R-PHYCOERYTHRINS HAVING 2 CONFORMATIONS FOR THE SAME AGGREGATE

The visible circular dichroism (CD) spectrum of an R-phycoerythrin (Po rphyra tenera) is composed of several positive bands. The protein in a queous buffer very slowly exhibits changes in the CD spectrum of its c hromophores, a band at 489 nm undergoes an increase in intensity and a red shift. When the band reached a 493 nm maximum, the spectrum becam e very stable. The aggregation state of the protein did not change dur ing this spectral conversion. The chromophore CD spectrum was also obt ained in the presence of a low concentration of urea or sodium thiocya nate, and the identical change in the CD was noted, but the change was much faster. The visible absorption and CD in the far UV spectra were unaffected by urea. Unchanged visible absorption and protein secondar y structure (61% alpha helix) contradicted by comparatively salient al terations in the visible CD spectra suggested very subtle structural c hanges are influencing some of the chromophores. For a second R-phycoe rythrin (Gastroclonium coulteri), the CD of the chromophores had a neg ative band on the blue edge of the spectrum. This is the first negativ e CD band observed for any R-phycoerythrin. Treatment of this protein with low concentrations of urea produced a change in the visible CD wi th the negative band being completely converted to a positive band. Fl uorescence studies showed that the treatment by urea did not affect en ergy migration. Deconvolution of the CD spectra were used to monitor t he chromophores. The results demonstrated that the same aggregate of e ach R-phycoerythrin could exist in two conformations, and this is a no vel finding for any red algal or cyanobacterial biliprotein. The two f orms of each protein would differ in tertiary structure, but retain th e same secondary structures.