Characterization of mitotic motors by their relative sensitivity to AMP-PNP

The relative sensitivities of the motors for mitotic chromosome movements and saltatory motion were compared using a nonhydrolyzable analog of ATP, AMP-PNP. K+AMP-PNP was microinjected into PtKl cells at the time of nuclear envelope disassembly or at anaphase onset. To produce a dose-response curve for the effect of AMP-PNP on the rate of movement, the intracellular concentration of AMP-PNP in individual cells was measured. The volume injected into each cell was determined by adding dextrans labeled with Lucifer Yellow to the injection buffer, measuring the injected cell's fluorescence intensity, and then comparing the value with the fluorescence intensity of known volumes of Lucifer Yellow dextran solution. AMP-PNP produced a 50% inhibition of spindle elongation at 0.2 mM, of saltatory motion at 0.8 mM, and of chromosome movement at 8.6 mM. Prometaphase chromosome movement and anaphase chromosome-to-pole movement were similarly inhibited by AMP-PNP. Equivalent volumes of injection buffer containing 1% Lucifer Yellow dextran had no effect on chromosome movement, spindle elongation or saltatory motion. Although AMP-PNP occasionally produced shorter anaphase spindles, tubulin immunofluorescence revealed the presence of abundant spindle microtubules. Metaphase cells treated with very high cell concentrations of AMP-PNP had spindles with unusually long astral microtubules; thus microtubules are stabilized rather than broken down by AMP-PNP. In conclusion, spindle elongation is four times more sensitive than saltatory motion to AMP-PNP and 40 times more sensitive than chromosome movement. When these sensitivities to AMP-PNP are considered with the results from other studies, it can be concluded that the molecular motors for spindle elongation, chromosome movement and saltatory motion are different.