Fig. 8. CrPRF inhibits nucleotide exchange. (A) Representative experiments show that profilins from diverse organisms have substantially different effects on the initial rate of nucleotide exchange for G-actin. The incorporation of 
-ATP by 0.5 µM G-actin (RSMA) in low salt buffer alone (curve 3) or in the presence of 0.1 µM HPRO1 (curve 1), 2.5 µM ZmPRO5 (curve 2), 0.5 µM CrPRF (curve 4) or 2.5 µM CrPRF (curve 5) was monitored over time. The curves shown are fits of raw data (not shown) with a single exponential function. Human profilin I dramatically enhanced the initial rate of nucleotide exchange, whereas ZmPRO5 had little effect and CrPRF significantly decreased the initial rate. (B) The effect of a range of CrPRF concentrations (0.1-20 µM) on the initial rate of nucleotide exchange of 2.0 µM G-actin (maize pollen) in low ionic strength buffer is shown. Initial rates were determined by fitting the first 240 seconds of curves similar to those shown in (A) to a single exponential function. The initial rates were plotted against the concentration of CrPRF and fitted to the equation described in Methods. The calculated dissociation constant was 0.11 µM.
