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 in flowers, stems, leaves, roots and siliques. Detected in apices and vascular tissues of leaves and roots, in the stigma and in the filaments and anthers of stamen. Not found in petioles or hypocotyls. Not induced by abscisic acid or auxin. Plants lacking ICMTA and ICMTB have altered phyllotaxis, fasciated stems and development of axillary flowers. ICMTB is more widely expressed and has a higher catalytic activity than ICMTA. Belongs to the class VI-like SAM-binding methyltransferase superfamily. Isoprenylcysteine carboxyl methyltransferase family.