Brightness of yellow fluorescent protein from coral (zFP538) depends on aggregation

The yellow fluorescent protein from coral (zFP538) forms aggregates in water solutions. According to dynamic light scattering and gel filtration data, the aggregation number is approximately 1000-10000 at pH 8-9 and protein concentration 1 mg/mL. Gel filtration demonstrated that dissociation of the aggregates takes place upon dilution, and the molecular weight of the aggregates decreases with pH. Atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) were used to obtain images of zFP538 in the solid state. It was shown that protein films are comprised of fluorescent ellipsoidal granules with a 50-300 nm major axis and a 30-130 nm minor axis. The dependence of zFP538 fluorescence on protein concentration between 1.2 x 10(-)(9) and 5.5 x 10(-)(7) M can be divided in two linear regions with different slopes indicating the existence of at least two different forms of zFP538. The fluorescence of zFP538 decreases with time upon acidification, and the decrease depends on pH and protein concentration. Between pH 3.5 and pH 5.5, relative residual fluorescence is higher for concentrated zFP538 solutions (about 10(-)(6) M) as compared with diluted ones (10(-)(7) M and below). Aggregation makes zFP538 more stable against fluorescence quenching upon acidification: the decrease in zFP538 fluorescence at protein concentration 1 mg/mL is completely reversible, unlike that observed for less concentrated solutions. This phenomenon may be due to the decrease in the freedom of chromophore mobility in zFP538 aggregates.