Phospholipid scramblase 3 (PLS3) is a newly recognized member of a family of proteins responsible for phospholipid translocation between two lipid compartments. A novel isoform of PLS3 is identified and characterized herein. The function of PLS3 in mitochondria was disrupted, yielding an inactive mutant PLS3(F258V). Cells transfected with PLS3(F258V) exhibited reduced proliferative capacity that was unaffected by the presence of Na.sub.3N. PLS3(F258V)-expressing cells exhibit abnormal mitochondrial metabolism and structure and were associated with decreased sensitivity to UV- and tBid-induced apoptosis, and diminished translocation of cardiolipin to the outer mitochondrial membrane. Cells transfected with wild-type PLS3 displayed increased sensitivity to apoptosis and enhanced cardiolipin translocation. These studies identify PLS3 as a regulator of mitochondrial structure and respiration, and cardiolipin transport in apoptosis.