Murine guanylate-binding protein: incomplete geranylgeranyl isoprenoid modification of an interferon-gamma-inducible guanosine triphosphate-binding protein

Farnesylation of Ras proteins is necessary for transforming activity. Although farnesyl transferase inhibitors show promise as anticancer agents, prenylation of the most commonly mutated Ras isoform, K-Ras4B, is difficult to prevent because K-Ras4B can be alternatively modified with geranylgeranyl (C20). Little is known of the mechanisms that produce incomplete or inappropriate prenylation. Among non-Ras proteins with CaaX motifs, murine guanylate-binding protein (mGBP1) was conspicuous for its unusually low incorporation of (3)H]mevalonate. Possible problems in cellular isoprenoid metabolism or prenyl transferase activity were investigated, but none that caused this defect was identified, implying that the poor labeling actually represented incomplete prenylation of mGBP1 itself. Mutagenesis indicated that the last 18 residues of mGBP1 severely limited C20 incorporation but, surprisingly, were compatible with farnesyl modification. Features leading to the expression of mutant GBPs with partial isoprenoid modification were identified. The results demonstrate that it is possible to alter a protein's prenylation state in a living cell so that graded effects of isoprenoid on function can be studied. The C20-selective impairment in prenylation also identifies mGBP1 as an important model for the study of substrate/geranylgeranyl transferase I interactions.