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A view of primitive organelles

CcmK subunits (purple and silver) pack into a hexamer with a small central pore. YEATES/AAAS

Though the textbook view of bacteria is one of a homogenous mixed bag of materials, many bacteria contain large polyhedral protein shells believed to serve as primitive organelles. The first of these bodies to be isolated, called a carboxysome, holds large amounts of RuBisCO, which uses CO₂ to form small sugars. Now, the first high resolution views of carboxysomes, from Cheryl Kerfeld, Todd Yeates (University of California, Los Angeles, CA), and colleagues, suggest that these structures do have many of the functional aspects of organelles.

Carboxysome proteins CcmK2 and CcmK4 crystallized in tightly packed hexameric units that assembled, also tightly, into sheets. "The six subunits leave very little space down the middle," says Yeates. "And the space left is highly [positively] charged." The proximity of so many charges is more than coincidence. For instance, the pore might regulate ion passage, allowing through small negatively charged molecules such as bicarbonate (which is converted to CO₂) but blocking oxygen (which inhibits RuBisCO). Mutagenesis of the pore-lining residues should allow the group to test this hypothesis.

Shells formed by CcmK homologues do more than fix carbon. The breakdown of propanediol in Salmonella, for instance, occurs inside polyhedral bodies, which might confine a toxic pathway intermediate. As they are not membrane bound, the compartments might not be considered true organelles. But they are, Yeates says, "if organelles are defined by purpose—spatial separation and control of what comes in and out. The bacteria are trying to achieve the same level of sophistication [found in eukaryotic organelles]."

Structurally, carboxysomes and other polyhedral bodies resemble viral particles, suggesting that perhaps bacteria co-opted viruses in the same way that led eukaryotic cells to contain bacteria-like organelles such as mitochondria. But small proteins with CcmK-like architectures seem to have evolved independently multiple times. There is, therefore, no sure way to know whether the primitive bacterial organelles were originally viruses or arose on their own.

Reference:

Kerfeld, C.A., et al. 2005. Science. 309:936–938.[Abstract/Free Full Text]

Nicole LeBrasseur

lebrasn{at}rockefeller.edu

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This Article Full Text (PDF, 422K) PPT slides of all figures Alert me when this article is cited Services Email this article Similar articles in this journal Alert me to new content in the JCB Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by LeBrasseur, N. Search for Related Content PubMed Articles by LeBrasseur, N. Social Bookmarking                            What's this?