Structural features of the precursor to mitochondrial XXXX aminotransferase responsible for binding to hsp70.

@entity1472 @entity55985 @entity95 @entity2733 @entity492 @entity3505
The precursor (pmAspAT) and mature ( @entity55985 ) forms of mitochondrial @entity492 aminotransferase interact with hsp70 very early during translation when synthesized in either @entity95 reticulocyte lysate or @entity3505 germ extract (Lain, B., Iriarte, A., and Martinez-Carrion. (1994) J. Biol. Chem. 269, 15588-15596). The nature of the structural elements responsible for recognition and binding of this protein to hsp70 has been studied by examining the folding and potential association with the chaperone of several engineered forms of this enzyme. Whereas pmAspAT and @entity55985 bind hsp70 very early during translation, the cytosolic form of this enzyme (cAspAT) does not interact with hsp70. A fusion protein consisting of the mitochondrial presequence peptide attached to the @entity1472 terminus of cAspAT associates with hsp70 only after the protein has acquired its native-like conformation, apparently through binding to the presequence exposed on the surface of the folded protein. Deletion of the @entity1472 -terminal segment of @entity55985 or its replacement with the corresponding domain from the cytosolic isozyme eliminates the cotranslational binding of hsp70 to the mitochondrial protein. We conclude that both the presequence and @entity2733 -terminal region of pmAspAT represent recognition signals for binding of hsp70 to the newly synthesized mitochondrial precursor. Results from competition studies with synthetic peptides support this conclusion. The ability of hsp70 to discriminate between these two highly homologous proteins probably involves the recognition of specific sequence elements in the @entity2733 -terminal portion of the mitochondrial protein and may relate to their separate localization in the cell. A slower folding rate and higher affinity for cytosolic chaperones may represent evolutionary adaptations of translocated mitochondrial proteins to ensure their efficient importation into the organelle.
