Import pathways of precursor proteins into mitochondria

The precursor of porin, a mitochondrial outer membrane protein,
competes for the import of precursors destined for the three other
mitochondrial compartments, including the Fe/S protein of the bc1-
complex (intermembrane space), the ADP/ATP carrier (inner
membrane), subunit 9 of the F0-ATPase (inner membrane), and subunit
beta of the F1- ATPase (matrix). Competition occurs at the level of
a common site at which precursors are inserted into the outer
membrane. Protease- sensitive binding sites, which act before the
common insertion site, appear to be responsible for the specificity
and selectivity of mitochondrial protein uptake. We suggest that
distinct receptor proteins on the mitochondrial surface
specifically recognize precursor proteins and transfer them to a
general insertion protein component (GIP) in the outer membrane.
Beyond GIP, the import pathways diverge, either to the outer
membrane or to translocation contact-sites, and then subsequently
to the other mitochondrial compartments.