Role of the N- and C-termini of porin in import into the outer membrane of Neurospora mitochondria.

The signals for targeting and assembly of porin, a protein of the mitochondrial outer membrane, have not been clearly defined. Targeting information has been hypothesized to be contained in the N-terminus, which may form an amphipathic alpha-helix, and in the C-terminal portion of the protein. Here, the role of the extreme N- and C-termini of porin from Neurospora crassa in its import into the mitochondrial outer membrane was investigated. Deletion mutants were constructed which lacked the N-terminal 12 or 20 residues or the C-terminal 15 residues. The porins truncated at their N-termini were imported in a receptor-dependent manner into the outer membrane of isolated mitochondria. When integrated into the outer membrane, these preproteins displayed an increased sensitivity to protease as compared to wild-type porin. In contrast, mutant porin truncated at its C-terminus did not acquire protease resistance upon incubation with mitochondria. Thus, unlike most other mitochondrial preproteins, porin appears to contain important targeting and/or assembly information at its C-terminus, rather than at the N-terminus.