Sulfocyanin

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 sequences of auracyanin B, halocyanin, and sulfocyanin deduced from their gene sequences reveal an unusually long N-terminal extension featuring a hydrophobic domain similar to signal peptides found in other... 

The redox partners of these proteins have yet to be identified, although it has been shown that auracyanins can donate electrons to the membrane-bound cytochrome c-554, which is the direct electron donor for the photooxidized bacterial reaction center P870+ (McManus et al., 1992). However, whether it is their proper in vivo function remains uncertain. The sulfocyanin gene is in the same operon with the components of the respiratory electron transfer chain and, since Su. acidocaladar-ius completely lack c-type cytochromes, it is implicated as a substrate for the CuA-containing terminal oxidase. Interestingly, the occurrence of... 

Halocyanin and sulfocyanin are archaebacterial cupre-doxins that are attached to peripheral membranes through a lipid anchor at their N-terminus. Halocyanin was the first cupredoxin purified from an archaeon, haloalkaliphilic Natronobacterium pharaonis These cells live in high pH (around 10-11) and in extreme salinity (30%) environments. The presence of the blue copper protein, sulfocyanin in Sul-folobus acidocaldarius was first predicted from its gene sequence. It has been subsequently purified as a recombinant protein and shown to bind a single copper ion with spectroscopic properties typical for a blue copper site. ... 

The sequences of halocyanin and sulfocyanin, deduced from their gene sequences, have unusually long N-terminal extensions. These extensions feature a hydrophobic signal peptide which is followed by a segment that in the case of halocyanin has an Asn-Gly doublet occurring consecutively seven times. In sulfocyanin, this segment is rich in Ser residues. It is believed that these extensions are covalently modified by a lipid moiety which anchors the proteins to the cell membrane. 

The function of these proteins is not known. The sulfocyanin gene is positioned in the same operon with the components of the respiratory electron-transfer chain, and it... 

Like halocyanin and sulfocyanin, auracyanin B has an N-terminal extension that is thought to anchor the protein to the cell membrane. In contrast to those proteins, however, this extension is rich in Pro, Ala, and Thr residues. In auracyanin A, they are Gly residues. ... 

Stock solutions of the sulfocyanine succinimidyl active esters may be made in dry DMF (0.3-1.0 mg active ester/100 ml) and are stable for days when stored at 4° in a desiccator. The active esters are also stable in distilled water for several hours provided the pH of the solution is not basic. Aqueous solutions of the dyes can be used for labeling antibodies if use of DMF is not suitable for certain antibodies. The concentration of cyanine fluorophore in the stock solution is determined by measuring the absorbance of an aliquot of the appropriately diluted stock solution in phosphate-buffered siline (PBS) and using the extinction coefficient of the dye (Table I). 

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