When the yeast protein Ypr140w was expressed in Escherichia coli, a lyso-PC acyltransferase activity was found associated with the membranes of the bacteria. To our knowledge, this is the first identification of a protein able to catalyse the acylation of lyso-PC molecules to form PC. Fluorescence microscopy analysis of living yeasts revealed that Ypr140w-GFP fusion protein is targeted to the mitochondria. Moreover, in contrast with wild type cells, in the absence of acyl-CoA, the yeast mutant deleted for the YPR140w gene has no lyso-PC acyltransferase activity associated with the mitochondrial fraction. When yeast cells were grown in the presence of lactate, the mutant synthesized two-fold more triglycerides than the wild type. Moreover its mitochondrial membranes contained a reduced amount of phosphatidylcholine and cardiolipin, and the fatty acid composition of these latter was greatly changed. These modifications were accompanied by a two-fold increase in the respiration rates (state 3 and state 4) of the mitochondria. The relationship between the deletion of the YPR140w gene and the lipid composition of the ypr140wDelta cells is discussed.