Role of the linker region of the anion-stimulated ATPase ArsA. Effect of deletion and point mutations in the linker region

Transition Metal Homeostasis

This chapter focuses on transition metals. All transition metal cations are toxic-those that are essential for Escherichia coli and belong to the first transition period of the periodic system of the element and also the "toxic-only" metals with higher atomic numbers. Common themes are visible in the metabolism of these ions. First, there is transport. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells. Control of the respective systems seems to be mainly through regulation of transport activity (flux control), with control of gene expression playing only a minor role. If these systems do not provide sufficient amounts of a needed ion to the cell, genes for ATP-hydrolyzing high-affinity but low-rate uptake systems are induced, e.g., ABC transport systems or P-type ATPases. On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. By combining different kinds of uptake and efflux systems with regulation at the levels of gene expression and transport activity, the concentration of a single ion in the cytoplasm and the composition of the cellular ion "bouquet" can be rapidly adjusted and carefully controlled. The toxicity threshold of an ion is defined by its ability to produce radicals (copper, iron, chromate), to bind to sulfide and thiol groups (copper, zinc, all cations of the second and third transition period), or to interfere with the metabolism of other ions. Iron poses an exceptional metabolic problem due its metabolic importance and the low solubility of Fe(III) compounds, combined with the ability to cause dangerous Fenton reactions. This dilemma for the cells led to the evolution of sophisticated multi-channel iron uptake and storage pathways to prevent the occurrence of unbound iron in the cytoplasm. Toxic metals like Cd2+ bind to thiols and sulfide, preventing assembly of iron complexes and releasing the metal from iron-sulfur clusters. In the unique case of mercury, the cation can be reduced to the volatile metallic form. Interference of nickel and cobalt with iron is prevented by the low abundance of these metals in the cytoplasm and their sequestration by metal chaperones, in the case of nickel, or by B12 and its derivatives, in the case of cobalt. The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism. E. coli solves this problem probably by preventing copper uptake, combined with rapid efflux if the metal happens to enter the cytoplasm.

Biochemical evidence for interaction between the two nucleotide binding domains of ArsA. Insights from mutants and ATP analogs

ArsA, the peripheral membrane component of the anion-translocating ATPase ArsAB, consists of two nucleotide binding domains (A1 and A2), which are connected by a linker sequence. Previous studies on ArsA have focused on the function of each nucleotide binding domain and the role of the linker, whereas the present study looks at the interactions between the binding domains and their interactions with the linker. It has previously been shown that the A1 domain of ArsA carries out unisite catalysis in the absence of antimonite, while A2 is recruited in multisite catalysis by antimonite in the presence of a functional A1 domain. Multisite catalysis thus seems to result from an interaction between A1 and A2 brought about by antimonite. In the present study, we provide direct biochemical evidence for interaction between the two nucleotide binding domains and show that the linker region acts as a transducer of the conformational changes between them. We find that nucleotide binding to the A2 domain results in a significant, detectable change in the conformation of the A1 domain. Two ATP analogs, FSBA and ATP gamma S, used in this study, were both found to bind preferentially to the A2 domain, and their binding resulted in changing the otherwise compact A1 domain into an open conformation. Point mutations in the A2 domain and the linker region also produced a similar effect on the conformation of A1, thus suggesting that events at A2 are relayed to A1 via the linker. We propose that nucleotide binding to A2 produces a two-tiered conformational change. The significance of these changes in the mechanism of ArsA is discussed.