Functional divergence of chloroplast Cpn60 alpha subunits during Arabidopsis embryo development

Chaperonins are a class of molecular chaperones that assist in the folding and assembly of a wide range of substrates. In plants, chloroplast chaperonins are composed of two different types of subunits, Cpn60 alpha and Cpn60 beta, and duplication of Cpn60 alpha and Cpn60 beta genes occurs in a high proportion of plants. However, the importance of multiple Cpn60 alpha and Cpn60 beta genes in plants is poorly understood. In this study, we found that loss-of-function of CPNA2 (AtCpn60 alpha 2), a gene encoding the minor Cpn60 alpha subunit in Arabidopsis thaliana, resulted in arrested embryo development at the globular stage, whereas the other AtCpn60 alpha gene encoding the dominant Cpn60 alpha subunit, CPNA1 (AtCpn60 alpha 1), mainly affected embryonic cotyledon development at the torpedo stage and thereafter. Further studies demonstrated that CPNA2 can form a functional chaperonin with CPNB2 (AtCpn60 beta 2) and CPNB3 (AtCpn60 beta 3), while the functional partners of CPNA1 are CPNB1 (AtCpn60 beta 1) and CPNB2. We also revealed that the functional chaperonin containing CPNA2 could assist the folding of a specific substrate, KASI (beta-ketoacyl-[acyl carrier protein] synthase I), and that the KASI protein level was remarkably reduced due to loss-of-function of CPNA2. Furthermore, the reduction in the KASI protein level was shown to be the possible cause for the arrest of cpna2 embryos. Our findings indicate that the two Cpn60 alpha subunits in Arabidopsis play different roles during embryo development through forming distinct chaperonins with specific AtCpn60 beta to assist the folding of particular substrates, thus providing novel insights into functional divergence of Cpn60 alpha subunits in plants.