Bacterial microcompartments are widespread bacterial organelles that are made of a protein shell that surrounds and encloses various enzymes. These compartments are typically about 100-200 nanometres across and made of interlocking proteins. They do not contain lipids since they are not surrounded by a membrane. Protein-enclosed compartments are also found in eukaryotes, such as the mysterious vault complex.
Protein families forming the microcompartment shell
The shells of diverse microcompartments are composed of members of three protein families: the BMC domain protein family, the inconsistently named CsoS4 / CcmL / EutN / OrfAB family, and the encapsulins/linocin-like proteins.
The BMC protein family
In microcompartment shells that have been studied, the major constituents are proteins belonging to the Bacterial Micro-Compartment (BMC) family. The crystal structures of a number of BMC proteins have been determined and invariably reveal assembly into cyclical hexamers with a small pore in the center.
The CsoS4 family
Recent structures have revealed either a pentameric or hexameric structure in this family. In icosahedral or quasi-icosahedral carboxysomes, it is likely that the pentameric form is positioned at the vertices.
Encapsulins are a large and widely distributed family of proteins and are present in most bacteria and have been identified in Candidatus methanoregula, a species of archaea. They were originally called linocin-like proteins and thought to be a group of bacterial antibiotics, since they showed bacteriostatic activity in culture. However, structural analysis showed these to form a spherical nanocompartment that contains enzymes involved in the defenses against oxidative stress.
A recent survey indicated seven different metabolic systems encapsulated by microcompartment shells. Three are characterized:
Some bacteria can use 1,2-propanediol as a carbon source. They express a microcompartment to encapsulate a number of enzymes used in this pathway. The Pdu compartment is constructed by a set of 21 genes in a single chromosomal locus. These genes are sufficient for assembly of the microcompartment since they can be transferred between bacteria and will produce a functional structure in the recipient.
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