Utilization of geranylgeraniol for protein isoprenylation in C6 glial cells

When rat C6 glial cells were incubated with [3H]geranylgeraniol (GGol), radioactivity was incorporated into a delipidated protein fraction. SDS-PAGE analysis of the protein fraction labeled by [3H]GGol revealed a 46 kDa polypeptide and a group of labeled polypeptides (19-27 kDa) in the same size range as the small GTP-binding proteins. A similar pattern of labeled polypeptides was seen when C6 cells were metabolically labeled with [14C]mevalonolactone. An isotopically labeled product, chromatographically identical to geranylgeranylcysteine (GG-Cys), was released by Pronase E digestion of the [3H]GGol-labeled protein. When the protein fraction from cells metabolically labeled with [3H]mevalonolactone was digested with Pronase E, two radiolabeled products were released with the chromatographic mobilities of farnesyl-cysteine (F-Cys) and GG-Cys. These studies suggest that C6 glial cells are capable of converting GGol to geranylgeranyl pyrophosphate (GG-P-P), or perhaps a novel "activated" form of the allylic isoprenol, that can be utilized for protein isoprenylation reactions.