Abstract
Many bacteria conditionally express proteinaceous organelles referred to here as microcompartments (Fig. 1). These microcompartments are thought to be involved in a least seven different metabolic processes and the number is growing. Microcompartments are very large and structurally sophisticated. They are usually about 100β150 nm in cross section and consist of 10,000β20,000 polypeptides of 10β20 types. Their unifying feature is a solid shell constructed from proteins having bacterial microcompartment (BMC) domains. In the examples that have been studied, the microcompartment shell encases sequentially acting metabolic enzymes that catalyze a reaction sequence having a toxic or volatile intermediate product. It is thought that the shell of the microcompartment confines such intermediates, thereby enhancing metabolic efficiency and/or protecting cytoplasmic components. Mechanistically, however, this creates a paradox. How do microcompartments allow enzyme substrates, products and cofactors to pass while confining metabolic intermediates in the absence of a selectively permeable membrane? We suggest that the answer to this paradox may have broad implications with respect to our understanding of the fundamental properties of biological protein sheets including microcompartment shells, Sβlayers and viral capsids. BioEssays 30:1084β1095, 2008. Β© 2008 Wiley Periodicals, Inc.