Statoliths in Phycomyces: Characterization of octahedral protein crystals

To identify the molecular mechanisms of gravitropism in the fungus Phycomyc es blakesleeanus we determined several biochemical and physical parameters of paracrystalline protein bodies, so-called octahedral crystals. The cryst als, which are present throughout the central vacuoles of the sporangiophor e, function as statoliths (Schimek et al,, 1999a,b), They possess an averag e volume of 9.96 mu m(3) and a specific mass of 1.26 g cm(-3). SDS-PAGE Of purified crystals shows three major proteins with relative molecular masses of 16, 46.5, and 55 kDa, These proteins are absent in gravitropism mutants which lack the crystals, Phototropism mutants (genotype mad) which are gra viresponsive (class 1) and those which are defective in gravitropism (class 2) contain the crystals and the three associated proteins. Absorption spec tra of isolated crystals and in situ absorption spectra of growing zones in dicate the presence of chromophores, probably oxidized and reduced flavins, The flavin nature of the chromophores is also indicated by their fluoresce nce properties, It appears likely that the chromophores represent an essent ial part of the statoliths and thus the gravitropic transduction chain. (C) 2000 Academic Press.