The pentameric vertex proteins are necessary for the icosahedral carboxysome shell to function as a CO2 leakage barrier.
BACKGROUND
Carboxysomes are polyhedral protein microcompartments found in many autotrophic bacteria; they encapsulate the CO(2) fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) within a thin protein shell and provide an environment that enhances the catalytic capabilities of the enzyme. Two types of shell protein constituents are common to carboxysomes and related microcompartments of heterotrophic bacteria, and the genes for these proteins are found in a large variety of bacteria.


RESULTS
We have created a Halothiobacillus neapolitanus knockout mutant that does not produce the two paralogous CsoS4 proteins thought to occupy the vertices of the icosahedral carboxysomes and related microcompartments. Biochemical and ultrastructural analyses indicated that the mutant predominantly forms carboxysomes of normal appearance, in addition to some elongated microcompartments. Despite their normal shape, purified mutant carboxysomes are functionally impaired, although the activities of the encapsulated enzymes are not negatively affected.


CONCLUSIONS
In the absence of the CsoS4 proteins the carboxysome shell loses its limited permeability to CO(2) and is no longer able to provide the catalytic advantage RubisCO derives from microcompartmentalization. This study presents direct evidence that the diffusion barrier property of the carboxysome shell contributes significantly to the biological function of the carboxysome.
Cai
Fei
F
Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi, United States of America.
Menon
Balaraj B
BB
Cannon
Gordon C
GC
Curry
Kenneth J
KJ
Shively
Jessup M
JM
Heinhorst
Sabine
S
D001419:Bacteria / Q000378:metabolism*; D002245:Carbon Dioxide / Q000737:chemistry*; D004269:DNA, Bacterial / Q000378:metabolism; D004058:Diffusion; D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial; D005838:Genotype; D044222:Halothiobacillus / Q000235:genetics*; D046529:Microscopy, Electron, Transmission; D016296:Mutagenesis; D009154:Mutation; D015388:Organelles / Q000378:metabolism; D010455:Peptides / Q000737:chemistry; D011994:Recombinant Proteins / Q000737:chemistry; D012273:Ribulose-Bisphosphate Carboxylase / Q000737:chemistry*
D004269:DNA, Bacterial; D010455:Peptides; D011994:Recombinant Proteins; D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase