Novel mode of phosphorylation-triggered reorganization of the nuclear lamina during nuclear egress of human cytomegalovirus.

The nucleocytoplasmic egress of viral capsids is a rate-limiting step in the replication of the human cytomegalovirus (HCMV). As reported recently, an HCMV-specific nuclear egress complex is composed of viral and cellular proteins, in particular protein kinases with the capacity to induce destabilization of the nuclear lamina. Viral protein kinase pUL97 and cellular protein kinase C (PKC) play important roles by phosphorylating several types of nuclear lamins. Using pUL97 mutants, we show that the lamin-phosphorylating activity of pUL97 is associated with a reorganization of nuclear lamin A/C. Either pUL97 or PKC has the potential to induce distinct punctate lamina-depleted areas at the periphery of the nuclear envelope, which were detectable in transiently transfected and HCMV-infected cells. Using recombinant HCMV, which produces green fluorescent protein-labeled viral capsids, the direct transition of viral capsids through these areas could be visualized. This process was sensitive to an inhibitor of pUL97/PKC activity. The pUL97-mediated phosphorylation of lamin A/C at Ser(22) generated a novel binding motif for the peptidyl-prolyl cis/trans-isomerase Pin1. In HCMV-infected fibroblasts, the physiological localization of Pin1 was altered, leading to recruitment of Pin1 to viral replication centers and to the nuclear lamina. The local increase in Pin1 peptidyl-prolyl cis/trans-isomerase activity may promote conformational modulation of lamins. Thus, we postulate a novel phosphorylation-triggered mechanism for the reorganization of the nuclear lamina in HCMV-infected cells.