Membrane Porters of ATP-Binding Cassette Transport Systems Are Polyphyletic

The ATP-binding cassette (ABC) superfamily consists of both importers and exporters. These transporters have, by tradition, been classified according to the ATP hydrolyzing constituents, which are monophyletic. The evolutionary origins of the transmembrane porter proteins/domains are not known. Using five distinct computer programs, we here provide convincing statistical data suggesting that the transmembrane domains of ABC exporters are polyphyletic, having arisen at least three times independently. ABC1 porters arose by intragenic triplication of a primordial two-transmembrane segment (TMS)-encoding genetic element, yielding six TMS proteins. ABC2 porters arose by intragenic duplication of a dissimilar primordial three-TMS-encoding genetic element, yielding a distinctive protein family, nonhomologous to the ABC1 proteins. ABC3 porters arose by duplication of a primordial four-TMS-encoding genetic element, yielding either eight- or 10-TMS proteins. We assign each of 48 of the 50 currently recognized families of ABC exporters to one of the three evolutionarily distinct ABC types. Currently available high-resolution structural data for ABC porters are fully consistent with our findings. These results provide guides for future structural and mechanistic studies of these important transport systems

ADP-ribosylation factors: a family of ~20-kDa guanine nucleotide-binding proteins that activate cholera toxin

ADP-ribosylation factors (ARFs) comprise a family of ∼20 kDa guanine nucleotide-binding proteins that were discovered as one of several cofactors required in cholera toxin-catalyzed ADP-ribosylation of Gsα, the guanine nucleotide-binding protein responsible for stimulation of adenylyl cyclase, and was subsequently found to enhance all cholera toxin-catalyzed reactions and to directly interact with, and activate the toxin. ARF is dependent on GTP or its analogues for activity, binds GTP with high affinity in the presence of dimyristoylphosphatidylcholine/cholate and contains consensus sequences for GTP-binding and hydrolysis. Six mammalian family members have been identified which have been classified into three groups (Class I, II, and III) based on size, deduced amino acid sequence identity, phylogenetic analysis and gene structure. ARFs are ubiquitous among eukaryotes, with a deduced amino acid sequence that is highly conserved across diverse species. They have recently been shown to associate with phospholipid and Golgi membranes in a GTP-dependent manner and are involved in regulating vesicular transport

Location, function, and ontogeny of pulmonary macrophages during the steady state

The lung is continuously exposed to potentially hazardous environmental challenges in the form of inert material and microbes. Pulmonary macrophages are critical in maintaining a low inflammatory context in the lung to facilitate optimal gas exchange. During infection, however, they mediate the immediate response to invading microorganisms in coordination with epithelial cells and other tissue-resident immune cells including dendritic cells, innate lymphocytes and memory T cells, and pulmonary interstitial macrophages. The balance between pulmonary M phi inhibition and activation is regulated by a complex set of receptors whose activation determines whether macrophages remain quiescent or undergo cellular activation. In addition, pulmonary macrophages perform tissue-specific functions such as surfactant catabolism necessary to prevent alveolar proteinosis and interstitial lung disease. This review summarizes current knowledge on different pulmonary macrophage types with an emphasis on their location, function, and available experimental models to manipulate them. Finally, we review recent developments on the dynamic ontogeny of pulmonary macrophages and how it may affect age-related diseases