A purified subfragment of yeast Atp11p retains full molecular chaperone activity

Atp11p is a molecular chaperone of the mitochondrial matrix that participates in the biogenesis pathway to form F1, the catalytic unit of the ATP synthase. Affinity tag pull-down assays and yeast two-hybrid screens have shown that Atp11p binds to free beta subunits of F1 (Wang, Z. G., and Ackerman, S. H. (2000) J. Biol. Chem. 275, 5767-5772). This binding action prevents the beta subunit from associating with itself in non-productive complexes and fosters the formation of a (alpha beta)3 hexamer. Following the premise that Atp11p action is mediated primarily through a surface (as opposed to specific amino acids, as in an enzyme active site), solving its three-dimensional structure so that we may learn how the shape of the protein influences its function is a high priority. Recombinant yeast Atp11p has proven refractory for such analysis because of the presence of a disordered region in the protein. In this article, we show that removal of 67 residues from the amino terminus of recombinant Atp11p yields a subfragment of the protein (called Atp11pTRNC) that retains molecular chaperone function as determined in vitro with both a surrogate substrate (reduced insulin) and the natural substrate (F1 beta). Moreover, preliminary 15N-1H heteronuclear single quantum coherence spectra obtained with Atp11pTRNC indicate that the truncated protein is well ordered and amenable to structure determination by nuclear magnetic resonance.