Human cathepsinW(CtsW) is a cysteine protease, which was identified in a genome-wide RNA interference (RNAi)
screen to be required for influenza A virus (IAV) replication. In this study, we show that reducing the levels of expression of CtsW reduces viral titers for different subtypes of IAV, and we map the target step of CtsW requirement to viral entry. Using a set of small interfering RNAs (siRNAs) targeting CtsW, we demonstrate that knockdown of CtsW results in a decrease of IAV
nucleoprotein accumulation in the nuclei of infected cells at 3 h postinfection. Assays specific for the individual stages of IAV entry further show that attachment, internalization, and early endosomal trafficking are not affected by CtsW knockdown. However, we detected impaired escape of viral particles from late endosomes in CtsW knockdown cells. Moreover, fusion analysis with a dual-labeled influenza virus revealed a significant reduction in fusion events, with no detectable impact on endosomal pH, suggesting that CtsW is required at the stage of viral fusion. The defect in IAV entry upon CtsW knockdown could be rescued by ectopic expression of wild-type CtsW but not by the expression of a catalytically inactive mutant of CtsW, suggesting that the proteolytic activity of CtsW is required for successful entry of IAV. Our results establish CtsW as an important host factor for
entry of IAV into target cells and suggest that CtsW could be a promising target for the development of future antiviral drugs.