The bacterial flagellar switch complex is getting more complex.

The mechanism of function of the bacterial flagellar switch, which determines the direction of flagellar rotation and is essential for chemotaxis, has remained an enigma for many years. Here we show that the switch complex associates with the membrane-bound respiratory protein fumarate reductase (FRD). We provide evidence that FRD binds to preparations of isolated switch complexes, forms a 1:1 complex with the switch protein FliG, and that this interaction is required for both flagellar assembly and switching the direction of flagellar rotation. We further show that fumarate, known to be a clockwise/switch factor, affects the direction of flagellar rotation through FRD. These results not only uncover a new component important for switching and flagellar assembly, but they also reveal that FRD, an enzyme known to be primarily expressed and functional under anaerobic conditions in Escherichia coli, nonetheless, has important, unexpected functions under aerobic conditions.

An enzymatic model of the growth hormone-releasing hormone oscillator incorporating neuronal synchronization.

Models of growth hormone (GH) rhythmogenesis which we and others have presented suggest that the GH pulses in the circulation are generated by a GH-releasing hormone (GHRH) oscillator with a 1h periodicity. Here we examine the possibility that this is an intrinsic neuronal rhythm resulting from enzymatic reactions occurring in the axon terminals. A "Baselator" feedback reaction sequence can generate an hourly chemical burst of a primer (presumably a low molecular weight peptide) regulating Ca(2+)-triggered exocytosis of GHRH-loaded vesicles. Accordingly we propose that the priming species is largely immobilized by binding within the terminals. Free unbound primer is able to diffuse and is alternately phosphorylated and dephosphorylated by a kinase and a phosphatase (or undergoes a similar pair of complementary reactions). Under appropriate conditions involving feedback control of one or other of the enzymes the levels of both unreacted and reacted free primer peak sharply at hourly intervals. It is self-evident that synchronization between the packed terminals of the GHRH neurons at the median eminence is necessary to generate highly ordered in vivo pulses of GH release. Gap junctions provide a means of interterminal communication for the primer. Simulations of clusters of 4 adjacent axon terminals in a linear array were performed to assess whether and when synchrony can occur. With gap junctions closed the axons were set to be 90 degrees out of phase, i.e. their chemical bursts were separated by 15 min. Opening the gap junctions, assuming either that only the unphosphorylated species permeates, or that both species permeate, resulted in rapid overall synchronization. The oscillatory systems undergo mutual entrainment and all peaks appeared simultaneously at an intermediate hourly interval. This result was independent of the mode of chemical feedback, whether positive or negative. Closing the gap junctions led to a gradual, but not immediate, loss of synchrony.

Exosomes biological significance: A concise review.

Exosomes were initially thought to be a mechanism for removing unneeded membrane proteins from reticulocytes. Current studies have shown that the process of exosome formation extends to many mammalian cells. This concise review highlights the findings reported at a Workshop on Exosomes. Full knowledge of the contribution of exosomes to intercellular information transmission and the potential medical application of this knowledge will depend on the ingenuity of future investigators and their insight into biological processes.

Revisiting the road to the discovery of exosomes.

A look-back in time is presented to review the 'Alice in Blunderland' approach which led to the discovery of exosomes in and their extrusion from maturing reticulocytes. The key experiments which established that these structures are naturally occurring in both nucleated and non-nucleated reticulocytes are reviewed. The protein content of reticulocyte exosomes is largely that of plasma membrane proteins which are known to diminish in the course of reticulocyte maturation. Thus their protein content may vary depending on the species origin of the reticulocyte. To date, all exosomes also contain HSP 70, a major cellular chaperone which is not transmembrane bound. Significantly, the proteins externalized are intact and retain catalytic activity as well as native ligand binding activity. Their fate remains unknown.