Differential Activation of Two Protease‐Activated Protein Kinases from Reticulocytes by a Ca2+‐Stimulated Protease and Identification of Phosphorylated Translational Components

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Abstract
Two protein kinases were partially purified from rabbit reticulocytes and shown to be activated by limited proteolysis with trypsin. Reticulocyte lysate was examined for protease activities which might be involved in activation of the protein kinases in vivo. Two neutral proteases, differentially activated by Fe2+ and Ca2+, were identified and partially purified. The Ca2+-stimulated protease specifically activated protease-activated kinase II; no effect was observed on protease-activated kinase I. The Fe2+-stimulated protease was not active on either protein kinase. The protease-activated kinases were examined using initiation factors (eIF) and 40-S ribosomal subunits as substrate. Protease-activated kinase I phosphorylated 1 subunit of eIF-3 (MW 130,000), eIF-4B and 40-S ribosomal protein S10. Protease-activated kinase II modified the .beta. subunit of eIF-2 (MW 53,000) and 40-S ribosomal protein S6. The substrate specificities are unique when compared with other cAMP-dependent and cAMP-independent protein kinases from reticulocytes.