In what form do proteins cross the mitochondrial membranes?

Mitochondrial proteins are not folded when they are synthesized in the cytosol. Instead, they are attached to Hsp 70 chaperone proteins which hold them in extended form for delivery to the mitochondria. On reaching the mitochondria the protein binds to an outer membrane translocase (TOM) protein. This binding is helped by Hsp70, but as the protein translocates across the membrane the chaperones are removed and do not enter the mitochondrion. Proteins transpoed to the matrix or inner membrane are passed straight from the TOM translocase to an inner membrane translocase (TIM) which transfers them, still in extended form, to the matrix, and to the inner membrane. In the matrix, they are finally folded by mitochondrial chaperones and chaperonins. Reference: Harper; 30th edition