| Type: | Family | Name: | Plant EC metallothionein-like protein, family 15 |
| Description: | Metallothioneins (MT) are small proteins that bind heavy metals, such as zinc, copper, cadmium, nickel, etc. They have a high content of cysteine residues that bind the metal ions through clusters of thiolate bonds [, , , ]. An empirical classification into three classes has been proposed by Fowler and coworkers [] and Kojima []. Members of class I are defined to include polypeptides related in the positions of their cysteines to equine MT-1B, and include mammalian MTs as well as MTs from crustaceans and molluscs. Class II groups MTs from a variety of species, including sea urchins, fungi, insects and cyanobacteria. Class III MTs are atypical polypeptides composed of gamma-glutamylcysteinyl units []. This original classification system has been found to be limited, in the sense that it does not allow clear differentiation of patterns of structural similarities, either between or within classes. Consequently, all class I and class II MTs (the proteinaceous sequences) have now been grouped into families of phylogenetically-related and thus alignable sequences. This system subdivides the MT superfamily into families, subfamilies, subgroups, and isolated isoforms and alleles. The metallothionein superfamily comprises all polypeptides that resemble equine renal metallothionein in several respects []: e.g., low molecular weight; high metal content; amino acid composition with high Cys and low aromatic residue content; unique sequence with characteristic distribution of cysteines, and spectroscopic manifestations indicative of metal thiolate clusters. A MT family subsumes MTs that share particular sequence-specific features and are thought to be evolutionarily related. The inclusion of a MT within a family presupposes that its amino acid sequence is alignable with that of all members. Fifteen MT families have been characterised, each family being identified by its number and its taxonomic range: e.g., Family 1: vertebrate MTs.Family 15 consists of planta MTs. Its members are recognised by the sequence pattern [YFH]-x(5,25)-C-[SKD]-C-[GA]-[SDPAT]-x(0,1)-C-x-[CYF] which yields all plant sequences, but also MTCU_HELPO and the non-MT ITB3_HUMAN. The taxonomic range of the members extends to planta. Planta MTs are 45-84 residue proteins, containing 17 conserved cysteines that bind 5 zinc ions. Generally, there are two Cys-rich regions (domain 1 and domain 3) separated by a Cys-poor region (domain 2) and only the domain 2 contains unusual residues. It is believed that the proteins may have a role in Zn2+homeostasis during embryogenesis. Family 15 includes the following subfamilies: p1, p2, p2v, p3, pec, p21. | Short Name: | Metallthion_15 |
