Nandini-Kishore S G, Frazier W A
Proc Natl Acad Sci U S A. 1981 Dec;78(12):7299-303. doi: 10.1073/pnas.78.12.7299.
Studies of the folate chemotactic receptor of vegetative Dictyostelium discoideum cells have been hampered by the presence of the degradative enzyme folate deaminase. The diaminopterin compounds aminopterin and methotrexate (MTX) are chemoattractants but are not attacked by the deaminase. [3',5',7,9-3H]methotrexate ([3H]MTX) is a nondegraded radioligand for the folate receptor. Binding to the receptor is rapid, reaching steady state in less than one min, and reversible in less than 15 s by an excess of unlabeled MTX. A single class of binding sites is found with a Kd of 2 x 10(-8) M, which correlates well with the concentration dependence of chemotaxis. Folate, aminopterin, and MTX all compete for [3H]MTX binding, whereas pterin, p-aminobenzoate, and nucleotides do not. Analysis of the receptor during differentiation indicates a decrease in site number by a factor of 3 with no change in affinity during the first 7 hr. During this time, the directional response (chemotaxis) to MTX and folate is lost, but a nondirectional stimulation of motility rate (chemokinesis) is retained. The response to cyclic AMP displays reciprocal behavior, first appearing as a chemokinetic response and then as a chemotactic response.
营养期盘基网柄菌细胞叶酸趋化受体的研究因降解酶叶酸脱氨酶的存在而受阻。二氨基蝶呤化合物氨基蝶呤和甲氨蝶呤（MTX）是趋化剂，但不会被脱氨酶攻击。[3',5',7,9 - 3H]甲氨蝶呤（[3H]MTX）是叶酸受体的一种非降解放射性配体。与受体的结合迅速，不到一分钟达到稳态，并且在不到15秒内可被过量的未标记MTX逆转。发现一类结合位点，其解离常数（Kd）为2×10(-8) M，这与趋化作用的浓度依赖性密切相关。叶酸、氨基蝶呤和MTX都竞争[3H]MTX的结合，而蝶呤、对氨基苯甲酸和核苷酸则不竞争。对分化过程中受体的分析表明，在前7小时内，结合位点数量减少了3倍，亲和力没有变化。在此期间，对MTX和叶酸的定向反应（趋化作用）丧失，但保留了对运动速率的非定向刺激（化学增活作用）。对环磷酸腺苷（cAMP）的反应表现出相反的行为，首先表现为化学增活反应，然后表现为趋化反应。
