Phytochelatins and Related Peptides (Structure, Biosynthesis, and Function)

Plants obtain micronutrients such as Cu and Zn from aquatic and terrestrial environments that may also provide the metals Cd, Pb, and Hg. These essential and nonessentia1 metals exist at low to high concentrations depending on natural and manmade disturbances. In a fluctuating environment the plants may experience shifting internal concentrations of these bioreactive metals. Therefore, it is beneficial for plants to have mechanisms that (a) maintain internal concentrations of essential metals between deficient and toxic limits and (b) keep nonessential metals below their toxicity thresholds. The proteins and peptides known as MTs sequester metals and thereby may accomplish cellular metal homeostasis and detoxification. These molecules are rich in Cys’s that provide thiols for binding the metals mentioned so far. The early work on plants was modeled after equine renal MT, a metal-induced protein with 20 Cys’s distributed uniquely within the sequence of 60 amino acids and M, of approximately 9000 (Robinson et al., 1993). Studies of yeasts, algae, and plants relied heavily on isolating the metal-induced components of M , of approximately 9000 that bound Cd or Cu. In many of these preparations Cys, Glu, and Gly accounted for 45 to 97% of the amino acids, which is inconsistent with a dose relationship to the archetypal form, equine renal MT. Two independent groups, one working with the fission yeast Schizosaccharomyces pombe (Kondo et al., 1984) and the other working with cultured cells of Rauvolfia serpentina (Grill et al., 1985), showed that the molecules binding Cd were a family of peptides with the primary structure (y-Glu-Cys),-Gly, where n = 2 to 7 depending on the organism. The peptide bond in the repeating Glu-Cys pairs is a y-carboxyamide linkage not synthesized on ribosomes. Consequently, these and related peptides are now designated class I11 MTs, which are defined as atypical, nontranslationally synthesized metal thiolate polypeptides (Robinson et al., 1993). No consensus has been reached concerning a trivial name for the peptides. The name cadystin holds priority (Kondo et al., 1984), with phytochelatin being popularly used for algae and plants (Grill et al., 1985). Recent progress on class I11 MTs