Bacterial microcompartments are primitive organelles composed entirely of protein subunits. The prototypical bacterial microcompartment is the carboxysome, a protein shell for sequestering carbon fixation reactions. These proteins for hexameric structure [1].

1: Protein structures forming the shell of primitive bacterial organelles. Kerfeld CA, Sawaya MR, Tanaka S, Nguyen CV, Phillips M, Beeby M, Yeates TO; Science 2005;309:936-938. PMID:16081736