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-cyclephase-dependent pattern of prenylation for various classes of cytosolic and nuclear matrix proteins in synchronized HepG2 cells. Characteristic [3H]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,) as cells approached mitosis. Prior to the rise in mevalonate incorporation into proteins, during early-to-mid G1, steady-state [14C]acetate incorporation into chromatographically resolved cholesterogenic lipid intermediates displayed a maximum only into cholesterol. However, during the late-G l/S interval, a singular peak of 14C 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 14C incorporation subsequent to mid G1. These results support the proposal that subsequent to peak cholesterol synthesis in early-to-mid G1, 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.The exposure of cultured cells in the G1 phase of the cell division cycle to competitive inhibitors of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, such as mevinolin or compactin, not only blocks mevalonic acid production, thereby inhibiting cholesterol synthesis, but in addition pleiotropically prevents DNA replication and cell cycle progression. Both phenomena are reversible upon exogenous addition of mevalonate [l-71. Thus, the operation of the cholesterogenic pathway, together with its mevalonate-dependent subroutes, is a prerequisite for cell growth (reviewed in [8 -lo]). Mevalonate assumes paramount importance in this regard since it is the key precursor intermediate required for the synthesis of sterol and various other classes of non-steroidal isoprenoids [ll], such as dolichol, the side chain of ubiquinone and isopentenyladenine. Mevalonate also is the source of the covalently modifying prenyl group(s) of a limited set of very interesting isoprenylated proteins [12].The production and metabolic utilization of mevalonate at two specific times during G1 is necessary for continuous cell c...

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