Cell‐cycle‐dependent, differential prenylation of proteins

Isoprenylated proteins related to cell growth have been detected during proliferation. Since cholesterogenesis (isoprenoid synthesis) is mandatory for cell proliferation, the observation of a temporally coordinated protein prenylation during the cell division cycle might constitute obligatory processes in the signalling pathway for initiating DNA replication and/or in maintaining the growing state. We have found such a definitive cell‐cycle‐phase‐dependent pattern of prenylation for various classes of cytosolic and nuclear matrix proteins in synchronized HepG2 cells. Characteristic [ 3 H]mevalonate incorporation began to increase during mid‐to‐late G 1 , just after cholesterol synthesis reached its apex, and peaked just prior to or coincident with mid S. Incorporation then declined subsequent to S (during G 2 ) as cells approached mitosis. Prior to the rise in mevalonate incorporation into proteins, during early‐to‐mid G 1 , steady‐state [ 14 C]acetate incorporation into chromatographically resolved cholesterogenic lipid intermediates displayed a maximum only into cholesterol. However, during the late‐G 1 /S interval, a singular peak of 14 C incorporation was found for the farnesyl moiety (farnesol/nerolidol plus farnesyl diphosphate). Except for the farnesyl moiety, none of the other polyisoprenoids detected by our procedures showed any fluctuation in 14 C incorporation subsequent to mid G 1 . These results support the proposal that subsequent to peak cholesterol synthesis in early‐to‐mid G 1 , the generation of a cholesterol‐pathway‐dependent set of post‐translationally modified, polyisoprenylated proteins could constitute an obligatory step leading to the duplication of the cellular genome, thereby impelling transit through the cell cycle. The well known high flux through cholesterogenesis in tumors, which manifests an intrinsic lack of sensitivity to feedback inhibition and operates continuously, is consonant with this proposal.