Heterogeneity of native rat liver elongation factor 2

Abstract

The high heterogeneity of native rat liver EF-2 prepared from either 105000 x g supernatant or microsome high-salt extract was detected by two-dimensional equilibrium isoelectric focusing-SDS-polyacrylamide gel electrophoresis in the presence of 9.5 M urea. Five spots were always detected, all of M _r 95000, which were not artefactual for their amount varied when EF-2 was specifically ADP-ribosylated by diphtheria toxin in the presence of NAD⁺, and/or phosphorylated on a threonine residue by a Ca²⁺/calmodulin-dependent protein kinase (most likely Ca²⁺/calmodulin-dependent protein kinase III described by others [(1987) J. Biol. Chem. 262, 17299–17303; (1988) Nature 334, 170–173]). Results of ADP-ribosylation and/or phosphorylation experiments with either unlabeled or labeled reagents ([¹⁴C]NAD and [³²P]ATP) strongly suggest that our preparation contained native ADP-ribosylated and native phosphorylated forms which could be estimated at about 20% and 40% of the whole EF-2. Phosphorylated and ADP-ribosylated forms of EF-2 could be ADP-ribosylated and phosphorylated, respectively, but a native form both ADP-ribosylated and phosphorylated was not detected. Our results also suggest the existence of a minor native form of EF-2 and of its phosphorylated and ADP-ribosylated derivatives.