NAP1L1 and NAP1L4 binding to Hypervariable Domain of Chikungunya Virus nsP3 Protein is bivalent and requires phosphorylation

Chikungunya virus (CHIKV) is one of the most pathogenic members of the Alphavirus genus in the Togaviridae family. Within the last two decades, CHIKV has expanded its presence to both hemispheres and is currently circulating in both Old and New Worlds. Despite the severity and persistence of the arthritis it causes in humans, no approved vaccines or therapeutic means have been developed for CHIKV infection. Replication of alphaviruses, including CHIKV, is determined not only by their nonstructural proteins, but also by a wide range of host factors, which are indispensable components of viral replication complexes (vRCs). Alphavirus nsP3s contain hypervariable domains (HVDs), which encode multiple motifs that drive recruitment of cell- and virus-specific host proteins into vRCs. Our previous data suggested that NAP1 family members are a group of host factors that may interact with CHIKV nsP3 HVD. In this study, we performed a detailed investigation of the NAP1 function in CHIKV replication in vertebrate cells. Our data demonstrate that i) the NAP1-HVD interactions have strong stimulatory effects on CHIKV replication; ii) both NAP1L1 and NAP1L4 interact with the CHIKV HVD; iii) NAP1 family members interact with two motifs, which are located upstream and downstream of the G3BP-binding motifs of CHIKV HVD; iv) NAP1 proteins interact only with a phosphorylated form of CHIKV HVD and HVD phosphorylation is mediated by CK2 kinase; v) NAP1 and other families of host factors redundantly promote CHIKV replication and their bindings have additive stimulatory effects on viral replication.