In Capsule Form

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Science  12 Sep 2008:
Vol. 321, Issue 5895, pp. 1421
DOI: 10.1126/science.321.5895.1421a

Intracellular membrane-bounded compartments—the mitochondrion, chloroplast, and nucleus—define the modern eukaryote. Bacteria make do without internal membranes, yet it is becoming evident that they do possess, nevertheless, intracellular nanosized environments. Sutter et al. describe the structure of the latest such oasis—an icosahedral shell 25 nm in diameter, formed by 60 monomers of the protein encapsulin. As one might intuit, encapsulin is structurally similar to viral capsid proteins, although any ancestral commonalities are no longer visible in their amino acid sequences. Compartments can be useful for sequestering small molecules, either because they are valuable or because they might cause harm if allowed to diffuse. Encapsulin appears to offer both kinds of functions, because biochemical experiments identified docking sites for a peroxidase and for a ferritin-like protein, with the latter catalyzing the storage of iron as ferrihydrite and the former detoxifying potentially toxic oxygen species. — GJC

Nat. Struct. Mol. Biol. 10.1038/nsmb.1473 (2008).

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