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 inner membrane. 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 SORT complex as the configuration that delivers inner membrane proteins. The TIMM21 subunit is required for this activity. In yeast, the N-terminal presequences of precursors first interact with TIM50 and TIM23 (TIMM50 and TIMM23 in human, Zhang et al. 2013). The TIM17 and TIM23 subunits (TIMM17 and TIMM23 in human) form a channel and are required to initiate translocation of precursors.
In yeast experimentally verified substrates of the TIM23 SORT complex targeted to the inner membrane include CYB2, DLD (LDHD in human), ATP9 (ATP5G1 in human), COQ2, TIM54 (TIMM54 in human), COX4, COX5A, and ATP2 (ATP5B in human). Many other inner membrane proteins are believed to be substrates of the TIMM23 complex.