Organization, structure, and assembly of alpha-carboxysomes determined by electron cryotomography of intact cells.
Carboxysomes are polyhedral inclusion bodies that play a key role in autotrophic metabolism in many bacteria. Using electron cryotomography, we examined carboxysomes in their native states within intact cells of three chemolithoautotrophic bacteria. We found that carboxysomes generally cluster into distinct groups within the cytoplasm, often in the immediate vicinity of polyphosphate granules, and a regular lattice of density frequently connects granules to nearby carboxysomes. Small granular bodies were also seen within carboxysomes. These observations suggest a functional relationship between carboxysomes and polyphosphate granules. Carboxysomes exhibited greater size, shape, and compositional variability in cells than in purified preparations. Finally, we observed carboxysomes in various stages of assembly, as well as filamentous structures that we attribute to misassembled shell protein. Surprisingly, no more than one partial carboxysome was ever observed per cell. Based on these observations, we propose a model for carboxysome assembly in which the shell and the internal RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) lattice form simultaneously, likely guided by specific interactions between shell proteins and RuBisCOs.
Iancu
Cristina V
CV
Division of Biology, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.
Morris
Dylan M
DM
Dou
Zhicheng
Z
Heinhorst
Sabine
S
Cannon
Gordon C
GC
Jensen
Grant J
GJ
D001419:Bacteria / Q000166:cytology* / Q000648:ultrastructure*; D020285:Cryoelectron Microscopy; D003594:Cytoplasmic Granules / Q000378:metabolism / Q000648:ultrastructure; D004602:Elements; D002479:Inclusion Bodies / Q000648:ultrastructure*; D011122:Polyphosphates / Q000378:metabolism; D014054:Tomography
D004602:Elements; D011122:Polyphosphates