Maximal efficiency of coupling between ATP hydrolysis and translocation of polypeptides mediated by SecB requires two protomers of SecA

SecA is the ATPase that provides energy for translocation of precursor polypeptides through the SecYEG translocon in Escherichia coli during protein export. We showed previously that when SecA receives the precursor from SecB, the ternary complex is fully active only when two protomers of SecA are bound. Here we used variants of SecA and of SecB that populate complexes containing two protomers of SecA to different degrees to examine both the hydrolysis of ATP and the translocation of polypeptides. We conclude that the low activity of the complexes with only one protomer is the result of a low efficiency of coupling between ATP hydrolysis and translocation.     

Hans Zinsser: a tale of two cultures

Hans Zinsser, president of the Society of American Bacteriologists in 1926, was known as much for his literary and textbook writing as for his scientific contributions. He was a widely known scientist and person of letters. His early interests in poetry and other forms of literature were maintained and developed during his career as a microbiologist, and his most enduring legacy is based on his writing about microbiology for a general readership as well as his reflective and philosophical autobiography.     

Molecular motors: a tale of two filaments

Cargos that are transported along actin frequently switch filaments. New work on single myosin V motors provides insight into this switching and its regulation, as well as revealing that myosin V diffuses on microtubules.     

Herpesvirus assembly: a tale of two membranes

Herpes virions are amongst the most complex virus particles: they comprise in excess of thirty virally encoded proteins, and also contain cellular components. Capsid formation and the cleavage and encapsidation of replicated viral DNA occur in the nucleus and resemble similar processes in tailed dsDNA (double-stranded DNA) bacteriophages, which indicates they might have common ancestry. In contrast, final virion maturation takes place in the cytoplasm. Nucleocapsids gain access to this compartment by envelopment at the inner nuclear membrane, which involves the interaction between viral and cellular proteins in order to locally alter nuclear architecture. Fusion of the primary viral envelope with the outer nuclear membrane results in translocation of the nucleocapsid to the cytoplasm. Here, the majority of the tegument - a structure, composed of a multitude of different proteins, that links the capsid and the envelope - is added to nucleocapsids, which obtain their final envelope by budding into glycoprotein-containing Golgi-derived vesicles. Thus, herpesvirus morphogenesis proceeds in two different cellular compartments, involving different viral and cellular proteins.     

Sleeping sickness: a tale of two diseases

Sleeping sickness presents clinically as two distinct diseases, reflecting the fact that two very different trypanosomes are responsible. The African Rift separating East and West Africa defines the distribution of the two diseases. In this review, Susan Welburn, Eric Fèvre, Paul Coleman, Martin Odiit and Ian Maudlin discuss the biology and distribution of these two diseases in relation to the evolution of hominids in Africa.     

Cell polarity: a tale of two Ts

The microtubule cytoskeleton plays an important role in cell polarity. Central to this process in fission yeast is tea1p, a marker of polarized cell growth that is delivered to the cell surface in a microtubule-dependent fashion. Recent studies suggest that the actin-binding protein bud6p may be a tea1p effector.     

Making a zebrafish kidney: a tale of two tubes

The kidney can be thought of as the pairing of two tubes: an epithelial tube (the nephron), carrying filtered blood and engaged in ion and water transport; and endothelial tubes (the blood vessels), delivering blood and carrying away recovered solute. The development of the nephron presents several interesting questions. How does an epithelial tube form and how is it patterned into functionally distinct components and segments? What guides the interaction between the vasculature and kidney epithelia? How are epithelial cell shape and lumen diameter maintained, and what goes wrong when kidney tubules balloon into cysts? Here, I outline the progress that has been made in answering these questions using the zebrafish pronephros as a simple, accessible model of nephron development.     

Sex determination: a tale of two Sox genes

Vertebrates use many different strategies to determine sex, but the Sox9 gene is a common thread, probably acting as the pivotal gene that controls the male-determining pathway. It now appears that Sox9 is not alone in this role, and that a closely related gene, Sox8, can partly substitute for Sox9. But is this a clever backup strategy to safeguard male development, or a relic of the past?     

Regulation of lipid metabolism: a tale of two yeasts

Eukaryotic cells synthesize multiple classes of lipids by distinct metabolic pathways in order to generate membranes with optimal physical and chemical properties. As a result, complex regulatory networks are required in all organisms to maintain lipid and membrane homeostasis as well as to rapidly and efficiently respond to cellular stress. The unicellular nature of yeast makes it particularly vulnerable to environmental stress and yeast has evolved elaborate signaling pathways to maintain lipid homeostasis. In this article we highlight the recent advances that have been made using the budding and fission yeasts and we discuss potential roles for the unfolded protein response (UPR) and the SREBP-Scap pathways in coordinate regulation of multiple lipid classes.     

Doublecortin and a tale of two serines

Doublecortin (DCX) is a microtubule-associated protein that interacts with and regulates the microtubule cytoskeleton and is required for neuronal migration in the cortex. Two papers in this issue of Neuron (Schaar et al. and Tanaka et al.) demonstrate a role for phosphorylation in the regulation of Doublecortin. Together with recent results showing that Doublecortin may play a role regulating the morphology of migrating neurons, these findings provide new insight into the mechanisms governing neuronal migration.     

S-phase and DNA-damage checkpoints: a tale of two yeasts

Many genes required for the S-phase and DNA-damage checkpoints have been identified in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This year many checkpoint genes have been sequenced, providing new information about the mechanism of checkpoint control. Several of these genes are conserved between the two yeasts but others are species-specific.