Confinement of enzymes in protein nanocompartments represents a potentially powerful strategy for controlling catalytic activity in cells. By using a simple electrostatically based tagging system for protein encapsulation, we successfully sequestered HIV protease, a toxic enzyme when produced cytoplasmically, within an engineered lumazine synthase capsid. The growth advantage resulting from protecting the Escherichia coli host from the protease enabled directed evolution of improved capsids. After four rounds of mutagenesis and selection, we obtained a variant with a 5- to 10-fold higher loading capacity than the starting capsid, which permitted efficient growth even at high intracellular concentrations of HIV protease. The superior properties of the evolved capsid can be ascribed to multiple mutations that increase the net negative charge on its luminal surface and thereby enhance engineered Coulombic interactions between host and guest. Such structures could be used for diverse biotechnological applications in living cells.

An engineered variant of lumazine synthase, a nonviral capsid protein with a negatively charged luminal surface, is shown to encapsulate up to 100 positively supercharged green fluorescent protein (GFP) molecules in vitro. Packaging can be achieved starting either from intact, empty capsids or from capsid fragments by incubation with cargo in aqueous buffer. The yield of encapsulated GFP correlates directly with the host/guest mixing ratio, providing excellent control over packing density. Facile in vitro loading highlights the unusual structural dynamics of this novel nanocontainer and should facilitate diverse biotechnological and materials science applications.

The founding members of the HD-domain protein superfamily are phosphohydrolases, and newly discovered members are generally annotated as such. However, myo-inositol oxygenase (MIOX) exemplifies a second, very different function that has evolved within the common scaffold of this superfamily. A recently discovered HD protein, PhnZ, catalyzes conversion of 2-amino-1-hydroxyethylphosphonate to glycine and phosphate, culminating a bacterial pathway for the utilization of environmentally abundant 2-aminoethylphosphonate. Using Mössbauer and EPR spectroscopies, X-ray crystallography, and activity measurements, we show here that, like MIOX, PhnZ employs a mixed-valent Fe II /Fe III cofactor for the O 2 -dependent oxidative cleavage of its substrate. Phylogenetic analysis suggests that many more HD proteins may catalyze yet-unknown oxygenation reactions using this hitherto exceptional Fe II /Fe III cofactor. The results demonstrate that the catalytic repertoire of the HD superfamily extends well beyond phosphohydrolysis and suggest that the mechanism used by MIOX and PhnZ may be a common strategy for oxidative C-X bond cleavage.

The class Ia ribonucleotide reductase (RNR) from Escherichia coli (Ec) employs a free-radical mechanism, which involves bidirectional translocation of a radical equivalent or “hole” over a distance of ∼35 Å from the stable diferric/tyrosyl-radical (Y122•) cofactor in the β subunit to cysteine 439 (C439) in the active site of the α subunit. This long-range, inter-subunit electron transfer occurs by a multi-step “hopping” mechanism via formation of transient amino acid radicals along a specific pathway and is thought to be conformationally gated and coupled to local proton transfers. Whereas constituent amino acids of the hopping pathway have been identified, details of the proton-transfer steps and conformational gating within the β sununit have remained obscure; specific proton couples have been proposed, but no direct evidence has been provided. In the key first step, the reduction of Y122• by the first residue in the hopping pathway, a water ligand to Fe1 of the diferric cluster was suggested to donate a proton to yield the neutral Y122. Here we show that forward radical translocation is associated with perturbation of the Mössbauer spectrum of the diferric cluster, especially the quadrupole doublet associated with Fe1. Density functional theory (DFT) calculations verify the consistency of the experimentally observed perturbation with that expected for deprotonation of the Fe1-coordinated water ligand. The results thus provide the first evidence that the diiron cluster of this prototypical class Ia RNR functions not only in its well-known role as generator of the enzyme's essential Y122•, but also directly in catalysis.