THE STRUCTURE OF A PHYTOCYANIN, THE BASIC BLUE PROTEIN FROM CUCUMBER,REFINED AT 1.8 ANGSTROM RESOLUTION

The crystal structure of the cucumber basic protein (CBP), a type 1 or blue copper protein, has been refined at 1.8 Angstrom resolution. The molecule resembles other blue copper proteins in having a Greek key b eta-barrel structure, except that the barrel is open on one side and i s better described as a ''beta-sandwich'' or ''beta-taco''. The Cu ato m has the normal blue copper NNSS' co-ordination with bond lengths Cu- N(His39) = 1.93 Angstrom, Cu-S(Cys79) = 2.16 Angstrom, Cu-N(His84) = 1 .95 Angstrom, Cu-S(Met89) = 2.61 Angstrom. The Cu-S(Met) bond is the s hortest so far observed in a blue copper protein. A disulphide link, ( Cys52)-S-S-(Cys85), appears to play an important role in stabilising t he molecular structure. It is suggested that the polypeptide fold is t ypical of a sub-family of blue copper proteins (phytocyanins) as well as a non-metalloprotein, ragweed allergen Ra3, with which CBP has a hi gh degree of sequence identity. The proteins currently identifiable as phytocyanins are CBP, stellacyanin, mavicyanin, umecyanin, a cucumber peeling cupredoxin, a putative blue copper protein in pea pads, and a blue copper protein from Arabidopsis thaliana, In all except CBP and the pea-pod protein, the axial methionine Ligand normally found at blu e copper sites is replaced by glutamine. The structure of CBP was orig inally solved by the multiple wavelength anomalous scattering method, using data recorded at four wavelengths. All these data were included in the restrained least squares refinement. The final model comprises 96 amino acid residues, 122 solvent molecules and a copper atom. Sever al residues are modelled as having more than one conformation. The res idual R is 0.141 for 41,910 observations (including Bijvoet-related ob servations) of 8.142 unique reflections in the resolution range 7 to 1 .8 Angstrom. (C) 1996 Academic Press Limited