Abstract Human T-cell leukemia virus type 1 encodes a number of "accessory" proteins of unclear function; one of these proteins, p13II, is targeted to mitochondria and disrupts mitochondrial morphology. The present study was undertaken to unravel the function of p13II through (i) determination of its submitochondrial localization and sequences required to alter mitochondrial morphology and (ii) an assessment of the biophysical and biological properties of synthetic peptides spanning residues 9–41 (p139–41), which include the amphipathic mitochondrial-targeting sequence of the protein. p139–41 folded into an α helix in micellar environments. Fractionation and immunogold labeling indicated that full-length p13II accumulates in the inner mitochondrial membrane. p139–41 induced energy-dependent swelling of isolated mitochondria by increasing inner membrane permeability to small cations (Na+, K+) and released Ca2+ from Ca2+-preloaded mitochondria. These effects as well as the ability of full-length p13II to alter mitochondrial morphology in cells required the presence of four arginines, forming the charged face of the targeting signal. The mitochondrial effects of p139–41 were insensitive to cyclosporin A, suggesting that full-length p13II might alter mitochondrial permeability through a permeability transition pore-independent mechanism, thus distinguishing it from the mitochondrial proteins Vpr and X of human immunodeficiency virus type 1 and hepatitis B virus, respectively
