TABLE 1.

Buffer and nucleotide effects on the critical concentration for polymerization of bacterial tubulin

Nucleotide concentrations were 1 mm for GTP and GDP and 0.1 mm for GMPCPP, GMPCP, and GTPγS. ND, not determined.

Buffer	Nucleotide	Cr	ΔGapp0a
		μm BtubA/B	kcal mol−1
Tris-KCl	GTP	2.05 ± 0.32b	−7.76 ± 0.10
	GDP	>20b	>−6.41
	GMPCPP	0.99 ± 0.20b	−8.19 ± 0.12
	GMPCP	>10b	>−6.82
	GTPγS	2.11 ± 0.26b	−7.74 ± 0.07
Tris-NaCl	GTP	10 < Cr < 20c	ND
Tris-KGlu	GTP	0.94 ± 0.10b0.76 ± 0.10c	−8.22 ± 0.06
Tris-KGlu	GDP	>10c	ND
Tris-0.2 m KCl	GTP	2.32 ± 0.30b	−7.69 ± 0.08
Pipes	GTP	10 < Cr < 20c	ND
	GMPCPP	10 < Cr < 30c	ND
Pipes-D2O	GTP	6 ± 1b,c	−7.2 ± 0.2
Pipes-0.2 m KCl	GTP	<14c	ND
Pipes-0.2 m NaCl	GTP	20 < Cr < 40c	ND
HMK	GTP	<10, aggregatesc	ND

^a Apparent elongation free energy changes calculated from the sedimentation Cr values (ΔG_app⁰ = RT lnCr⁻¹).

^b Determined by sedimentation at 25 °C.

^c Determined by light scattering at 30 °C.