Bacterial microcompartment

Bacterial microcompartments are widespread bacterial organelles characterized by a protein shell composed of members of two protein families that encapsulates various enzymes.

Protein families forming the microcompartment shell
All microcompartments are composed of members of two protein families: the BMC domain protein family and the inconsistently named CsoS4 / CcmL / EutN / OrfAB family.

The BMC protein family
The crystal structures of a number of BMC domain 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.

Types of microcompartment
A recent survey indicated seven different metabolic systems encapsulated by microcompartment shells. Three are characterized:

Carboxysomes
Carboxysomes encapsulate RuBisCo and carbonic anhydrase in carbon-fixing bacteria as part of a carbon concentrating mechanism.

Pdu microcompartments
Some bacteria can used 1,2-propanediol as a carbon source. They express a microcompartment to encapsulate a number of enzymes used in this pathway.