Fig. 1

Schematic relationship between molecular chaperones and the ORANGE protein family. Molecular chaperones cover a diverse group of protein families with the common role of regulating protein quality control (PQC) and proteostasis. The HSP70 (Heat shock protein 70) family plays large roles in PQC in plants, particularly in chloroplasts. DnaJ co-chaperones provide HSP70 target specificity via interacting with both HSP70 and their targets. Classes DnaJA-C depend on HSP70 for functions. The closely related classes DnaJD-F evolved from the former and appear to regulate PQC independent of HSP70 proteins. The DnaJE1 class is of particular interest for the involvement of its members in chloroplast protein stability and complex formation. One such member is the ORANGE (OR) family, consisting of OR in algae and higher plants with ORANGE-like (ORL) appearing only in higher plants. Two isoforms of OR, termed OR (wild type) and OR^OR (mutant including OR^His) are of noteworthy study because of their specific roles in regulating carotenoid levels and chromoplast biogenesis