Human cytomegalovirus US21 protein is a viroporin that modulates calcium homeostasis and protects cells against apoptosis

The human cytomegalovirus (HCMV) US12 gene family comprises a set of 10 contiguous genes (US12 to US21) with emerging roles in the regulation of virus cell tropism, virion composition, and immunoevasion. Of all of the US12 gene products, pUS21 shows the highest level of identity with two cellular transmembrane BAX inhibitor motif-containing (TMBIM) proteins: Bax inhibitor-1 and Golgi anti-apoptotic protein, both of which are involved in the regulation of cellular Ca2+ homeostasis and adaptive cell responses to stress conditions. Here, we report the US21 protein to be a viral-encoded ion channel that regulates intracellular Ca2+ homeostasis and protects cells against apoptosis. Indeed, we show pUS21 to be a 7TMD protein expressed with late kinetics that accumulates in ER-derived vesicles. Deletion or inactivation of the US21 gene resulted in reduced HCMV growth, even in fibroblasts, due to reduced gene expression. Ratiometric fluorescence imaging assays revealed that expression of pUS21 reduces the Ca2+ content of intracellular ER stores. An increase in cell resistance to intrinsic apoptosis was then observed as an important cytobiological consequence of the pUS21-mediated alteration of intracellular Ca2+ homeostasis. Moreover, a single point mutation in the putative pore of pUS21 impaired the reduction of ER Ca2+ concentration and attenuated the antiapoptotic activity of pUS21wt, supporting a functional link with its ability to manipulate Ca2+ homeostasis. Together, these results suggest pUS21 of HCMV constitutes a TMBIM-derived viroporin that may contribute to HCMV's overall strategy to counteract apoptosis in infected cells.