Catalyzes the post-translational methylation of isoprenylated C-terminal cysteine residues, resulting in the modulation of the function of prenylated proteins. Involved in negative regulation of abscisic acid signaling. Carboxyl methylation is a reversible and potentially regulated step in the post-translational modification of prenylated proteins. [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 Divalent metal cations. Probably Zn(2+). Inhibited by farnesylthioacetic acid (FTAA) and N-acetyl-S-trans, trans-farnesyl-l-cysteine (AFC). Expressed primarily in flowers, stems, leaves and roots. Almost not expressed in siliques. Detected in root tips and vascular tissues of roots, cotyledons, petiols, hypocotyls, filaments, pollen grains and the distal and proximal portions of the gynoecium. Not induced by abscisic acid or auxin. No visible phenotype; due to redundancy with ICMTB. Icmta and icmtb double mutants have altered phyllotaxis, fasciated stems and development of axillary flowers. ICMTA is less widely expressed and has a lower catalytic activity than ICMTB. Belongs to the class VI-like SAM-binding methyltransferase superfamily. Isoprenylcysteine carboxyl methyltransferase family. Extended C-terminus.