As inferred from the yeast TIM23 complex, the human TIMM23 complex resides in the inner membrane of the mitochondrion and transfers precursor proteins to the matrix. The TIMM23 complex appears to adopt different configurations (and perhaps different subunit compositions) depending on whether the substrate is destined for the inner membrane or the matrix. Here we refer to the TIMM23 PAM complex as the configuration that delivers inner membrane proteins. The PAM17 subcomplex is required for this activity. The N-terminal presequence of precursors first interacts with TIMM50 and TIMM23 (Zhang et al. 2013). The TIMM17 and TIMM23 subunits form a channel and are required to initiate translocation of precursors.
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.