Distribution of Nsp100 and Nsp100 kinase, a nerve growth factor-sensitive phosphorylation system, in rat tissues

Identification of Nsp100 as elongation factor 2 (EF-2)

The nerve growth factor-sensitive phosphoprotein from PC12 cells, previously designated Nsp100, has been shown to be elongation factor 2 (EF-2). The criteria used for this identification include: (i) similarity of N-terminal sequence; (ii) phosphorylation by the same kinase; (iii) ADP-ribosylation mediated by diphtheria toxin; (iv) comparable function in cell-free protein synthesis. According to these criteria, Nsp100 and EF-2 are identical and the kinase that phosphorylates Nsp100 in PC12 cells is calcium/calmodulin kinase III.

Phosphorylation of elongation factor 2 in the rat superior cervical ganglion

Elongation factor 2 (EF-2) and its associated kinase, Ca2+/calmodulin-dependent protein kinase III, have recently been identified as a major protein phosphorylation system in mammalian tissues. We have measured the phosphorylation of EF-2 in 32P-labeled superior cervical ganglia. Phosphorylation of EF-2 was increased by preganglionic stimulation or by treatment of the ganglion with dimethylphenylpiperazinium or veratridine. Increases in EF-2 phosphorylation presumably reflect the activation of Ca2+/calmodulin-dependent protein kinase III by nicotinic stimulation and depolarization. The phosphorylation of EF-2 may help to coordinate neuronal protein synthesis with neuronal activity.

Nerve growth factor stimulates the phosphorylation of a 250 kDa cytoskeletal protein in cell-free extracts of PC12 cells

Nerve growth factor (NGF) rapidly stimulates the phosphorylation of a 250 kDa cytoskeletally-associated protein (pp250) by a protein kinase which is also associated with structural elements of the cell. We have solubilized these proteins and demonstrated that NGF-stimulated phosphorylation can be observed in cell free extracts of cytoskeletons from NGF-treated PC12 cells. The pp250 substrate and the 250-kinase were solubilized from PC12 cytoskeletons by treatment with 2 M urea. Phosphorylation of pp250 was maximally stimulated following treatment of the cells for 5 min with NGF. This effect was transient, diminishing with longer exposure of the cells to hormone. The 250-kinase preferred Mn2+ over Mg2+ and was inhibited by both Na+ and K+. The phosphorylation of pp250 was not affected by Ca2+. Upon fractionation of the urea-soluble cytoskeletal proteins by gel filtration, the 250-kinase eluted in two peaks; one peak of enzyme activity coeluting with the pp250 substrate, and a second peak of enzyme activity eluting with an apparent Mr of approximately 60 kDa. Treatment of the PC12 cells with the phorbol ester TPA also stimulated the phosphorylation of pp250, although this effect was not as great as that produced by NGF. This cell free system should be a valuable tool in the investigation of the mechanisms of NGF action.

Protein kinase C as a component of a nerve growth factor-sensitive phosphorylation system in PC12 cells

The treatment of PC12 cells with either nerve growth factor or phorbol 12-myristate 13-acetate caused a decrease in the phosphorylation of a soluble 100-kDa protein (Nsp100). After treatment with nerve growth factor, the activity of Ca2+/phospholipid-dependent protein kinase (protein kinase C) in the cytosol was increased. When the cytosol from untreated PC12 cells was preincubated with purified protein kinase C and its cofactors, the phosphorylation of Nsp100 was decreased. The preincubation of cytosol from nerve growth factor-treated PC12 cells with protein kinase C did not decrease Nsp100 phosphorylation further. Moreover, preincubation of partially purified Nsp100 kinase with protein kinase C decreased its ability to phosphorylate Nsp100. These results suggest that the binding of nerve growth factor to its receptor on PC12 cells causes an increase in the activity of protein kinase C in the cytosol and phosphorylation of Nsp100 kinase, which in turn lowers its ability to phosphorylate Nsp100.