Farnesyltransferase and Geranylgeranyltransferase I are very similar proteins.
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Instead, Rab proteins are bound by the Rab escort protein (REP) over a more conserved region of the Rab protein and then presented to the Rab geranylgeranyltransferase.
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Small molecules can also undergo prenylation, such as in the case of prenylflavonoids.
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In the case of the K- and N-Ras forms of Ras, when cells are treated with FTIs, these forms of Ras can undergo alternate prenylation in the form of geranylgeranylation.