ADP:ATP antiporter that mediates import of ADP into the mitochondrial matrix for ATP synthesis, and export of ATP out to fuel the cell (By similarity). Cycles between the cytoplasmic-open state (c-state) and the matrix-open state (m-state): operates by the alternating access mechanism with a single substrate-binding site intermittently exposed to either the cytosolic (c-state) or matrix (m-state) side of the inner mitochondrial membrane (By similarity). Specifically required during spermatogenesis, probably to mediate ADP:ATP exchange in spermatocytes (PubMed:17137571, PubMed:17681941, PubMed:19556438). Large ATP supplies from mitochondria may be critical for normal progression of spermatogenesis during early stages of meiotic prophase I, including DNA double-strand break repair and chromosomal synapsis (PubMed:19556438). In addition to its ADP:ATP antiporter activity, also involved in mitochondrial uncoupling and mitochondrial permeability transition pore (mPTP) activity (PubMed:31489369). Plays a role in mitochondrial uncoupling by acting as a proton transporter: proton transport uncouples the proton flows via the electron transport chain and ATP synthase to reduce the efficiency of ATP production and cause mitochondrial thermogenesis (By similarity). Proton transporter activity is inhibited by ADP:ATP antiporter activity, suggesting that SLC25A31/ANT4 acts as a master regulator of mitochondrial energy output by maintaining a delicate balance between ATP production (ADP:ATP antiporter activity) and thermogenesis (proton transporter activity) (By similarity). Proton transporter activity requires free fatty acids as cofactor, but does not transport it (By similarity). Also plays a key role in mPTP opening, a non-specific pore that enables free passage of the mitochondrial membranes to solutes of up to 1.5 kDa, and which contributes to cell death (PubMed:31489369). It is however unclear if SLC25A31/ANT4 constitutes a pore-forming component of mPTP or regulates it (PubMed:31489369). ADP(in) + ATP(out) = ADP(out) + ATP(in) H(+)(in) = H(+)(out) The matrix-open state (m-state) is inhibited by the membrane-permeable bongkrekic acid (BKA). The cytoplasmic-open state (c-state) is inhibited by the membrane-impermeable toxic inhibitor carboxyatractyloside (CATR) (By similarity). Proton transporter activity is inhibited by ADP:ATP antiporter activity (By similarity). Monomer. In sperm flagellum this protein is located in the fibrous sheath, a non-mitochondrial region (By similarity). May localize to non-mitochondrial membranes (By similarity). Specifically expressed in undifferentiated embryonic stem cells and germ cells (PubMed:16051982, PubMed:31489369). Expression is down-regulated after embryonic stem cells differentiation (PubMed:16051982). In adults, only expressed in developing gametes in testis (PubMed:16051982). In testis, expressed at higher level in spermatocytes. Expression is probably associated with entry of the male germ cells into meiosis (PubMed:17681941). Expressed at very low level in Sertoli cells (PubMed:17681941). In testis, expression increases upon transition of premeiotic type B spermatogonia into the early stages of meiosis as represented by preleptotene spermatocytes (PubMed:17681941). Continues to increase through the leptotene and zygotene spermatocyte stages, peaking in early pachytene spermatocytes (PubMed:17681941). Expression decreases in late pachytene spermatocytes and in later round spermatids (PubMed:17681941). Expression is repressed by E2F6. The transmembrane helices are not perpendicular to the plane of the membrane, but cross the membrane at an angle. Odd-numbered transmembrane helices exhibit a sharp kink, due to the presence of a conserved proline residue. Male mice display a significant reduction in testicular size and are sterile, due to impaired spermatogenesis (PubMed:17681941). Males show increased levels of apoptosis within the spermatocyte layer of the seminiferous epithelium, accompanied by the absence of spermatids and spermatozoa within the seminiferous epithelium and lumen respectively (PubMed:17681941). Early meiotic arrest: an accumulation of leptotene spermatocytes, a decrease in pachytene spermatocytes and an absence of diplotene spermatocytes are observed in spermatocytes (PubMed:19556438). Deletion of Slc25a4/Ant1, Slc25a5/Ant2 and Slc25a31/Ant4 in liver completely inhibits mitochondrial permeability transition pore (mPTP) (PubMed:31489369). Mice lacking Slc25a4/Ant1, Slc25a5/Ant2, Slc25a31/Ant4 and Ppif lack Ca(2+)-induced mPTP formation (PubMed:31489369). Belongs to the mitochondrial carrier (TC 2.A.29) family.