Protein-folding location can regulate manganese-binding versus copper- or zinc-binding

Abstract

Metals are needed by at least one-quarter of all proteins. Although metallochaperones insert the correct metal into some proteins, they have not been found for the vast majority, and the view is that most metalloproteins acquire their metals directly from cellular pools. However, some metals form more stable complexes with proteins than do others. For instance, as described in the Irving-Williams series, Cu(2+) and Zn(2+) typically form more stable complexes than Mn(2+). Thus it is unclear what cellular mechanisms manage metal acquisition by most nascent proteins. To investigate this question, we identified the most abundant Cu(2+)-protein, CucA (Cu(2+)-cupin A), and the most abundant Mn(2+)-protein, MncA (Mn(2+)-cupin A), in the periplasm of the cyanobacterium Synechocystis PCC 6803. Each of these newly identified proteins binds its respective metal via identical ligands within a cupin fold. Consistent with the Irving-Williams series, MncA only binds Mn(2+) after folding in solutions containing at least a 10(4) times molar excess of Mn(2+) over Cu(2+) or Zn(2+). However once MncA has bound Mn(2+), the metal does not exchange with Cu(2+). MncA and CucA have signal peptides for different export pathways into the periplasm, Tat and Sec respectively. Export by the Tat pathway allows MncA to fold in the cytoplasm, which contains only tightly bound copper or Zn(2+) (refs 10-12) but micromolar Mn(2+) (ref. 13). In contrast, CucA folds in the periplasm to acquire Cu(2+). These results reveal a mechanism whereby the compartment in which a protein folds overrides its binding preference to control its metal content. They explain why the cytoplasm must contain only tightly bound and buffered copper and Zn(2+).

Molecular basis for manganese sequestration by calprotectin and roles in the innate immune response to invading bacterial pathogens

“…Hence, it is surprising that the canonical S100 site with three histidines and an aspartate (S2) is not responsible for tight binding of Mn in CP. This is all the more remarkable given that a number of proteins, including superoxide dismutase from E. coli and MncA from Synechocystis PCC 6803, bind Mn tightly via three histidines and an aspartate along with a coordinating water molecule (41,42). Mn typically binds to proteins at partially buried sites with a coordination number of five or greater where water molecules often serve as ligands in addition to side chains of the protein (41,42).…”

“…This is all the more remarkable given that a number of proteins, including superoxide dismutase from E. coli and MncA from Synechocystis PCC 6803, bind Mn tightly via three histidines and an aspartate along with a coordinating water molecule (41,42). Mn typically binds to proteins at partially buried sites with a coordination number of five or greater where water molecules often serve as ligands in addition to side chains of the protein (41,42). In the Mn site of CP, we propose that the lack of preferred oxygen ligands involved in Mn coordination is overcome by the creation of an almost perfect octahedral binding site that is relatively inaccessible to solvent.…”

Direct observation of structurally encoded metal discrimination and ether bond formation in a heterodinuclear metalloprotein

“…Metallochaperones are required for correct metallation of some metalloproteins (19)(20)(21)(22), but to date no chaperones have been identified for manganese, and it is unclear whether they are required for diiron clusters (16). Protein-folding location or general metal status can also control metallation (16,23). However, these mechanisms cannot be used for proteins with mixed-metal cofactors.…”

History of biological metal utilization inferred through phylogenomic analysis of protein structures

“…This array of outer-sphere chemistry provides significant catalytic diversity, yet has consequences for both biological and environmental chemistry. Within the cell, metals compete for protein binding sites; hence, extensive protein networks involving transporters and metalsensing regulatory proteins are required to maintain the proper subcellular concentrations of each element, and in some cases directly shuttle each metal to its requisite metalloprotein in the proper cellular compartment (1,5,6). It has been hypothesized that the establishment of a metal homeostasis system is required for distinct phylotypes of cells to develop (7).…”