Compartmentalized bacteria

[Hercules Rockefeller]

I thought this is fascinating, so I'm sharing.

BIOCHEMISTRY: In Capsule Form

Gilbert Chin

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).

I'm wondering: is the structural similarity of encapsulin to viral capsid proteins due solely to function, or does this protein have anything to do with the “evolution” of viruses from living cells? The text states that any “ancestral commonalities are no longer visible in their amino acid sequences” – this comment doesn’t seem to preclude that they might have had ancestral commonalities in the past.

 

[Walter L. Wagner]

Interesting.

It appears that the author is suggesting an analog for bacteria similar to the symbiotic evolution of engulfed bacteria inside of eukaryotes transforming over time into cellular components such as mitochondria and chloroplasts.

In this case, it would be an engulfed virus with protein-shell being transformed into 'encapsulin', but no longer having the other DNA/RNA components of the virus, just the encapsulin coding portion that is now different from normal viral protein coats.

I believe that further work might elucidate whether this is an example of evolution, or simply an example of function-dictates-form.