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 F-1, 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 F-1 (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 (alphabeta)(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 Atp11p(TRNC)) that retains molecular chaperone function as determined in vitro with both a surrogate substrate ( reduced insulin) and the natural substrate (F-1 beta). Moreover, preliminary N-15-H-1 heteronuclear single quantum coherence spectra obtained with Atp11p(TRNC) indicate that the truncated protein is well ordered and amenable to structure determination by nuclear magnetic resonance.