Alpha-beta heterodimeric human GGTase-I transfers a geranylgeranyl moiety to the Cys of a C-terminal CAAX motif in candidate proteins as a means of membrane localization. The conserved 48-kDa alpha subunit of GGTase-I is identical to the alpha subunit (FNTA) of Farnesyl Protein Transferase. The conserved 42-kDa Protein Geranylgeranyltransferase Type I Beta Subunit (PGGT1B Gene), containing 4 PFTB repeats, is distinct from the beta subunit (FNTB) of Farnesyl Protein Transferase; FNTB and PGGT1B are catalytic for specific substrates. The mammalian counterpart of CDC43 (S. cerevisiae) and responsible for peptide binding, PGGT1B acts on Rac1, Rac2, Rap1A, and Rap1B. Interacting with TGF-beta and Activin Type I receptors, FNTA is likely a key component of p21ras signaling, a farnesyltransferase substrate. (NCI) [ ]