Protein synthesis in rabbit reticulocytes: Mechanism of protein synthesis inhibition by heme-regulated inhibitor

Authors

• A. Das, University of Nebraska - Lincoln
• O. Ralston, University of Nebraska at Lincoln
• M. Grace, University of Nebraska - Lincoln
• R. Roy, University of Nebraska - Lincoln
• P. Ghosh-Dastidar, University of Nebraska - Lincoln
• H. K. Das, University of Nebraska-Lincoln
• B. Yaghmai, University of Nebraska-Lincoln
• S. Palmieri, University of Nebraska-Lincoln
• N. K. Gupta, University of Nebraska - Lincoln

Document Type

Article

Date of this Version

1979

Comments

Published in Proc. Natl. Acad. Sci. USA Vol. 76, No. 10, pp. 5076-5079, October 1979. Used by Permission

Abstract

Partially purified Met-tRNA_f binding factor, eIF-2, was phosphorylated by using heme-regulated inhibitor (HRI). Phosphorylated eIF-2 was freed from HRI by phosphocellulose column chromatography. Analysis by isoelectric focusing showed 100% phosphorylation of the 38,000-dalton subunit of eIF-2. Both eIF-2 and eIF-2(P) formed ternary complexes with Met-tRNA_f and GTP with almost the same efficiency, and in both cases the ternary complex formation was drastically inhibited by prior addition of Mg²⁺. However, whereas the ternary complexes formed with eIF-2 could be stimulated by Co-eIF-2C at 1 mM Mg²⁺ and dissociated by Co-eIF-2B at 5 mM Mg²⁺, the ternary complexes formed with eIF-2(P) were unresponsive to both Co-eIF-2B and Co-eIF-2C. Also under conditions of eIF-2 phosphorylation, HRI drastically inhibited AUG-dependent Met-tRNA_f binding to 40S ribosomes. However, HRI (in the presence of ATP) had no effect on the joining of preformed Met-tRNA_f•40S•AUG complex to the 60S ribosomal subunit to form Met-tRNA_f•80S•AUG complex. These studies suggest that HRI inhibits protein synthesis initiation by phosphorylation of the 38,000-dalton subunit of eIF-2. HRIphosphorylated eIF-2 does not interact with at least two other protein factors, Co-eIF-2B and Co-eIF-2C, and is thus inactive in protein synthesis initiation.