Van Haastert P J, Wang M, Bominaar A A, Devreotes P N, Schaap P
Department of Biochemistry, University of Groningen, The Netherlands.
Mol Biol Cell. 1992 Jun;3(6):603-12. doi: 10.1091/mbc.3.6.603.
Surface cAMP receptors on Dictyostelium cells are linked to several second messenger systems and mediate multiple physiological responses, including chemotaxis and differentiation. Activation of the receptor also triggers events which desensitize signal transduction. These events include the following: 1) loss of ligand binding without loss of receptor protein; 2) phosphorylation of the receptor protein, which may lead to impaired signal transduction; 3) redistribution and degradation of the receptor protein; and 4) decrease of cyclic AMP (cAMP) receptor mRNA levels. These mechanisms of desensitization were investigated with the use of mutant synag7, with no activation of adenylyl cyclase; fgdC, with no activation of phospholipase C; and fgdA, with defects in both pathways. cAMP-induced receptor phosphorylation and loss of ligand binding activity was normal in all mutants. In contrast, cAMP-induced degradation of the receptor was absent in all mutants. The cAMP-induced decrease of cAMP-receptor mRNA levels was normal in mutant synag7, but absent in mutant fgdC. Finally, the cAMP analogue (Rp)-cAMPS induced loss of ligand binding without inducing second messenger responses or phosphorylation, redistribution, and degradation of the receptor. We conclude that 1) loss of ligand binding can occur in the absence of receptor phosphorylation; 2) loss of ligand binding and receptor phosphorylation do not require the activation of second messenger systems; 3) cAMP-induced degradation of the receptor may require the phosphorylation of the receptor as well as the activation of at least the synag7 and fgdC gene products; and 4) cAMP-induced decrease of receptor mRNA levels requires the activation of the fgdC gene product and not the synag7 gene product. These results imply that desensitization is composed of multiple components that are regulated by different but partly overlapping sensory transduction pathways.
盘基网柄菌细胞表面的环磷酸腺苷（cAMP）受体与多种第二信使系统相连，并介导多种生理反应，包括趋化性和分化。受体的激活还会引发使信号转导脱敏的事件。这些事件包括：1）配体结合丧失但受体蛋白不丧失；2）受体蛋白磷酸化，这可能导致信号转导受损；3）受体蛋白的重新分布和降解；4）环磷酸腺苷（cAMP）受体mRNA水平降低。利用无腺苷酸环化酶激活的突变体synag7、无磷脂酶C激活的fgdC以及两条途径均有缺陷的fgdA，对这些脱敏机制进行了研究。在所有突变体中，cAMP诱导的受体磷酸化和配体结合活性丧失均正常。相反，在所有突变体中均不存在cAMP诱导的受体降解。cAMP诱导的cAMP受体mRNA水平降低在突变体synag7中正常，但在突变体fgdC中不存在。最后，cAMP类似物（Rp）-cAMPS诱导配体结合丧失，而不诱导第二信使反应或受体的磷酸化、重新分布和降解。我们得出以下结论：1）在没有受体磷酸化的情况下可能发生配体结合丧失；2）配体结合丧失和受体磷酸化不需要第二信使系统的激活；3）cAMP诱导的受体降解可能需要受体磷酸化以及至少synag7和fgdC基因产物的激活；4）cAMP诱导的受体mRNA水平降低需要fgdC基因产物的激活，而不是synag7基因产物的激活。这些结果表明，脱敏由多个组分组成，这些组分由不同但部分重叠的感觉转导途径调节。
