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

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Abstract
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 α‐helix, and in the C‐terminal portion of the protein. Here, the role of the extreme N‐ and C‐termini of porin from Neurospora crussa 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.