Essential component of the TIM23 complex, a complex that mediates the translocation of transit peptide-containing proteins across the mitochondrial inner membrane. The vast majority of mitochondrial proteins are encoded by nuclear genes, synthesized in the cytosol and imported into the organelle by a multi-protein import complex. The major mitochondrial import pathway transports preproteins with cleavable N-terminal presequences. The preproteins are recognized by the receptors TOMM20 and TOMM22 of the translocase of the outer mitochondrial membrane (TOM complex) and handed over from the TOM complex to the presequence translocase of the inner mitochondrial membrane (TIM23 complex) via a two-membrane-spanning super-complex intermediate. Humans have two TIM23 orthologs. TIMM23 codes for a single protein, TIMM23, which functions as part of the TIM23 complex. TIMM23B was identified by massive sequencing and codes for two proteins, TIMM23B1 and TIMM23B2, which are highly similar to TIMM23