A view of primitive organelles

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...