Essential component of the TIM23 complex, a complex that mediates the translocation of transit peptide-containing proteins across the mitochondrial inner membrane. Component of the TIM23 complex at least composed of Tim23, Tim17 (Tim17a1, Tim17a2 or Tim17b1) and a Tim50. The complex interacts with the Tim44 component of the PAM complex (By similarity). Belongs to the Tim17/Tim22/Tim23 family.