The 70-Kilodalton Heat Shock Cognate Can Act as a Molecular Chaperone during the Membrane Translocation of a Plant Secretory Protein Precursor.

When a model secretory precursor was synthesized in vitro and analyzed by rate-zonal sedimentation, it appeared to be associated with other proteins present in a wheat germ extract. At least one of the associated proteins is a member of the 70-kD family of stress proteins. It was possible to immunoprecipitate the secretory precursor with anti-heat shock cognate 70 (Hsc70) antibodies in the absence but not in the presence of ATP, suggesting that the association was specific. ATP-sensitive association is one diagnostic characteristic of molecular chaperone-type proteins. Increasing incubation temperature decreased the amount of precursor associated with Hsc70. A method was developed for the removal of Hsc70 from a wheat germ in vitro translation mixture by immunoprecipitation. Cotranslational translocation and processing of the secretory precursor by maize endosperm microsomes were inefficient in the Hsc70-depleted system but were greatly stimulated by addition of purified preparations of various heat shock 70 proteins (Hsp70s). Cytosolic Hsc70 from maize endosperm was capable of autophosphorylation in vitro. Phosphorylated Hsc70 was much less efficient in promoting membrane translocation of the secretory precursor. These results suggest that chaperone function in vivo could be regulated by phosphorylation.