Mitochondrial transporter that imports/exports pyrimidine nucleotides into and from mitochondria (PubMed:25320081). Transports preferentially uracil, thymine, and cytosine (deoxy)nucleoside di- and triphosphates by an antiport mechanism (PubMed:25320081). Also transports guanine but not adenine (deoxy)nucleotides (PubMed:25320081). Is inhibited strongly by pyridoxal 5'-phosphate, 4,7-diphenyl-1,10-phenanthroline, tannic acid, and mercurials (mercury dichloride, mersalyl acid, p-hydroxymercuribenzoate) (PubMed:25320081). Participates in mitochondrial genome maintenance, regulation of mitochondrial membrane potential and mitochondrial respiration (PubMed:20453889). Upon INS or IGF1 stimulation regulates cell growth and proliferation by controlling mitochondrial DNA replication and transcription, the ratio of mitochondria-to nuclear-encoded components of the electron transport chain resulting in control of mitochondrial ROS production (PubMed:20453889, PubMed:17596519). Participates in dendritic cell endocytosis and may associate with mitochondrial oxidative phosphorylation (PubMed:14715278). The inhibitory constants (Ki) of these compounds are 174 um (UTP), 195 um (CTP), 318 um (GTP), and 333 um (ITP). UTP is the best substrate. Expressed in the central nervous system. Also expressed in testis and skeletal muscle. Weakly expressed in heart, liver, kidney, prostate, colon and peripheral blood leukocytes. By INS or IGF1 through the PI-3 kinase/mTOR pathway. Belongs to the mitochondrial carrier (TC 2.A.29) family.