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.