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