The growth of an individual is deeply influenced by the regulation of cell growth and division, both of which also contribute to a wide variety of pathological conditions, including cancer, diabetes, and inflammation. To identify a major regulator of human growth, we performed positional cloning in an autosomal recessive type of profound short stature, anauxetic dysplasia. Homozygosity mapping led to the identification of novel mutations in the RMRP gene, which was previously known to cause two milder types of short stature with susceptibility to cancer, cartilage hair hypoplasia, and metaphyseal dysplasia without hypotrichosis. We show that different RMRP gene mutations lead to decreased cell growth by impairing ribosomal assembly and by altering cyclin-dependent cell cycle regulation. Clinical heterogeneity is explained by a correlation between the level and type of functional impairment in vitro and the severity of short stature or predisposition to cancer. Whereas the cartilage hair hypoplasia founder mutation affects both pathways intermediately, anauxetic dysplasia mutations do not affect B-cyclin messenger RNA (mRNA) levels but do severely incapacitate ribosomal assembly via defective endonucleolytic cleavage. Anauxetic dysplasia mutations thus lead to poor processing of ribosomal RNA while allowing normal mRNA processing and, therefore, genetically separate the different functions of RNase MRP.

In the yeast Saccharomyces cerevisiae, RNase mitochondrial RNA processing (MRP) is an essential endoribonuclease that consists of one RNA component and at least nine protein components. Characterization of the complex is complicated by the fact that eight of the known protein components are shared with a related endoribonuclease, RNase P. To fully characterize the RNase MRP complex, we purified it to apparent homogeneity in a highly active state using tandem affinity purification. In addition to the nine known protein components, both Rpr2 and a protein encoded by the essential gene YLR145w were present in our preparations of RNase MRP. Precipitation of a tagged version of Ylr145w brought with it the RNase MRP RNA, but not the RNase P RNA. A temperature-sensitive ylr145w mutant was generated and found to exhibit a rRNA processing defect identical to that seen in other RNase MRP mutants, whereas no defect in tRNA processing was observed. Homologues of the Ylr145w protein were found in most yeasts, fungi, and Arabidopsis. Based on this evidence, we propose that YLR145w encodes a novel protein component of RNase MRP, but not RNase P. We recommend that this gene be designated RMP1, for RNase MRP protein 1. RNase MRP1 is a highly conserved and essential ribonucleoprotein endoribonuclease that cleaves substrates in at least two intracellular compartments. Most RNase MRP is localized to the nucleolus (1), where a role in processing of rRNA precursors has been identified (2). RNase MRP-mediated cleavage at the A 3 site of pre-rRNA ultimately leads to the generation of 5.8S(S) rRNA (3-5). A fraction of RNase MRP RNA finds its way to the mitochondrion (6, 7). Mitochondrial RNase MRP processes RNA transcripts, which serve as primers for the leading strand of mitochondrial DNA replication (8, 9). Recent data also support a role for RNase MRP mRNA degradation, whereby cleavage of the 5Ј-untranslated region (UTR) of CLB2 mRNA, which encodes a B-type cyclin, leads to its rapid degradation and aids cell cycle progression (10).In Saccharomyces cerevisiae, all the known components of RNase MRP are essential for viability. To date, a single RNA component and at least nine protein components of nuclear RNase MRP have been identified. The subunit composition of RNase MRP closely resembles that of a related ribonucleoprotein endoribonuclease, RNase P, which processes tRNA precursors to generate mature 5Ј termini. Eight of the proteins associated with RNase MRP (Pop1, Pop3, Pop4, Pop5, Pop6, Pop7, Pop8, and Rpp1) are also components of RNase P (11-14). An RNA-binding protein, encoded by the gene SNM1, is the only known protein component that associates with RNase MRP RNA but not RNase P RNA (15). Similarly, Rpr2p has been identified as a unique protein component of the RNase P complex (14).The similarities between RNase MRP and RNase P extend beyond that of shared protein components. The RNA subunits of RNase MRP and RNase P are evolutionarily and structurally related (16,17). They share only weak sequence homology, but they fold into...

Adenine nucleotide translocase (Ant) is the most abundant protein on the mitochondrial inner membrane (MIM) primarily involved in ADP/ATP exchange. Ant also possesses a discrete membrane uncoupling activity. Specific mis-sense mutations in the human Ant1 cause autosomal dominant Progressive External Ophthalmoplegia (adPEO), mitochondrial myopathy and cardiomyopathy, which are commonly manifested by fractional mitochondrial DNA (mtDNA) deletions. It is currently thought that the pathogenic mutations alter substrate preference (e.g. ATP versus ADP) thereby dominantly disturbing adenine nucleotide homeostasis in mitochondria. This may interfere with mtDNA replication, consequently affecting mtDNA stability and oxidative phosphorylation. Here, we showed that the adPEO-type A128P, A106D and M114P mutations in the yeast Aac2p share the following common dominant phenotypes: electron transport chain damage, intolerance to moderate over-expression, synthetic lethality with low Deltapsi(m) conditions, hypersensitivity to the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) and mtDNA instability. More interestingly, the aac2(A137D) allele mimicking ant1(A123D) in mitochondrial myopathy and cardiomyopathy exhibits similar dominant phenotypes. Because Aac2(A137D) is known to completely lack transport activity, it is strongly argued that the dominant mitochondrial damages are not caused by aberrant nucleotide transport. The four pathogenic mutations occur in a structurally dynamic gating region on the cytosolic side. We provided direct evidence that the mutant alleles uncouple mitochondrial respiration. The pathogenic mutations likely enhance the intrinsic proton-conducting activity of Ant, which excessively uncouples the MIM thereby affecting energy transduction and mitochondrial biogenesis. mtDNA disintegration is a phenotype co-lateral to mitochondrial damages. These findings provide mechanistic insights into the pathogenesis of the Ant1-induced diseases.