We have characterized 1,2,3-benzenetricarboxylic acid-sensitive, mersalyl-insensitive
citrate uptake by mitochondria from two strains of Saccharomyces cerevisiae by describing
the time course, Km and Vmax values, pH dependence, and response to inhibitors. In
unloaded mitochondria from PSY142 CS1- cells, a mutant that lacks mitochondrial citrate
synthase, both citrate uptake and efflux were reduced 7- and 8-fold, respectively,
compared with the parental strain. No malate uptake was detectable in mitochondria
from CS1- cells, while in the parental strain, uptake was 5.4 nmol/min/mg of protein.
In contrast, mutations in peroxisomal citrate synthase (CS2-) or in other tricarboxylic
acid cycle enzymes did not result in changes in mitochondrial citrate transport, suggesting
a specific functional role for mitochondrial citrate synthase in citrate transport.
More important, liposomes containing protein extracts from CS1- mitochondria showed
the same citrate and malate transport rates as liposomes made from protein extracts
of parental strain mitochondria. Thus, an apparently normal amount of both the citrate
transporter and the dicarboxylate carrier is present in CS1- mitochondria, but both
function abnormally in undisrupted mitochondria. We suggest that cooperation between
the citrate transporter and mitochondrial citrate synthase is necessary for normal
function of the transporter.