N-TERMINAL HYDROPHOBIC SORTING SIGNALS OF PREPROTEINS CONFER MITOCHONDRIAL HSP70 INDEPENDENCE FOR IMPORT INTO MITOCHONDRIA

The requirement of mitochondrial hsp70 (mt-hsp70) for the import of a series af preproteins containing hydrophobic sorting signals into isol ated yeast mitochondria was investigated. Here we demonstrate that the presence of such a sorting signal in proximity to the N-terminal matr ix-targeting sequence of 21 preprotein can secure a translocating poly peptide chain in the fashion channel in a manner that does nea; requir e mt-hsp70 activity. Trapping the translocating chain in this fashion leads to efficient processing by the mitochondrial processing peptidas e and to complete translocation across the outer mitochondrial membran e into the inner membrane space. These mt-hsp70-independent effects ap pear to be exerted at the level of the inner membrane through an inter action of the hydrophobic core of the sorting signal with component(s) of the translocase of the inner membrane. Hydrophobic sorting signals of inner membrane proteins inserted into the membrane from the matrix , as well as those of intermembrane space proteins, are capable of cau sing this mt-hsp70-independent stabilization, demonstrating that this phenomenon is not unique to those preproteins normally sorted to the i ntermembrane space.