Statoliths in Phycomyces : spectrofluorometric characterization of octahedral protein crystals

The giant sporangiophore of the unicellular zygomycete Phycomyces blakesleeanus displays negative gravitropism. Vacuolar paracrystalline protein bodies, so-called octahedral crystals, function as statoliths of gravitropism. The spectrofluorometric analysis indicates the presence of pterin- and flavin-like pigments that are non-covalently associated with the crystals. The redox states of the pterin- and flavin-like pigments differ in that the pterins are mainly oxidized while the flavins are reduced. Fluorescence polarization spectra indicate parallel transition dipole moments for the two pigments, which allows for internal energy transfer from the pterin- to the flavin-like pigments. The buoyant density of the crystals is unusually high for proteins and varies between 1.247 and 1.271 g cm−3.