Isoprenoid alcohols restore protein isoprenylation in a time‐dependent manner independent of protein synthesis

Mevalonic acid derivatives are required for the isoprenylation of a variety of growth-regulating proteins. Treatment of NIH3T3 cells with lovastatin (LOV), an HMG-CoA reductase inhibitor, depletes cells of these derivatives and impairs isoprenylation of RAS and RAS-related proteins. In LOV-treated cells, farnesol (FOH) and geranylgeraniol (GGOH) restore RAS and Rap1 isoprenylation, respectively. In this study, we further characterize the manner in which these isoprenoid alcohols are utilized for protein isoprenylation. Over a 48-h time span, FOH is unable to maintain RAS isoprenylation in the continuing presence of LOV, whereas GGOH is able to maintain Rap1 isoprenylation in the presence of LOV at all times tested. When cells are pretreated with LOV, the ability of both FOH and GGOH to restore protein isoprenylation is time dependent; as the LOV pretreatment time increases, the time required for FOH and GGOH to restore isoprenylation also increases. Despite this time dependence, the ability of FOH and GGOH to restore protein isoprenylation is not dependent on new protein synthesis and does not require alcohol dehydrogenase. These data support the existence of and further characterize the isoprenoid shunt, a novel metabolic pathway that utilizes FOH and GGOH for protein isoprenylation. The enzymes of the isoprenoid shunt are constitutively expressed, their activity may be modulated by isoprenoid depletion, and they are differentially regulated.