Abstract
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, Cpn60α and Cpn60β, and duplication of Cpn60α and Cpn60β genes occurs in a high proportion of plants. However, the importance of multiple Cpn60α and Cpn60β genes in plants is poorly understood. In this study, we found that loss-of-function of CPNA2 (AtCpn60α2), a gene encoding the minor Cpn60α subunit in Arabidopsis thaliana, resulted in arrested embryo development at the globular stage, whereas the other AtCpn60α gene encoding the dominant Cpn60α subunit, CPNA1 (AtCpn60α1), mainly affected embryonic cotyledon development at the torpedo stage and thereafter. Further studies demonstrated that CPNA2 can form a functional chaperonin with CPNB2 (AtCpn60β2) and CPNB3 (AtCpn60β3), while the functional partners of CPNA1 are CPNB1 (AtCpn60β1) and CPNB2. We also revealed that the functional chaperonin containing CPNA2 could assist the folding of a specific substrate, KASI (β-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 Cpn60α subunits in Arabidopsis play different roles during embryo development through forming distinct chaperonins with specific AtCpn60β to assist the folding of particular substrates, thus providing novel insights into functional divergence of Cpn60α subunits in plants.
Highlights
Chaperonins are a class of molecular chaperones that are characterized by a barrel-shaped architecture formed by two stacked oligomeric rings consisting of several subunits of approximately 60 kDa
Further studies demonstrated that AtCpn60α2 could form functional chaperonins with AtCpn60β2 and AtCpn60β3 to assist in folding of the substrate KASI, which is important for the formation of heart-shaped embryos
Our results suggest that duplication of Cpn60α genes in higher plants can increase the potential number of chloroplast chaperonin substrates and provide chloroplast chaperonins with more roles in plant growth and development, revealing the relationship between duplication and functional specialization of chaperonin genes
Summary
Chaperonins are a class of molecular chaperones that are characterized by a barrel-shaped architecture formed by two stacked oligomeric rings consisting of several subunits of approximately 60 kDa. An alternative model, known as the symmetric “football” model, was recently proposed In this model, the exchange of ADP to ATP is extremely rapid in the presence of abundant substrate protein, resulting in formation of a symmetric “football” intermediate that has GroES bound to both rings and can assist in protein folding simultaneously in both rings. The exchange of ADP to ATP is extremely rapid in the presence of abundant substrate protein, resulting in formation of a symmetric “football” intermediate that has GroES bound to both rings and can assist in protein folding simultaneously in both rings This “football” intermediate would be reverted to the “bullet” conformation upon ATP hydrolysis [5,6,7]. In the previous study, Wang and coworkers found that some specific mutations of GroEL can improve the folding of GFP, but the mutated GroEL has a reduced ability to function as general chaperones, suggesting a conflict between the increased ability of GroEL to fold particular substrates and its general ability to fold a wide range of substrates, and this conflict would be resolved by duplication and variation of chaperonin genes [9]
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