Focus on cellular biochemistry - Mechanisms of mitochondrial protein import

Import of nuclear encoded precursor proteins into and across the mitochondrial inner membrane is mediated by two distinct translocases, the TIM23 complex and the TIM22 complex, which both cooperate with the general preprotein translocase of the outer membrane, TOM complex. The TIM23 complex mediates import of preproteins with a positively charged matrix-targeting signal, while the TIM22 complex mediates the import of a class of integral inner-membrane proteins which do not carry a matrix-targeting signal. The focus of this review is on the structural organization and function of components that act along these import pathways and their conservation between species.