title: Regulation of the microtubule-lysosome interaction: activation by Mg2+ and inhibition by ATP.
abstract: We developed a sedimentation assay to characterize and quantify the association of purified lysosomes to reconstituted microtubules (Mithieux, G., Audebet, C. and Rousset. B. (1988) Biochim. Biophys. Acta 969, 121-130). In the present work, we have examined the potential regulatory role of ATP and Mg2+ on the microtubule-lysosome interaction. The formation of microtubule-lysosome complexes takes place in the absence of Mg2+, but is activated by the addition of Mg2+; both the rate of the interaction and the amount of complexes formed are increased. The maximal effect is observed between 1.5 and 3.5 mM free Mg2+. Measured at the plateau of the interaction, the proportion of microtubules bound to lysosomes increases as a function of free Mg2+ concentration; at optimal concentration of free Mg2+, 90% of the microtubules present in the incubation mixture are bound to lysosomes. ATP induces a concentration-dependent inhibition of the formation of microtubule-lysosome complexes. The half-maximal effect is obtained at an ATP concentration of 0.83 +/- 0.11 mM (n = 7). The effect of ATP is not related to ATP hydrolysis, since ATP exerts its inhibitory action in the presence of EDTA. The ATP effect is mimicked by GTP, p[NH]ppA and tripolyphosphate, ADP and pyrophosphate, but not by AMP or phosphate. In the presence of 1 mM ATP, a Mg2+ concentration of 3 mM (corresponding to 2 mM free Mg2+) is required to overcome the inhibition caused by ATP; above 3 mM, Mg2+ exerts its activating effect. Since the modulating effects of ATP and Mg2+ are obtained at concentrations closed to those occurring in intact cells, we conclude that the regulation of the microtubule-lysosome interaction reported in this paper could be of physiological significance.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D004492:Edetic Acid / Q000494:pharmacology; D008247:Lysosomes / Q000187:drug effects* / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology*; D008278:Magnesium Sulfate / Q000494:pharmacology; D008868:Microtubule Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008870:Microtubules / Q000187:drug effects* / Q000378:metabolism; D013552:Swine
chemical_list: D008868:Microtubule Proteins; D008278:Magnesium Sulfate; D000255:Adenosine Triphosphate; D004492:Edetic Acid; D008274:Magnesium