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 (By similarity). (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 (By similarity). Phosphorylated. Phosphorylation is mediated by MUSK upon AGRIN stimulation and results in the activation of FNTA (By similarity). Belongs to the protein prenyltransferase subunit alpha family. Contaminating sequence. Potential poly-A sequence.