Casarosa Paola, Gruijthuijsen Yvonne K, Michel Detlef, Beisser Patrick S, Holl Jens, Fitzsimons Carlos P, Verzijl Dennis, Bruggeman Cathrien A, Mertens Thomas, Leurs Rob, Vink Cornelis, Smit Martine J
Leiden/Amsterdam Center for Drug Research, Division of Medicinal Chemistry, Faculty of Chemistry, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
J Biol Chem. 2003 Dec 12;278(50):50010-23. doi: 10.1074/jbc.M306530200. Epub 2003 Sep 30.
The human cytomegalovirus (HCMV) UL33 gene is conserved among all beta-herpesviruses and encodes a protein that shows sequence similarity with chemokine receptors belonging to the family of G protein-coupled receptors. Here, we show that HCMV UL33 is predominantly transcribed as a spliced mRNA of which the 5' terminus is localized 55 bp upstream of the start codon. Like its homolog from rat cytomegalovirus (RCMV), R33, UL33 activates multiple signaling pathways in a ligand-independent manner. Although both receptors constitutively activate phospholipase C via G(q/11), and partially via G(i/o)-mediated pathways, they exhibit profound differences in the modulation of cAMP-responsive element (CRE) activation. R33 constitutively inhibits, whereas UL33 constitutively enhances CRE-mediated transcription. For R33, the inhibition of CRE-driven transcription is entirely G(i/o)-mediated. For UL33, however, CRE-mediated transcription is modulated not only through coupling to Galpha(i/o) but also through coupling to Galphas. In addition, UL33 was found to enhance CRE activation through the Rho/p38 pathway, via Gbetagamma. Interestingly, by studying chimeric UL33/R33 proteins, we found the C-terminal cytoplasmic tail of UL33, but not that of R33, to be responsible for the activation of G(i/o) proteins. A UL33-deficient variant of HCMV was generated to analyze UL33-signaling properties in a physiologically relevant model system. Data obtained with infected cells show that HCMV induces CRE activation, and this effect is, at least in part, dependent on UL33 expression. Taken together, our data indicate that constitutive signaling of UL33 differs from that of R33 by promiscuous activation of G proteins of the Gq, G(i/o), as well as Gs class. Thus, HCMV may effectively use UL33 to orchestrate multiple signaling networks within infected cells.
人类巨细胞病毒（HCMV）的UL33基因在所有β疱疹病毒中都是保守的，它编码一种蛋白质，该蛋白质与属于G蛋白偶联受体家族的趋化因子受体具有序列相似性。在此，我们表明HCMV UL33主要转录为一种剪接的mRNA，其5'末端位于起始密码子上游55 bp处。与大鼠巨细胞病毒（RCMV）的同源物R33一样，UL33以不依赖配体的方式激活多种信号通路。尽管这两种受体都通过G(q/11)组成型激活磷脂酶C，并部分通过G(i/o)介导的途径激活，但它们在调节环磷酸腺苷反应元件（CRE）激活方面表现出显著差异。R33组成型抑制，而UL33组成型增强CRE介导的转录。对于R33，对CRE驱动转录的抑制完全由G(i/o)介导。然而，对于UL33，CRE介导的转录不仅通过与Gα(i/o)偶联进行调节，还通过与Gαs偶联进行调节。此外，发现UL33通过Rho/p38途径，经由Gβγ增强CRE激活。有趣的是，通过研究嵌合的UL33/R33蛋白，我们发现UL33的C末端胞质尾巴而非R33的C末端胞质尾巴负责激活G(i/o)蛋白。我们构建了一种HCMV的UL33缺陷变体，以在生理相关的模型系统中分析UL33的信号特性。用感染细胞获得的数据表明，HCMV诱导CRE激活，并且这种效应至少部分依赖于UL33的表达。综上所述，我们的数据表明，UL33的组成型信号传导与R33的不同，它能杂乱地激活Gq、G(i/o)以及Gs类的G蛋白。因此，HCMV可能有效地利用UL33来协调受感染细胞内的多个信号网络。
