SHELTERED DISRUPTION OF NEUROSPORA-CRASSA MOM22, AN ESSENTIAL COMPONENT OF THE MITOCHONDRIAL PROTEIN IMPORT COMPLEX

MOM22 is a component of the protein import complex of the mitochondria l outer membrane of Neurospora crassa. Using the newly developed proce dure of 'sheltered disruption', we created a heterokaryotic strain har boring two nuclei, one with a null aIlele of the mom-22 gene and the o ther with a wild-type allele. Homokaryons bearing the mom-22 disruptio n could not be isolated, suggesting that mom-22 is an essential gene. The mutant nucleus can be forced to predominate in the heterokaryon th rough the use of specific nutritional and inhibitor resistance markers . Cultivation of the heterokaryon under conditions favoring the mutant nucleus resulted in selective depletion of MOM22. MOM22-depleted cell s did not grow and contained mitochondria with an altered morphology a nd protein composition. Protein import into isolated, MOM22-depleted m itochondria was abolished for most precursor proteins destined for all subcompartments. In contrast, precursors of MOM19, MOM22 and MOM72 be came inserted normally into the outer membrane, defining a novel MOM22 -independent import pathway which remained intact in mutant mitochondr ia. Furthermore, the specific binding of the ADP/ATP carrier to the ou ter membrane was unaffected, but subsequent transport across the outer membrane did not occur. Our data show that MOM22 is an essential comp onent of Neurospora cells specifically required for the biogenesis of mitochondria.