Evidence that Hsc70 negatively modulates the activation of the heme‐regulated eIF‐2α kinase in rabbit reticulocyte lysate

Abstract

The role of the heat-shock cognate protein, Hsc70, in regulating the activity of the heme-regulated eIF-2alpha kinase (HRI) in hemin-supplemented rabbit reticulocyte lysate (RRL) in response to heat and oxidative stress was examined and compared with the effect of Hsc70 on HRI activation in response to heme deficiency. Hsc70 suppressed eIF-2alpha phosphorylation and maintained the guanine nucleotide exchange activity of eIF-2B in heme-deficient RRL and in hemin-supplemented RRL exposed to elevated temperatures (42 degrees C), denatured protein (reduced carboxymethylated bovine serum albumin, RCM-BSA), oxidized glutathione or Hg2+. The ability of Hsc70 to inhibit HRI activation was mediated through its ability to inhibit the hyper-autophosphorylation of transformed HRI, which causes the hyperactivation of HRI. Maintenance of protein-synthesis rate was observed to be an unreliable indicator of the ability of Hsc70 to suppress HRI activation in response to stress. While Hsc70 completely reversed protein synthesis inhibition caused by Hg2+. Hsc70 only partially reversed translational inhibition caused by oxidized glutathione (GSSG) or heat shock. The inability of Hsc70 to fully protect protein synthesis from inhibition induced by heat shock or GSSG was due to inability of Hsc70 to protect eIF-4 E from heat-induced dephosphorylation, and its inability to protect translational elongation from GSSG-induced inhibition, respectively. Activation of HRI in heat-shocked hemin-supplemented lysate correlated with a marked decrease in the pool of Hsc70 that was available to bind RCM-BSA and the loss of the interaction of Hsc70 with HRI. These observations indicate that heat shock induced the accumulation of a sufficient quantity of Hsc70 binding substrates (e.g., denatured protein) to sequester Hsc70 and inhibit the ability of Hsc70 to suppress HRI activation. Our results indicate that Hsc70 not only negatively modulates the activation of HRI in heme-deficienct RRL, but also in hemin-supplemented RRL in response to heat and oxidative stress.