Department of Hematology and Oncology Lorenzo e Ariosto Seràgnoli, University of Bologna, Medical School, Bologna, Italy.
Virology. 2011 Sep 1;417(2):259-67. doi: 10.1016/j.virol.2011.06.015. Epub 2011 Jul 8.
The processivity factor of human cytomegalovirus DNA polymerase, phosphoprotein ppUL44, is essential for viral replication. During viral infection ppUL44 is phosphorylated by the viral kinase pUL97, but neither the target residues on ppUL44 nor the effect of phosphorylation on ppUL44's activity are known. We report here that ppUL44 is phosphorylated when transiently expressed in mammalian cells and coimmunoprecipitates with cellular kinases. Of three potential phosphorylation sites (S413, S415, S418) located upstream of ppUL44's nuclear localization signal (NLS) and one (T427) within the NLS itself, protein kinase CK2 (CK2) specifically phosphorylates S413, to trigger a cascade of phosphorylation of S418 and S415 by CK1 and CK2, respectively. Negative charge at the CK2/CK1 target serine residues facilitates optimal nuclear accumulation of ppUL44, whereas negative charge on T427, a potential cyclin-dependent 1 phosphorylation site, strongly decreases nuclear accumulation. Thus, nuclear transport of ppUL44 is finely tuned during viral infection through complex phosphorylation events.
人巨细胞病毒 DNA 聚合酶磷酸蛋白 ppUL44 的持续合成因子对于病毒复制是必需的。在病毒感染过程中，ppUL44 被病毒激酶 pUL97 磷酸化，但 ppUL44 的磷酸化靶位残基以及磷酸化对 ppUL44 活性的影响均未知。我们在此报告，ppUL44 在哺乳动物细胞中转染瞬时表达时会发生磷酸化，并与细胞激酶共免疫沉淀。ppUL44 的核定位信号（NLS）上游的三个潜在磷酸化位点（S413、S415、S418）和 NLS 内的一个（T427）中，蛋白激酶 CK2（CK2）特异性磷酸化 S413，分别触发 CK1 和 CK2 对 S418 和 S415 的级联磷酸化。CK2/CK1 靶位丝氨酸残基上的负电荷有助于 ppUL44 的最佳核积累，而 T427（潜在的周期蛋白依赖性激酶 1 磷酸化位点）上的负电荷则强烈降低核积累。因此，在病毒感染过程中，ppUL44 的核转运通过复杂的磷酸化事件进行精细调节。
