Mitochondrial outer membrane proteome of Trypanosoma brucei reveals macromolecular transport machineries and novel factors required to maintain mitochondrial morphology

The mitochondrial outer membrane (MOM) separates the mitochondria from the cytoplasm, serving both as a barrier and as a gateway. Protein complexes — believed to be universally conserved in all eukaryotes — reside in the MOM to orchestrate and control metabolite exchange, lipid metabolism and uptake of biopolymers such as protein and RNA. African trypanosomes are the causative agent of the sleeping sickness in humans. The parasites are among the earliest diverging eukaryotes that have bona fide mitochondria capable of oxidative phosphorylation. Trypanosomes have unique mitochondrial biology that concerns their mitochondrial metabolism and their unusual mitochondrial morphology that differs to great extent between life stages. Another striking feature is the organization of the mitochondrial genome that does not encode any tRNA genes, thus all tRNAs needed for mitochondrial translation have to be imported. However, the MOM of T. brucei is essentially unchartered territory. It lacks a canonical protein import machinery and facilitation of tRNA translocation remains completely elusive. Using biochemical fractionation and label-free quantitative mass spectrometry for correlated protein abundance-profiling we were able to identify a cluster of 82 candidate proteins that can be localized to the trypanosomal MOM with high confidence. This enabled us to identify a highly unusual, potentially archaic protein import machinery that might also transport tRNAs. Moreover, two-thirds of the identified polypeptides present on the MOM have never been associated with mitochondria before. 40 proteins share homology with proteins of known functions. The function of 42 proteins remains unknown. 11 proteins are essential for the disease-causing bloodstream form of T. brucei and therefore may be exploited as novel drug targets. A comparison with the outer membrane proteome of yeast defines a set of 17 common proteins that are likely present in the MOM of all eukaryotes. Known factors involved in the regulation of mitochondrial morphology are virtually absent in T. brucei. Interestingly, RNAi-mediated ablation of three outer membrane proteins of unknown function resulted in a collapse of the network-like mitochondrion of insect-stage parasites and therefore directly or indirectly are involved in the regulation of mitochondrial morphology.

Prevalence of paunch calf syndrome carriers in Italian Romagnola cattle.

The term 'paunch calf syndrome' encompasses the multi-organic lethal developmental dysplasia reported in the Romagnola breed of cattle and is characterised by facial deformities, an enlarged and floating abdomen containing considerable abdominal effusion, and hepatic fibrosis. Paunch calf syndrome is caused by a missense mutation in the KDM2B gene (c.2503G>A) that is thought to lead to an amino acid exchange (p.D835N). In this study, the prevalence of carriers of the mutant KDM2B allele (and thus the frequency of the allele) was assessed in selected subpopulations of Romagnola cattle. The prevalence of carriers within top-ranked Romagnola sires over the years 2007-2012 was 29.3% (allele frequency 14.6%). In young bull calves, 30.9% were carriers with an allele frequency of 15.4%.

A Call for Systematic Research on Solute Carriers

Solute carrier (SLC) membrane transport proteins control essential physiological functions, including nutrient uptake, ion transport, and waste removal. SLCs interact with several important drugs, and a quarter of the more than 400 SLC genes are associated with human diseases. Yet, compared to other gene families of similar stature, SLCs are relatively understudied. The time is right for a systematic attack on SLC structure, specificity, and function, taking into account kinship and expression, as well as the dependencies that arise from the common metabolic space.

Ion channel macromolecular complexes in cardiomyocytes: roles in sudden cardiac death.

The movement of ions across specific channels embedded on the membrane of individual cardiomyocytes is crucial for the generation and propagation of the cardiac electric impulse. Emerging evidence over the past 20 years strongly suggests that the normal electric function of the heart is the result of dynamic interactions of membrane ion channels working in an orchestrated fashion as part of complex molecular networks. Such networks work together with exquisite temporal precision to generate each action potential and contraction. Macromolecular complexes play crucial roles in transcription, translation, oligomerization, trafficking, membrane retention, glycosylation, post-translational modification, turnover, function, and degradation of all cardiac ion channels known to date. In addition, the accurate timing of each cardiac beat and contraction demands, a comparable precision on the assembly and organizations of sodium, calcium, and potassium channel complexes within specific subcellular microdomains, where physical proximity allows for prompt and efficient interaction. This review article, part of the Compendium on Sudden Cardiac Death, discusses the major issues related to the role of ion channel macromolecular assemblies in normal cardiac electric function and the mechanisms of arrhythmias leading to sudden cardiac death. It provides an idea of how these issues are being addressed in the laboratory and in the clinic, which important questions remain unanswered, and what future research will be needed to improve knowledge and advance therapy.