As inferred from the yeast TIM23 complex, the human TIMM23 complex transports precursor proteins across the inner membrane and into the matrix. As in yeast, subunits TIMM50, TIMM17, and TIMM23 are probably necessary for initiating translocation while the PAM complex with mtHSP70 (HSPA9, yeast SSC1) provides the motive force that drives the transport. mtHSP70 binding to the precursor pulls the protein into the matrix in a reaction requiring ATP hydrolysis. The yeast reaction appears to use a Brownian ratchet mechanism (Yamano et al. 2008).
In yeast experimentally verified substrates of TIM23 PAM include Hsp60 (HSP60 in human) and Yfh1 (Frataxin, FXN in human). Many other matrix proteins are believed to be substrates of the TIMM23 complex