Influence of Cytoplasmatic Folding on Mitochondrial Import

The eukaryotic cell, with its organelle organization, represents a challenge for protein traffic. Contrary to what occurs in the endoplasmic reticulum, mitochondrial protein import is proposed to occur postranslationaly, as proteins are synthesized in cytoplasmic ribosomes and only then imported to the organelle. Because the diameter of the Tom and Tim pores is too narrow for the passage of a folded protein, it is assumed that polypeptides must be already in an unfolded, import competent, state for organelle entry. However, it has been suggested that mitochondria might be able to actively unfold proteins itself at the outer membrane. Here we discuss the influence of cytoplasmatic protein folding on mitochondrial import. Despite the contribution of active mitochondrial unfolding to protein import is not excluded, this mechanism is inconsistent with a number of experimental evidences. Accordingly, other alternative models for mitochondrial import are here discussed. Understanding the molecular constraints regulating this process is of crucial importance, since its failure can lead to a number of pathological situations.