The crystal structure of mitochondrial F1-ATPase indicates that the alpha and beta subunits fold into a structure defined by three domains: the top beta-barrel domain, the middle nucleotide-binding domain, and the C-terminal alpha-helix bundle domain (Abrahams et al., 1994); Bianchet et al., 1998). The beta-barrel domains of the alpha and beta subunits form a crown structure at the top of F1, which was suggested to stabilize it (Abrahams et al. 1994). In this study, the role of the beta-barrel domain in the alpha and beta subunits of the yeast Saccharomyces cerevisiae F1, with regard to its folding and assembly, was investigated. The beta-barrel domains of yeast F1alpha and beta subunits were expressed individually and together in Escherichia coli. When expressed separately, the beta-barrel domain of the beta subunit formed a large aggregate structure, while the domain of the alpha subunit was predominately a monomer or dimer. However, coexpression of the beta-barrel domain of alpha subunit with the beta-barrel domain of beta subunit, greatly reduced the aggregation of the beta subunit domain. Furthermore, the two domains copurified in complexes with the major portion of the complex found in a small molecular weight form. These results indicate that the beta-barrel domain of the alpha and beta subunits interact specifically with each other and that these interactions prevent the aggregation of the beta-barrel domain of the beta subunit. These results mimic in vivo results and suggest that the interactions of the beta-barrel domains may be critical during the folding and assembly of F1.
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