A CONSTITUTIVE FORM OF HEAT-SHOCK PROTEIN 70 IS LOCATED IN THE OUTER MEMBRANES OF MITOCHONDRIA FROM RAT-LIVER

HSP73, the constitutive form of heat-shock protein 70, has been implic ated in the translocation of preproteins across the mitochondrial memb ranes, being required for maintaining mitochondrial preproteins in an import competent conformation. Here we report that highly purified mit ochondrial outer membranes contain a protein indistinguishable from HS P73 as a tightly associated peripheral component of the membrane. This membrane form of HSP73 was photolabelled with [alpha-P-32]ATP and cou ld be released from the outer membrane with sodium carbonate, but not after incubation of the membranes with salt or with ATP. A sensitive i mmunoassay with an anti-HSP73 monoclonal antibody, revealed the associ ation of HSP73 with mitochondrial outer membrane vesicles at a level s imilar to that of preprotein import receptors.