Georg Nagel

A reevaluation of substrate specificity of the rat cation transporter rOCT1.

The substrate specificity of the previously cloned rat cation transporter rOCT1, which is expressed in kidney, liver, and small intestine, was reevaluated. rOCT1 is the first member of a new protein family comprising electrogenic and polyspecific cation transporters that transport hydrophilic cations like tetraethylammonium, choline, and monoamine neurotransmitters. Previous electrical measurements suggested that cations like quinine, quinidine, and cyanine 863, which have been classified as type 2 cations in the liver, are also transported by rOCT1, since they may induce inward currents in rOCT1 expressingXenopus oocytes (Busch, A. E., Quester, S., Ulzheimer, J. C., Waldegger, S., Gorboulev, V., Arndt, P., Lang, F., and Koepsell, H. (1996) J. Biol. Chem. 271, 32599–32604). Tracer flux measurements with oocytes and with stably transfected human embryonic kidney cells showed that [3H]quinine and [3H]quinidine are not transported by rOCT1. The voltage dependence observed for the quinine- or quinidine-induced inward currents in rOCT1-expressing oocytes, and tracer efflux measurements indicate that the inward currents by type 2 cations are generated by the inhibition of electrogenic efflux of transported type 1 cations. Therefore, rOCT1 cannot contribute to transport of type 2 cations in the liver and the hepatic transporter for type 2 cations remains to be identified.

Differential function of the two nucleotide binding domains on cystic fibrosis transmembrane conductance regulator

The genetic disease cystic fibrosis is caused by defects in the chloride channel cystic fibrosis transmembrane conductance regulator (CFTR). CFTR belongs to the family of ABC transporters. In contrast to most other members of this family which transport substrates actively across a membrane, the main function of CFTR is to regulate passive flux of substrates across the plasma membrane. Chloride channel activity of CFTR is dependent on protein phosphorylation and presence of nucleoside triphosphates. From electrophysiological studies of CFTR detailed models of its regulation by phosphorylation and nucleotide interaction have evolved. These investigations provide ample evidence that ATP hydrolysis is crucial for CFTR gating. It becomes apparent that the two nucleotide binding domains on CFTR not only diverge strongly in sequence, but also in function. Based on previous models and taking into account new data from pre-steady-state experiments, a refined model for the action of nucleotides at two nucleotide binding domains was recently proposed.

Algen, Froscheier und Putzfimmel bei Fliegen

Um auf Veränderungen der äußeren Bedingungen reagieren zu können, müssen in einem Organismus verschiedene Signalketten geschaltet werden. Meist wird ein Signal zunächst von einem Rezeptor erkannt und dann über so genannte sekundäre Botenstoffe weitergeleitet. Bei diesen handelt es sich um kleine, wasserlösliche Moleküle, die sich durch Diffusion in Zellen leicht ausbreiten. Ca2+-Ionen und cAMP (cyclisches Adenosinmonophosphat) sind solche Botenstoffe. cAMP überträgt beispielsweise das vom Stresshormon Adrenalin übermittelte Signal ins Zellinnere, wo es dann zur Mobilisierung von Traubenzucker kommt. Hierbei bindet Adrenalin an einen Rezeptor auf der Plasmamembran einer Leberoder Muskelzelle, worauf in der Zelle cAMP gebildet wird. Dabei ist Adrenalin nur eines von vielen Hormonen und anderen Signalmolekülen, die die cAMP-Produktion in Gang setzen. cAMP beeinflusst Funktionen wie Herzschlag, Sekretion, Zellwachstum, und vermutlich auch Intelligenz und Gedächtnis, ist somit also ein sehr wichtiger Botenstoff. Um die Auswirkungen von cAMP zu erforschen, wäre es ideal, wenn es sich schnell und wiederholbar in der lebenden Zelle bzw. im lebenden Tier erzeugen ließe.