Moorthamer M, Zumstein-Mecker S, Stephan C, Mittl P, Chaudhuri B
Oncology Research, Novartis Pharma AG, Basel, Switzerland.
Biochem Biophys Res Commun. 1998 Dec 18;253(2):305-10. doi: 10.1006/bbrc.1998.9737.
The cyclin-dependent kinase 5 (Cdk5) catalytic subunit is expressed in both cycling and noncycling cells and is present in many tissues. Neuronal and muscle cells contain the highest amount of this protein. The p35 protein, which is expressed solely in the brain, activates Cdk5. Cdk5 activity is involved in terminal differentiation of neurons and muscle cells. We attempted to clone cdk5 by PCR from a human fetal brain cDNA library. Surprisingly, we amplified two forms of the cdk5 gene, the wild type and a cdk5 variant that lacks the complete kinase domain VI. The variant is also found in SH-SY-5Y neuroblastoma cells but not in T-cells, HeLa cells, the thymus, and placental tissue. The protein encoded by the cdk5 variant, the Cdk5 isoform (Cdk5i), purifies with p35 when coexpressed in insect cells. The activity associated with the heterodimer Cdk5i/p35 is found to be appreciably weaker than the wild-type Cdk5/p35 kinase. Moreover, Cdk5i/p35 cannot autophosphorylate its two subunits as with Cdk5/p35. Interestingly, kinase-defective Cdk5i can abolish the activity of wild-type Cdk5 when both are coexpressed with p35 in insect cells, suggesting that Cdk5i may have a function in regulating Cdk5 activity in human cells too.
细胞周期蛋白依赖性激酶5（Cdk5）催化亚基在处于细胞周期和非细胞周期的细胞中均有表达，且存在于多种组织中。神经元和肌肉细胞中这种蛋白的含量最高。仅在大脑中表达的p35蛋白可激活Cdk5。Cdk5活性参与神经元和肌肉细胞的终末分化。我们试图通过聚合酶链反应（PCR）从人胎脑cDNA文库中克隆cdk5。令人惊讶的是，我们扩增出了两种形式的cdk5基因，即野生型和一种缺少完整激酶结构域VI的cdk5变体。该变体也存在于SH-SY-5Y神经母细胞瘤细胞中，但在T细胞、HeLa细胞、胸腺和胎盘组织中未发现。当在昆虫细胞中共表达时，由cdk5变体编码的蛋白，即Cdk5同工型（Cdk5i），可与p35一起纯化。发现与异二聚体Cdk5i/p35相关的活性明显弱于野生型Cdk5/p35激酶。此外，Cdk5i/p35不能像Cdk5/p35那样自磷酸化其两个亚基。有趣的是，当激酶缺陷型Cdk5i与野生型Cdk5在昆虫细胞中与p35共表达时，激酶缺陷型Cdk5i可消除野生型Cdk5的活性，这表明Cdk5i在人类细胞中可能也具有调节Cdk5活性的功能。
