Translation and phosphorylation of wheat germ lysate: Phosphorylation of wheat germ initiation factor 2 by casein kinase II and in N-ethylmaleimide-treated lysates

摘要

Previously, we observed that N-ethylmaleimide (NEM), a thiol-alkylating agent, was found to stimulate the phosphorylation of several proteins in translating wheat germ (WG) lysates, including the phosphorylation of alpha, the p41-42 doublet subunit, and beta, the p36 subunit, of the WG initiation factor 2 (eIF2). We find now that NEM increases phosphorylation of several proteins significantly in lysates which are moderate or low in their translation compared to optimally active lysates. Heat treatment, which stimulates oxidation of protein sulfhydryls, decreases the translation and phosphorylation ability of WG lysates. The decrease in phosphorylation, but not translation, that occurs in heat-treated lysates is prevented very efficiently by NEM and partially by reducing agents such as dithiothreitol (DTT) and GSH. DTT prevents, however, completely the loss of sulfhydryl content of heat-treated WG lysates and does not at all prevent heat-induced inhibition of translation. In contrast, DTT prevents completely the diamide-induced translational inhibition and also the loss of sulfhydryl content. These findings therefore suggest that in addition to the maintenance of sulfhydryl groups, heat-labile proteins and their interactions with other proteins play an important role in overall translation and phosphorylation. It is also observed here that heat treatment stimulates the phosphorylation of rabbit reticulocyte eIF2alpha but not the alpha subunit (p41-42 doublet) of WG eIF2alpha. A phosphospecific anti-eIF2alpha antibody recognizes the WG eIF2alpha(P) that is phosphorylated by an authentic eIF2alpha kinase such as double-stranded RNA-dependent protein kinase, but it is unable to recognize the eIF2alpha that is phosphorylated in NEM-treated lysates. These findings therefore suggest that phosphorylation of WG eIF2alpha in NEM-treated lysates occurs on a site different from the serine 51 residue that is phosphorylated by authentic eIF2alpha kinases. In addition, it also suggests that WG eIF2alpha, unlike reticulocyte eIF2alpha, is phosphorylated by eIF2alpha kinases and also by other kinases. Consistent with this idea, it has been observed here that casein kinase II (CKII) phosphorylates WG eIF2alpha and the phosphorylation is enhanced by NEM in vitro and in lysates. The phosphopeptide analysis suggests that WG eIF2alpha has separate phosphorylation sites for CKII and heme-regulated eIF2alpha kinase (a well-characterized mammalian eIF2alphakinase), and NEM-induced phosphorylation in WG lysates resembles CKII-mediated phosphorylation. (C) 2002 Elsevier Science (USA). [References: 60]