2013). As a multifunctional protein, HnRNPA1 widely participates in the regulation of RNA transcription, splicing, nuclear shuttle and the translation of cellular and viral proteins. Similarly, the N protein, as an RNA binding protein, is implicated in forming the replication and transcription complex of coronaviruses, and plays a crucial role in the replication of viruses. Especially, the binding of hnRNPA1 to the N proteins of other coronaviruses like MHV and SARS-CoV, has been confirmed by a series of experiments (W. Y & X, 1999). Likewise, the binding and co-localization of hnRNPA1 with the N protein of PEDV have been verified, implying the involvement of hnRNPA1 in the formation of PEDV replication-transcription complex (L. Z et al., 2018). It remains to be determined whether the same binding site of the PEDV N protein is used to interact with hnRNPA1 as that of other coronaviruses such as MHV and SARS-CoV [75, 76].
Assembly and release
Successful PEDV replication requires the coordinated production, processing and assembly of each protein and nucleic acid of the virus, as well as the release of progeny viruses capable of infecting new cells from infected cells. Initially, the interaction of the same type M proteins provides a scaffold for the morphogenesis of the virus, while the interaction between the M-S and M-N facilitates the recruitment of the structural components of the virus to the assembly site (Y. Y & BG, 2007). Finally, the newly generated virus particles are transported in the smooth vesicles and released by the exocytic pathway of exocytosis. In this aspect, multiple host factors also take part in these processes of coronavirus including PEDV.