InterPro domain: IPR003245

Among the blue copper proteins with a single type I (or "blue") mononuclearcopper site, the plant-specific phytocyanins constitute a distinct subfamilythat can be further subdivided into the families of uclacyanins, stellacyanins,plantacyanins, and early nodulins. Stellacyanins have a blue coppercoordinated by two His, one Cys and one Gln. In plantacyanins and uclacyanins,the ligands of the type-I Cu sites are two His, one Cys and one Met [ 1 , 2 , 3 , 4 ]. Early nodulins lack amino acid residues that coordinate Cu, so they are believed to be involved in unknown processes without binding Cu [ 5 ]. Phytocyanins are found in chloropasts of higher plants.

The phytocyanin domain has a core of seven polypeptide strands arranged as abeta-sandwich comprising two beta-sheets, beta-sheet I and beta-sheet II. Beta-sheet I consists of three beta-strands and beta-sheet IIconsists of four beta-strands. A disulfide bridge close the metal centre ischaracteristic for phytocyanins, in contrast to azurins, pseudoazurins, andplastocyanins, where a disulfide bond is located on the distal side of thebeta-barrel. This disuldide bridge may play a crucial role in maintaining thetertiary structure of the protein and/or the formation of the copper bindingcentre because one of the His ligands of copper is followed directly by abridging Cys residue [ 6 , 6 , 6 , 6 ]. Some members of this family (P93328) may not bind copper due to the lack of key residues.

Some proteins known to contain a phytocyanin domain are listed below:

• Cucumber basic protein (CBP).
• Spinach basic protein (SBP).
• Cucumber stellacyanin (CST).
• Zucchini mavicyanin.
• Horseradish umecyanin [ 6 , 7 ].

Some of the proteins in this family are allergens. The allergens in this family include allergens with the following designations: Amb a 3.

1. Uclacyanins, stellacyanins, and plantacyanins are distinct subfamilies of phytocyanins: plant-specific mononuclear blue copper proteins. Protein Sci. 7, 1915-29
2. Crystal structure of plantacyanin, a basic blue cupredoxin from spinach. J. Biol. Inorg. Chem. 5, 666-72
3. Structural reorganization of the copper binding site involving Thr15 of mavicyanin from Cucurbita pepo medullosa (zucchini) upon reduction. J. Biochem. 137, 455-61
4. Crystal structures of oxidized and reduced stellacyanin from horseradish roots. J. Am. Chem. Soc. 127, 158-66
5. Genome-wide identification, structure and expression studies, and mutant collection of 22 early nodulin-like protein genes in Arabidopsis. Biosci. Biotechnol. Biochem. 73, 2452-9
6. Structural properties of umecyanin. A copper protein from horseradish root. Biochim. Biophys. Acta 236, 246-52
7. On the state of copper in the blue protein umecyanin. FEBS Lett. 12, 260-262