期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1981
卷号：78
期号：12
页码：7299-7303
DOI：10.1073/pnas.78.12.7299
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：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.
