Yang C, Brar S K, Desbarats L, Siu C H
Banting and Best Department of Medical Research, University of Toronto, Ontario, Canada.
Differentiation. 1997 Aug;61(5):275-84. doi: 10.1046/j.1432-0436.1997.6150275.x.
In Dictyostelium discoideum, the cadA gene encodes the cell adhesion molecule DdCAD-1, a protein of M(r) 24,000, which mediates Ca(2+)-dependent cell-cell adhesion during development. We have examined the effects of cAMP, cell-cell contact, and growth conditions on cadA expression. cadA has a unique pattern of expression, which appears to be a combination of the expression patterns of early genes and aggregation-stage genes. Expression of the cadA gene in bacterially grown cells is activated at the beginning of the developmental cycle, followed by a period of rapid DdCAD-1 accumulation. The mRNA level reaches its maximum at 9 h of development and then declines to the basal level at approximately 18 h, while the protein level remains constant after reaching its maximum at 12 h. Pulse-chase experiments have demonstrated that DdCAD-1 has a significantly longer half-life than the average cellular protein. Transcription of the cadA gene is stimulated by exogenous cAMP pulses, leading to a 3- to 5-fold increase in the transcription rate. In the fgdA mutant, which lacks a functional G alpha 2, cAMP fails to enhance cadA expression, suggesting that cAMP stimulates cadA transcription via a G protein-dependent pathway. However, inhibition of cell-cell contact has no effect on the synthesis of DdCAD-1. Growth conditions also have a major influence on cadA expression. Axenically grown cells produce a high level of cadA transcripts during vegetative growth. The mRNA level shows a steady decrease during development and is reduced to the basal level by 12 h. In contrast, the level of DdCAD-1 remains relatively high throughout development, suggesting that axenic growth affects the accumulation of cadA mRNA but not the stability of the protein. These results indicate that multiple mechanisms are involved to maintain a high level of DdCAD-1 during development.
在盘基网柄菌中，cadA基因编码细胞黏附分子DdCAD-1，这是一种相对分子质量为24,000的蛋白质，在发育过程中介导依赖于Ca(2+)的细胞间黏附。我们研究了环磷酸腺苷（cAMP）、细胞间接触和生长条件对cadA表达的影响。cadA具有独特的表达模式，它似乎是早期基因和聚集期基因表达模式的组合。在细菌中生长的细胞中，cadA基因的表达在发育周期开始时被激活，随后是DdCAD-1快速积累的时期。mRNA水平在发育9小时时达到最大值，然后在大约18小时时下降到基础水平，而蛋白质水平在12小时达到最大值后保持恒定。脉冲追踪实验表明，DdCAD-1的半衰期明显长于平均细胞蛋白。cadA基因的转录受到外源性cAMP脉冲的刺激，导致转录速率增加3至5倍。在缺乏功能性Gα2的fgdA突变体中，cAMP无法增强cadA表达，这表明cAMP通过依赖G蛋白的途径刺激cadA转录。然而，抑制细胞间接触对DdCAD-1的合成没有影响。生长条件对cadA表达也有重大影响。在无菌条件下生长的细胞在营养生长期间产生高水平的cadA转录本。mRNA水平在发育过程中稳步下降，到12小时时降至基础水平。相比之下，DdCAD-1的水平在整个发育过程中保持相对较高，这表明无菌生长影响cadA mRNA的积累，但不影响蛋白质的稳定性。这些结果表明，在发育过程中涉及多种机制来维持高水平的DdCAD-1。
