Structure and ion-release mechanism of PIB-4-type ATPases.

Transition metals, such as zinc, are essential micronutrients in all organisms, but also highly toxic in excessive amounts. Heavy-metal transporting P-type (PIB) ATPases are crucial for homeostasis, conferring cellular detoxification and redistribution through transport of these ions across cellular membranes. No structural information is available for the PIB-4-ATPases, the subclass with the broadest cargo scope, and hence even their topology remains elusive. Here, we present structures and complementary functional analyses of an archetypal PIB-4-ATPase, sCoaT from Sulfitobacter sp. NAS14-1. The data disclose the architecture, devoid of classical so-called heavy-metal-binding domains (HMBDs), and provide fundamentally new insights into the mechanism and diversity of heavy-metal transporters. We reveal several novel P-type ATPase features, including a dual role in heavy-metal release and as an internal counter ion of an invariant histidine. We also establish that the turnover of PIB-ATPases is potassium independent, contrasting to many other P-type ATPases. Combined with new inhibitory compounds, our results open up for efforts in for example drug discovery, since PIB-4-ATPases function as virulence factors in many pathogens.

Heavy metals such as zinc and cobalt are toxic at high levels, yet most organisms need tiny amounts for their cells to work properly. As a result, proteins studded through the cell membrane act as gatekeepers to finetune import and export. These proteins are central to health and disease; their defect can lead to fatal illnesses in humans, and they also help bacteria infect other organisms. Despite their importance, little is known about some of these metal-export proteins. This is particularly the case for P_IB-4-ATPases, a subclass found in plants and bacteria and which includes, for example, a metal transporter required for bacteria to cause tuberculosis. Intricate knowledge of the three-dimensional structure of these proteins would help to understand how they select metals, shuttle the compounds in and out of cells, and are controlled by other cellular processes. To reveal this three-dimensional organisation, Grønberg et al. used X-ray diffraction, where high-energy radiation is passed through crystals of protein to reveal the positions of atoms. They focused on a type of PIB-4-ATPases found in bacteria as an example. The work showed that the protein does not contain the metal-binding regions seen in other classes of metal exporters; however, it sports unique features that are crucial for metal transport such as an adapted pathway for the transport of zinc and cobalt across the membrane. In addition, Grønberg et al. tested thousands of compounds to see if they could block the activity of the protein, identifying two that could kill bacteria. This better understanding of how PIB-4-ATPases work could help to engineer plants capable of removing heavy metals from contaminated soils, as well as uncover new compounds to be used as antibiotics.

Phoxilium(®) reduces hypophosphataemia and magnesium supplementation during continuous renal replacement therapy.

BACKGROUND: Although associated with severe clinical complications, phosphate remains a neglected ion. Additionally, phosphate balance during continuous renal replacement therapy (CRRT) is complex and multifunctional. The present retrospective study investigated the effects of phosphate-containing CRRT fluid on phosphate homeostasis. METHODS: We retrospectively analysed 112 patients treated with CRRT at Skåne University Hospital, Sweden. The control group was treated with Hemosol(®) B0 (no phosphate; n = 36) as dialysis and replacement fluid, while the study group received Phoxilium(®) (phosphate; n = 76) as dialysis fluid and Hemosol(®) B0 as replacement fluid. RESULTS: Hypophosphataemia (<0.7 mM) occurred in 15% of the treatment days in the control group compared with 7% in the study group (P = 0.027). Magnesium substitution was reduced by 40% in the study group (P < 0.001). No differences in acid-base parameters were detected between the groups. CONCLUSIONS: In this larger cohort, we could confirm that Phoxilium(®) reduced the episodes of hypophosphataemia during CRRT. A beneficial effect on magnesium balance could also be observed.