Inner membrane complex

Abstract: Publisher Summary The genetic structure of Toxoplasma gondii is notable chiefly for being relatively conventional— similar to that of its mammalian host cells with respect to gene organization, codon usage, and nucleotide bias. These observations have led several investigators to examine the feasibility of molecular transformation in this parasite. This chapter outlines the use of several of the molecular genetic tools that have recently been developed for the T. gondii system. An introduction to parasite culture techniques is also provided. Recombinant molecules can be expressed either transiently or as stable transformants, as episomes or integrated into the parasite genome, and as single copy or multicopy transgenes. Stable integration can be produced by random nonhomologous recombination, single-site homologous recombination, or perfect gene replacement. Many of these outcomes can be selected specifically using appropriate vectors and transformation conditions. The extraordinarily high frequencies of stable transformation observed permit cloning by complementation, insertional mutagenesis/marker rescue, gene knock-outs, and allelic replacement. In combination with available classical and “cell-genetic” possibilities and physical and genetic mapping strategies, these tools provide a powerful arsenal for investigations into the biology of intracellular parasitism.

Abstract: Integral outer membrane receptors for iron chelates and vitamin B12 carry out specific ligand transport against a concentration gradient. Energy for active transport is obtained from the proton-motive force of the inner membrane through physical interaction with TonB-ExbB-ExbD, an inner membrane complex. Here we report the crystal structure of an active transport, outer membrane receptor at 2.4 A resolution. Two distinct functional domains are revealed: (i) a 22-stranded beta-barrel that spans the outer membrane and contains large extracellular loops which appear to function in ligand binding; and (ii) a globular N-terminal domain that folds into the barrel pore, inhibiting access to the periplasm and contributing two additional loops for potential ligand binding. These loops could provide a signaling pathway between the processes of ligand recognition and TonB-mediated transport. The blockage of the pore suggests that the N-terminal domain must undergo a conformational rearrangement to allow ligand transport into the periplasm.

Abstract: Mitochondria are eukaryotic organelles that generate most of the energy in the cell by oxidative phosphorylation (OXPHOS). Each mitochondrion contains multiple copies of a closed circular double-stranded DNA genome (mtDNA). Human (mammalian) mtDNA encodes 13 essential subunits of the inner membrane complex responsible for OXPHOS. These mRNAs are translated by the mitochondrial protein synthesis machinery, which uses the 22 species of mitochondrial tRNAs (mt tRNAs) encoded by mtDNA. The unique structural features of mt tRNAs distinguish them from cytoplasmic tRNAs bearing the canonical cloverleaf structure. The genes encoding mt tRNAs are highly susceptible to point mutations, which are a primary cause of mitochondrial dysfunction and are associated with a wide range of pathologies. A large number of nuclear factors involved in the biogenesis and function of mt tRNAs have been identified and characterized, including processing endonucleases, tRNA-modifying enzymes, and aminoacyl-tRNA synthetases. These nuclear factors are also targets of pathogenic mutations linked to various diseases, indicating the functional importance of mt tRNAs for mitochondrial activity.

Abstract: The Apicomplexa are a phylum of diverse obligate intracellular parasites including Plasmodium spp., the cause of malaria; Toxoplasma gondii and Cryptosporidium parvum, opportunistic pathogens of immunocompromised individuals; and Eimeria spp. and Theileria spp., parasites of considerable agricultural importance. These protozoan parasites share distinctive morphological features, cytoskeletal organization, and modes of replication, motility, and invasion. This review summarizes our current understanding of the cytoskeletal elements, the properties of cytoskeletal proteins, and the role of the cytoskeleton in polarity, motility, invasion, and replication. We discuss the unusual properties of actin and myosin in the Apicomplexa, the highly stereotyped microtubule populations in apicomplexans, and a network of recently discovered novel intermediate filament-like elements in these parasites.