Membrane regulation of the Na+, K+‐ATPase during the neuroblastoma cell cycle: Correlation with protein lateral mobility

The pumping activity of the plasma membrane‐bound Na+, K+‐ATPase shows considerable variation during the cell cycle of mouse neuroblastoma Neuro‐2A cells. Addition of external ATP at millimolar concentrations, which selectively enhances the plasma membrane permeability of Neuro‐2A cells for sodium ions, stimulates the Na+, K+‐ATPase pumping activity at all phases of the cell cycle from a factor of 1.05 in mitosis up to 2.2 in G1 phase. Determination of the number of Na+, K+‐ATPase copies per cell by direct 3H‐ouabain binding studies in the presence of external ATP shows a gradual increase in the number of pump sites on passing from mitosis to the late S/G2‐phase by approximately a factor of 2. From these data the pumping activity per copy of Na+, K+‐ATPase, optimally stimulated with respect to its various substrate ions, has been determined during the various phases of the cell cycle. This optimally stimulated pumping activity per enzyme copy, which is a reflection of the physicochemical state of the plasma membrane, is high in mitosis, almost twofold lower in early G1 phase, and increases gradually again during the other phases of the cell cycle. This shows that the observed regulation of Na+, K+‐ATPase activity during the cell cycle is caused by a combination of three independent factors–namely variation in intracellular substrate availability (Na+), changes in number of enzyme copies per cell, and modulation of the plasma membrane environment of the protein molecules. The modulation of the optimal pumping activity per enzyme copy shows a good correlation (ρ = 0.96) with the known modulation of protein lateral mobility during the cell cycle, such that a high protein lateral mobility correlates with a low enzyme activity. It is concluded that changes in plasma membrane properties take place during the Neuro‐2A cell cycle that result in changes in the rate of protein lateral diffusion and Na+, K+‐ATPase activity in a directly correlated way.