Phytocyanins

VVV-NQGG VAV-NGRG VKV-DSGS VAV-DAGG VKV - SQEE VIV-NKAN 

Cucumber stellacyanin and spinach plantacyanin are the only phyto-cyanins for which sequence information is currently available for both the mature proteins, determined by protein sequencing, and the precursor proteins, deduced from the cDNA (Mann et al., 1992, 1996 Nersissian et al., 1996, 1998). With the exception of the plantacyanins, most phyto-cyanins are chimeric proteins in their predicted or known mature forms. They are composed of two structurally distinct sequence domains, a 100-to 109-amino-acid BCB domain followed by a domain that varies in length between 30 and 220 amino acids, lacks any obvious consensus sequence, and resembles heavily glycosylated arabinogalactan proteins (AGP) (Nothnagel, 1997). 

The classification of the four subfamilies, stellacyanins, plantacyanins, uclacyanins, and early nodulins, is based (i) on their spectroscopic features, (ii) on precursor as well as mature protein domain organization. 

Plantacyanins are strongly basic proteins with an isoelectric point close to 11. First isolated from cucumber in 1974, plantacyanins have since been characterized from various other plant species (Markossian et al., 1974 Aikazyan and Nalbandyan, 1981). They exhibit relatively high sequence identity to stellacyanin and spectroscopic properties similar 

Crystal structures of three phytocyanins are currently available. Two are for plantacyanins, from cucumber (also known as cucumber basic protein) (Guss et al., 1988, 1996) and from spinach (Einsle et al., 2000), and one is for the recombinant BCB domain of cucumber stella-cyanin (Hart et al., 1996). The three proteins display folding topology identical to one another, suggesting that phytocyanins fold into a uniform structure, which can be designated as a phytocyanin fold. As a historical note, the crystallization of the cucumber basic protein and its preliminary crystallographic data were reported in 1977, before any structure of a blue copper protein was available (Colman et al., 1977). However, the structure was solved in 1988 only by application of the then newly 

---

The molecular structure of another blue protein, the phytocyanin (phytocyanins are electron carriers found in the non-photosynthetic part of plants) cucumber basic protein (FW=10 100), also known as plantacyanin, is shown in Figure 33.60... 

A further cupredoxin we consider is mavicyanin from green zucchini peelings (FW = 12 700),70 a positively charged phytocyanin (p/= 8.85) the copper site of which is very similar to that of the above discussed... 

The first class is cupredoxins—single-domain blue copper proteins composed of only one BCB domain. These proteins include plastocyanin, azurin, pseudoazurin, amicyanin, auracyanins, rusticyanin, halocyanin, and sulfocyanin (see Section IV). Plantacyanin of the phytocyanin family (Section V), subunit II of the cytochrome c oxidase, and the recently characterized nitrosocyanin also fall into this class. The last two are single BCB domain polypeptides closely related structurally to cupredoxins, but harboring, respectively, a binuclear copper site known as CuA and a novel type of copper-binding site called red (see Sections IX and X). 

The third class consists of proteins that are composed of one or more BCB domains fused to a sequence domain(s) characteristic of evolutionarily unrelated protein families. Such a mosaic domain organization has been found in the phytocyanin protein family, stellacyanins, uclacyanins, and the recently characterized dicyanins (Section V) in blood coagulation factor VIII (Section VIII) and in nitrous oxide reductase (Section IX). 

Solvent is usually excluded from the blue copper site, which is buried 6 A inside the protein, having only the His ligand from the copperbinding loop exposed to the surface. The phytocyanins, stellacyanin and plantacyanin (cucumber basic protein), are exceptions, in which both His ligands are solvent exposed and the copper ion is only 3 A beneath the protein surface. This situation makes the copper center in this family of blue copper proteins more accessible to low-molecular-weight solutes (see Section V). 

Putative functions of phytocyanins based on literature data published over the past few years can be grouped into three different categories ... 

Interestingly, plant AGPs have been also impbcated in cellular processes that are identical to those described above for phytocyanins (Noth-nagel, 1997). As we have already mentioned, most phytocyanins in their mature form are predicted to be composed of a BCB domain and a domain with sequence characteristics reminiscent of those of described for AGPs. Thus, phytocyanins are interesting examples of the recently developed Rosetta stone sequence concept, which postulates that when two different proteins also occur in parallel as a fused, larger composite protein, it is an indication that they are functionally related and may even physically interact (Eisenberg et al., 2000 Marcotte, 2000). 

Ceruloplasmin is synthesized in the hver as a precursor with a signal peptide directing it to the endoplasmic reticulum. It incorporates Cu provided by the Wilsons disease-associated Cu-ATPase, ATP7B, and is secreted into the bloodstream. The copper-deficient ceruloplasmin in Wilson s disease patients has been found to rapidly degrade. A GPI-anchored form of ceruloplasmin has been recently identified as the product of an alternately spliced ceruloplasmin transcript that generates a hydrophobic C-terminal GPI-anchoring signal, similar to those found in phytocyanins (Patel and David, 1997). The GPI-anchored form of ceruloplasmin is preferentially expressed in brain. 

---