Essential subunit of both the farnesyltransferase and the geranylgeranyltransferase complex. Contributes to the transfer of a farnesyl or geranylgeranyl moiety from farnesyl or geranylgeranyl diphosphate to a cysteine at the fourth position from the C-terminus of several proteins having the C-terminal sequence Cys-aliphatic-aliphatic-X. May positively regulate neuromuscular junction development downstream of MUSK via its function in RAC1 prenylation and activation. (2E,6E)-farnesyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-(2E,6E)-farnesyl-L-cysteinyl-[protein] geranylgeranyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-geranylgeranyl-L-cysteinyl-[protein] Activated by the AGRIN-induced phosphorylation which is mediated by MUSK. Heterodimer of FNTA and FNTB (farnesyltransferase). Heterodimer of FNTA and PGGT1B (geranylgeranyltransferase). Interacts with MUSK; the interaction is direct and mediates AGRIN-induced phosphorylation of FNTA. Phosphorylated. Phosphorylation is mediated by MUSK upon AGRIN stimulation and results in the activation of FNTA. Belongs to the protein prenyltransferase subunit alpha family.