The US21 viroporin of human cytomegalovirus stimulates cell migration and adhesion

The human cytomegalovirus (HCMV) US12 gene family contributes to virus-host interactions by regulating the virus’ cell tropism and its evasion of host innate immune responses. US21, one of the 10 US12 genes (US12–US21), is a descendant of a captured cellular transmembrane BAX inhibitor motif-containing gene. It encodes a 7TMD endoplasmic reticulum (ER)-resident viroporin (pUS21) capable of reducing the Ca2+ content of ER stores, which, in turn, protects cells against apoptosis. Since regulation of Ca2+ homeostasis affects a broad range of cellular responses, including cell motility, we investigated whether pUS21 might also interfere with this cytobiological consequence of Ca2+ signaling. Indeed, deletion of the US21 gene impaired the ability of HCMV-infected cells to migrate, whereas expression of US21 protein stimulated cell migration and adhesion, as well as focal adhesion (FA) dynamics, in a way that depended on its ability to manipulate ER Ca2+ content. Mechanistic studies revealed pUS21-mediated cell migration to involve calpain 2 activation since its inhibition prevented the viroporin’s effects on cell motility. Pertinently, pUS21 expression stimulated a store-operated Ca2+ entry (SOCE) mechanism that may determine the activation of calpain 2 by promoting Ca2+ entry. Furthermore, pUS21 was observed to interact with talin-1, a calpain 2 substrate, and crucial protein component of FA complexes. A functional consequence of this interaction was confirmed by talin-1 knockdown, which abrogated the pUS21-mediated increase in cell migration. Together, these results indicate the US21-encoded viroporin to be a viral regulator of cell adhesion and migration in the context of HCMV infection.