Human T cell leukemia virus type 1 (HTLV-1) encodes p13, an 87-amino-acid protein that accumulates in the inner mitochondrial membrane. Recent studies performed using synthetic p13 and isolated mitochondria demonstrated that the protein triggers an inward potassium (K(+)) current and inner membrane depolarization. The present study investigated the effects of p13 on mitochondrial inner membrane potential (Deltapsi) in living cells. Using the potential-dependent probe tetramethyl rhodamine methyl ester (TMRM), we observed that p13 induced dose-dependent mitochondrial depolarization in HeLa cells. This effect was abolished upon mutation of 4 arginines in p13's alpha-helical domain that were previously shown to be essential for its activity in in vitro assays. As Deltapsi is known to control mitochondrial calcium (Ca(2+)) uptake, we next analyzed the effect of p13 on Ca(2+) homeostasis. Experiments carried out in HeLa cells expressing p13 and organelle-targeted aequorins revealed that the protein specifically reduced mitochondrial Ca(2+) uptake. These observations suggest that p13 might control key processes regulated through Ca(2+) signaling such as activation and death of T cells, the major targets of HTLV-1 infection
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