The complexity of chloroplast chaperonins

• The multiplicity of chloroplast chaperonins allows for specificity and regulation.
• Dynamic equilibrium facilitates interchange between different functional oligomers.
• The Cpn60α subunits seem to play a special role in Rubisco biogenesis.
• Some chaperonins have moonlighting functions unrelated to protein folding.

Type I chaperonins are large oligomeric protein ensembles that are involved in the folding and assembly of other proteins. Chloroplast chaperonins and co-chaperonins exist in multiple copies of two distinct isoforms that can combine to form a range of labile oligomeric structures. This complex system increases the potential number of chaperonin substrates and possibilities for regulation. The incorporation of unique subunits into the oligomer can modify substrate specificity. Some subunits are upregulated in response to heat shock and some show organ-specific expression, whereas others possess additional functions that are unrelated to their role in protein folding. Accumulating evidence suggests that specific subunits have distinct roles in biogenesis of ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco).