Post-translational modification of proteins by 15-carbon and 20-carbon isoprenoids in three mammalian cell lines

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A number of cellular proteins, including p21ras, lamin B, and the G-protein gamma subunits, undergo post-translational modification by 15-carbon farnesyl or 20-carbon geranylgeranyl isoprenoid moieties derived from pyrophosphate intermediates of the cholesterol biosynthetic pathway. In this study, isoprenylated proteins in three mammalian cell lines (Hela cells, Rat-6 fibroblasts and COS cells) were radiolabeled with an isoprenoid precursor, [3H]mevalonate, and resolved by SDS gel electrophoresis. Groups of proteins with different molecular masses were eluted from the gels and the chain-lengths of the radiolabeled isoprenyl groups, released from the proteins by Raney-nickel-catalyzed desulfurization, were established by gel permeation chromatography. 15-Carbon and 20-carbon isoprenyl groups were found in separate classes of proteins within each cell line. With the exception of p21ras, which incorporated a 15-carbon group when expressed in COS cells, the proteins in the region of the 21-28 kDa ras-related GTP binding proteins contained mostly 20-carbon isoprenyl chains. In contrast, proteins belonging to the 66-72 kDa nuclear lamin family, as well as unidentified proteins with molecular masses of 41-46 kDa and 53-55 kDa, contained predominantly 15-carbon isoprenyl chains. The chain-lengths of the isoprenoids associated with particular classes of proteins did not vary from one cell line to another, suggesting that the nature of the isoprenoid modification (farnesyl versus geranylgeranyl) is determined by intrinsic structural features of the proteins, rather than the cell type in which the proteins are expressed.