Mitochondrial ATP synthase subunit 8 of the yeast Saccharomyces cerevisiae and of the filamentous fungus Aspergillus nidulans have the same length and similar structural motifs. However, the two proteins share only 50% identical residues, with the conserved residues being concentrated in the N- and C-terminal domains. We have investigated whether it is amino acid sequence or overall structural motifs that are required for subunit 8 function. PCR was used to construct a gene encoding A. nidulans subunit 8 fused to an N-terminal cleavable mitochondrial targeting sequence. Following expression in the nucleus of a yeast strain deficient in subunit 8, the chimaeric precursor targeted the subunit 8 protein back to the mitochondrion. The A. nidulans subunit 8 was found to be able to restore growth on non-fermentable substrate at 18 degrees C and 28 degrees C, but not at 36 degrees C. Given the sequence divergence between subunit 8 of A. nidulans and that of S. cerevisiae, this finding suggests that common structural motifs are important for subunit 8 function.