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 presequences of proteins targeted to the inner membrane are transferred to the matrix where they are cleaved. Sequences in the mature regions of the proteins then interact with the TIMM23 complex to halt transfer across the inner membrane and the proteins are released laterally into the inner membrane. TIMM21 is required.
In yeast experimentally verified substrates of the TIM23 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.