Essential component of the TIM23 complex, a complex that mediates the translocation of transit peptide-containing proteins across the mitochondrial inner membrane. Required to direct preproteins in transit and direct them to the channel protein TIM23, and possibly facilitates transfer of the translocating proteins from the TOM complex to the TIM23 complex (By similarity). Component of the TIM23 complex, at least composed of TIM23, TIM17, TIM50 and TIM21. Interacts with preproteins in transit (By similarity). Belongs to the TIM50 family.