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. If X is Ser, Ala, Met, Cys, or Gln, then the protein is farnesylated, if X is Leu or Phe, then the protein is geranylgeranylated. The alpha subunit wraps partly around the beta subunit forming part of the active site. The beta subunit is responsible for isoprenoid and peptide-binding. (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] Farnesyltransferase and geranyl-geranyltransferase share a common alpha subunit (RAM2) (Probable). Protein farnesyltransferase is a heterodimer of alpha subunit RAM2 and beta subunit RAM1 (PubMed:1763050). Protein geranyl-geranyltransferase type-1 is a heterodimer of alpha subunit RAM2 and beta subunit CDC43 (PubMed:1400380, PubMed:9174352). Present with 396 molecules/cell in log phase SD medium. Belongs to the protein prenyltransferase subunit alpha family.