Direct Detection of Bound Ammonium Ions in the Selectivity Filter of Ion Channels by Solid-State NMR.

The flow of ions across cell membranes facilitated by ion channels is an important function for all living cells. Despite the huge amount of structural data provided by crystallography, elucidating the exact interactions between the selectivity filter atoms and bound ions is challenging. Here, we detect bound 15N-labeled ammonium ions as a mimic for potassium ions in ion channels using solid-state NMR under near-native conditions. The non-selective ion channel NaK showed two ammonium peaks corresponding to its two ion binding sites, while its potassium-selective mutant NaK2K that has a signature potassium-selective selectivity filter with four ion binding sites gave rise to four ammonium peaks. Ions bound in specific ion binding sites were identified based on magnetization transfer between the ions and carbon atoms in the selectivity filters. Magnetization transfer between bound ions and water molecules revealed that only one out of four ions in the selectivity filter of NaK2K is in close contact with water, which is in agreement with the direct knock-on ion conduction mechanism where ions are conducted through the channel by means of direct interactions without water molecules in between. Interestingly, the potassium-selective ion channels investigated here (NaK2K and, additionally, KcsA-Kv1.3) showed remarkably different chemical shifts for their bound ions, despite having identical amino acid sequences and crystal structures of their selectivity filters. Molecular dynamics simulations show similar ion binding and conduction behavior between ammonium and potassium ions and identify the origin of the differences between the investigated potassium channels.

A tainted trade? Moral ambivalence and legitimation work in the private security industry.

The private security industry is often represented - and typically represents itself - as an expanding business, confident of its place in the world and sure of its ability to meet a rising demand for security. But closer inspection of the ways in which industry players talk about its past, present and future suggests that this self-promotion is accompanied by unease about the industry's condition and legitimacy. In this paper, we analyse the self-understandings of those who sell security - as revealed in interviews conducted with key industry players and in a range of trade materials - in order to highlight and dissect the constitutive elements of this ambivalence. This analysis begins by describing the reputational problems that are currently thought to beset the industry and the underlying fears about its status and worth that these difficulties disclose. We then examine how security players seek to legitimate the industry using various narratives of professionalization. Four such narratives are identified - regulation, education, association and borrowing - each of which seeks to justify private security and enhance the industry's social worth. What is striking about these legitimation claims is that they tend not to justify the selling of security in market terms. In conclusion we ask why this is the case and argue that market justifications are 'closed-off' by a moral ambivalence that attaches to an industry trading in products which cannot guarantee to deliver the condition that its consumers crave.