TABLE 1.

Summary of sedimentation properties

Sedimentation velocity analysis using the fluorescence detection system (see “Experimental Procedures”) was carried out on the indicated protein samples. The data were analyzed using Sedfit with the following parameters: partial specific volume = 0.7300, buffer density = 0.99823 g/ml, and viscosity = 0.01002 Poise. The sedimentation coefficient (s) values are the mean ± S.E. for n separate determinations.

Protein sample	n	sa	RSb	Axial ratio (a/b)c	Predicted sd
			cm
Pαβγγ	32	7.9 ± 0.02	6.3 × 10−7	6.8
Pαβ	24	7.5 ± 0.02*	6.1 × 10−7	6.6
Pαβ + 2γ	12	7.9 ± 0.03	6.3 × 10−7	6.8
Pαβ + cGMP	20	7.5 ± 0.04*	6.1 × 10−7	6.7
Pγ	13	1.0 ± 0.02	2.2 × 10−7	5.9
PrBP/δ	24	2.0 ± 0.04	2.0 × 10−7	1.3	1.9
2PrBP/δ + Pαβγγ	20	8.1 ± 0.05	7.2 × 10−7	8.6
Tα/GTPγS	13	3.1 ± 0.04	3.1 × 10−7	3.7	3.2
Tα/GTPγS + Pγ	26	3.5 ± 0.08**	3.4 × 10−7	4.1
PDE5 dimer	16	7.7 ± 0.04	6.4 × 10−7	7.2	4.3/7.1
PDE5 dimer + cGMP	12	7.4 ± 0.04*	6.7 × 10−7	8.4
PDE5-CAT monomer	6	3.4 ± 0.12	3.1 × 10−7	3.2	3.1
PDE5-CAT + cGMP	6	3.3 ± 0.04	3.2 × 10−7	3.6

^a To determine statistical significance referenced to the boldface entry in each section, a two-tailed t test was performed, and the degree of significance noted by an asterisk (p < 0.001), a double asterisk (p < 0.01), or no asterisk (not statistically significant).

^b R_S was calculated using SEDNTERP, a knowledge of the composition molecular weight, and the partial specific volume (assuming solvation = (0.4 g of H₂O/g of protein)).

^c Axial ratio was calculated assuming a prolate ellipsoid model of the hydrodynamic properties, assuming the monomeric state except for PDE5 and PDE6 catalytic dimers.

^d Predicted sedimentation coefficient is based on hydrodynamic modeling using SOMO (35) with the following Protein Data Bank structure files: PrBP/δ (1KSH), Tα-GTPγS (1TND), PDE5 monomer/dimer (modeled using the dimeric PDE2 crystal structure, 3IBJ), PDE5 catalytic domain (no ligand bound, 1T9R).