During intracellular Ca2+ signalling mitochondria accumulate significant amounts of Ca2+ from the cytosol. Mitochondrial Ca2+ uptake controls the rate of energy production, shapes the amplitude and spatio-temporal patterns of intracellular Ca2+ signals, and is instrumental to cell death. This Ca2+ uptake is undertaken by the mitochondrial Ca2+ uniporter (MCU) located in the organelle's inner membrane. The uniporter passes Ca2+ down the electrochemical gradient maintained across this membrane without direct coupling to ATP hydrolysis or transport of other ions. Carriers are characterized by turnover numbers that are typically 1,000-fold lower than ion channels, and until now it has been unclear whether the MCU is a carrier or a channel. By patch-clamping the inner mitochondrial membrane, we identified a previously unknown Ca2+-selective ion channel sensitive to inhibitors of mitochondrial Ca2+ uptake. Our data indicate that this unique channel binds Ca2+ with extremely high affinity (dissociation constant < or =2 nM), enabling high Ca2+ selectivity despite relatively low cytoplasmic Ca2+ concentrations. The channel is inwardly rectifying, making it especially effective for Ca2+ uptake into energized mitochondria. Thus, we conclude that the properties of the current mediated by this novel channel are those of the MCU.

TRP proteins are cation channels responding to receptor-dependent activation of phospholipase C. Mammalian (TRPC) channels can form hetero-oligomeric channels in vitro, but native TRPC channel complexes have not been identified to date. We demonstrate here that TRPC1 and TRPC5 are subunits of a heteromeric neuronal channel. Both TRPC proteins have overlapping distributions in the hippocampus. Coexpression of TRPC1 and TRPC5 in HEK293 cells resulted in a novel nonselective cation channel with a voltage dependence similar to NMDA receptor channels, but unlike that of any reported TRPC channel. TRPC1/TRPC5 heteromers were activated by G(q)-coupled receptors but not by depletion of intracellular Ca(2+) stores. In contrast to the more common view of the TRP family as comprising store-operated channels, we propose that many TRPC heteromers form diverse receptor-regulated nonselective cation channels in the mammalian brain.

Heart rate is slowed in part by acetylcholine-dependent activation of a cardiac potassium (K+) channel, IKACh. Activated muscarinic receptors stimulate IKACh via the G-protein beta gamma-subunits. It has been assumed that the inwardly rectifying K(+)-channel gene, GIRK1, alone encodes IKACh. It is now shown that IKACh is a heteromultimer of two distinct inwardly rectifying K(+)-channel subunits, GIRK1 and a newly cloned member of the family, CIR.

Mammalian spermatozoa become motile at ejaculation, but before they can fertilize the egg, they must acquire more thrust to penetrate the cumulus and zona pellucida. The forceful asymmetric motion of hyperactivated spermatozoa requires Ca 2؉ entry into the sperm tail by an alkalinization-activated voltage-sensitive Ca 2؉ -selective current (ICatSper). Hyperactivation requires CatSper1 and CatSper2 putative ion channel genes, but the function of two other related genes (CatSper3 and CatSper4) is not known. Here we show that targeted disruption of murine CatSper3 or CatSper4 also abrogated I CatSper , sperm cell hyperactivated motility and male fertility but did not affect spermatogenesis or initial motility. Direct protein interactions among CatSpers, the sperm specificity of these proteins, and loss of I CatSper in each of the four CatSper ؊/؊ mice indicate that CatSpers are highly specialized flagellar proteins.calcium ͉ contraception ͉ flagella S permatozoa first acquire the potential for motility in the epididymis. They are capacitated in the female reproductive tract (1), where they acquire hyperactivated motility and other attributes that enable fertilization (2). During hyperactivation, the sperm tail motion changes from symmetric, fast, and low amplitude (sinusoidal) to asymmetric, slow, and large amplitude (whip-like; refs. 3-5). Hyperactivation is required for fertilization, providing the force needed to free the sperm cell from the oviductal reservoir and to penetrate the cumulus and zona pellucida surrounding the egg (1, 6, 7).Sperm cells become motile and progress directionally once they enter the female reproductive tract. Ca 2ϩ -independent flagellar dynein and ATP orchestrate the low-amplitude sinusoidal-activated motility of the tail. As the sperm cells encounter a more alkaline environment in the higher female reproductive tract, they hyperactivate, a process that requires Ca 2ϩ entry (3,8,9). Studies with antibodies or nucleotide probes have labeled several Ca 2ϩ -permeant channels, including voltage-sensitive Ca 2ϩ -selective channels (CatSpers and CaVs), cyclic nucleotide-gated channels, and transient receptor potential channels, in spermatocytes or spermatozoa (10-18). However, recent patch-clamp recordings of mouse epididymal spermatozoa (19) show that the predominant Ca 2ϩ -carrying current requires the CatSper1 gene that encodes a six-transmembrane-spanning protein of the voltage-gated ligand ion channel superfamily (20). In both whole-cell and perforatedpatch configurations, the Ca 2ϩ -selective current (I CatSper ) originated from the principal piece of the sperm tail and was absent in spermatozoa from CatSper1 Ϫ/Ϫ mice. CatSper1 Ϫ/Ϫ and CatSper2 Ϫ/Ϫ male mice are infertile (11,21), and sperm cells from CatSper1 Ϫ/Ϫ and CatSper2 Ϫ/Ϫ mice are unable to hyperactivate (4, 21). I CatSper was dramatically potentiated by a rise in intracellular pH, suggesting that the alkalinization that occurs during sperm capacitation activates I CatSper to increase intracellular [Ca 2ϩ ] and induce hypera...