TABLE 3.

Effect of glucose and of the ptk2 and yor267c mutations on the kinetic properties of the plasma membrane H⁺-ATPase

Genotypea	Km (mM)b		Vmax (μmol/min per mg of protein)b
	GS	GF	GS	GF
PTK2 YOR267c	4.0	1.2	0.15	1.5
ptk2 YOR267c	4.0	4.0	0.13	1.9
PTK2 yor267c	4.0	2.5	0.14	1.7

^aStrains W303-1B (PTK2 YOR267c), FPY1434 (ptk2 YOR267c), and FPY1506 (PTK2 yor267c) are described in Table 1. 

^bATPase activity of purified plasma membranes was assayed at pH 6.5 with a range of Mg and ATP concentrations from 0.8 to 6 mM and an excess Mg over ATP of 5 mM (10). The apparent K_m and V_max were extrapolated from double-reciprocal plots fitted using a standard least-squares method (linear regression coefficients, r = 0.96 to 0.98). Similar values (within 10%) were obtained with two different plasma membrane preparations isolated independently. GS, glucose-starved cells; GF, glucose-fermenting cells. Cells were grown, collected, and treated or not with glucose before homogenization, and plasma membranes were isolated as described in Materials and Methods.