Methylates the C-terminal cysteine residues of small GTPases and the heterotrimeric G protein gamma subunit in response to cAMP. The methylation is required for intercellular signaling and regulation of cAMP waves propagation. It also seems to induce the activity of car1, a G protein-coupled receptor which senses extracellular cAMP during the aggregation phase of development. [protein]-C-terminal S-[(2E,6E)-farnesyl]-L-cysteine + S-adenosyl-L-methionine = [protein]-C-terminal S-[(2E,6E)-farnesyl]-L-cysteine methyl ester + S-adenosyl-L-homocysteine Induced by starvation and expressed throughout development, with a peak at 8 hours after starvation. Cells lacking both icmt-1 and icmt-2 do not propagate cAMP waves in a sustained manner and fail to aggregate, affecting differentiation and development. Belongs to the class VI-like SAM-binding methyltransferase superfamily. Isoprenylcysteine carboxyl methyltransferase family. The gene for this protein is duplicated in strains AX3 and AX4. These strains contain a duplication of a segment of 750 kb of chromosome 2 compared to the corresponding sequence in strain AX2.