A Chaperonin Subunit with Unique Structures Is Essential for Folding of a Specific Substrate

Type I chaperonins are large, double-ring complexes present in bacteria (GroEL), mitochondria (Hsp60), and chloroplasts (Cpn60), which are involved in mediating the folding of newly synthesized, translocated, or stress-denatured proteins. In Escherichia coli, GroEL comprises 14 identical subunits and has been exquisitely optimized to fold its broad range of substrates. However, multiple Cpn60 subunits with different expression profiles have evolved in chloroplasts. Here, we show that, in Arabidopsis thaliana, the minor subunit Cpn60 beta 4 forms a heterooligomeric Cpn60 complex with Cpn60 alpha 1 and Cpn60 beta 1-beta 3 and is specifically required for the folding of NdhH, a subunit of the chloroplast NADH dehydrogenase-like complex (NDH). Other Cpn60 beta subunits cannot complement the function of Cpn60 beta 4. Furthermore, the unique C-terminus of Cpn60 beta 4 is required for the full activity of the unique Cpn60 complex containing Cpn60 beta 4 for folding of NdhH. Our findings suggest that this unusual kind of subunit enables the Cpn60 complex to assist the folding of some particular substrates, whereas other dominant Cpn60 subunits maintain a housekeeping chaperonin function by facilitating the folding of other obligate substrates.