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Most mitochondrial preproteins are maintained in a loosely folded
import-competent conformation by cytosolic chaperones, and are imported into
mitochondria by translocator complexes containing a preprotein receptor, termed
translocase of the outer membrane of mitochondria (Tom) 20. Using two-hybrid
screening, we identified arylhydrocarbon receptor-interacting protein (AIP), an
FK506-binding protein homologue, interacting with Tom20. The extreme
COOH-terminal acidic segment of Tom20 was required for interaction with
tetratricopeptide repeats of AIP. An in vitro import assay indicated that AIP
prevents preornithine transcarbamylase from the loss of import competency. In
cultured cells, overexpression of AIP enhanced preornithine transcarbamylase
import, and depletion of AIP by RNA interference impaired the import. An in
vitro binding assay revealed that AIP specifically binds to mitochondrial
preproteins. Formation of a ternary complex of Tom20, AIP, and preprotein was
observed. Hsc70 was also found to bind to AIP. An aggregation suppression assay
indicated that AIP has a chaperone-like activity to prevent substrate proteins
from aggregation. These results suggest that AIP functions as a cytosolic
factor that mediates preprotein import into mitochondria.
