The first complete amino-acid sequence of a mitochondrial chaperonin 10 is reported. The amino-terminal alanine residue is acetylated, a modification that may be required for the interaction with heptameric chaperonin 60. Part of the sequence constitutes a potential dinucleotide binding motif and is identical with 7 out of 10 residues in the GTP-binding site of p21ras. This similarity may be the structural basis for the recently discovered complex between p21ras and chaperonin 60 in intact cells (Ikawa, S. and Weinberg, R.A. (1992) Proc. Natl. Acad. Sci. USA 89, 2012-2016).