A crucial role of the mitochondrial protein import receptor MOM19 for the biogenesis of mitochondria

The novel genetic method of "sheltered RIP" (repeat induced point
mutation) was used to generate a Neurospora crassa mutant in which
MOM19, a component of the protein import machinery of the
mitochondrial outer membrane, can be depleted. Deficiency in MOM19
resulted in a severe growth defect, but the cells remained viable.
The number of mitochondrial profiles was not grossly changed, but
mutant mitochondria were highly deficient in cristae membranes,
cytochromes, and protein synthesis activity. Protein import into
isolated mutant mitochondria was decreased by factors of 6 to 30
for most proteins from all suborganellar compartments. Proteins
like the ADP/ATP carrier, MOM19, and cytochrome c, whose import
into wild-type mitochondria occurs independently of MOM19 became
imported normally showing that the reduced import activities are
solely caused by a lack of MOM19. Depletion of MOM19 reveals a
close functional relationship between MOM19 and MOM22, since loss
of MOM19 led to decreased levels of MOM22 and reduced protein
import through MOM22. Furthermore, MOM72 does not function as a
general backup receptor for MOM19 suggesting that these two
proteins have distinct precursor specificities. These findings
demonstrate that the import receptor MOM19 fulfills an important
role in the biogenesis of mitochondria and that it is essential for
the formation of mitochondria competent in respiration and
phosphorylation.