Domain organization of the monomeric form of the Tom70 mitochondrial import receptor

Tom70 is a mitochondrial protein import receptor composed of 11 tetratricopeptide repeats (TPRs). The first three TPRs form an N-terminal domain that recruits heat shock protein family chaperones, while the eight C-terminal TPRs form a domain that receives, from the bound chaperone, mitochondrial precursor proteins destined for import. Analytical ultracentrifugation and solution small-angle X-ray scattering (SAXS) analysis characterized Tom70 as an elongated monomer. A model for the Tom70 monomer was proposed based on the alternate interpretation of the domain pairings observed in the crystal structure of the Tom70 dimer and refined against the SAXS data. In this open model of the Tom70 monomer, the chaperone- and precursor-binding sites are exposed and lay side by side on one face of the molecule. Fluorescence anisotropy measurements indicated that monomeric Tom70 can bind both chaperone and precursor peptides and that chaperone peptide binding does not alter the affinity of Tom70 for the precursor peptide. SAXS was unable to detect any shape change in Tom70 upon chaperone binding. However, molecular modeling indicated that chaperone binding is incompatible with Tom70 dimer formation. It is proposed that the Tom70 monomer is the functional unit mediating initial chaperone docking and precursor recognition.