The Cajal body protein p80-coilin forms a complex with the adenovirus L4-22K protein and facilitates the nuclear export of adenovirus mRNA

Cajal bodies (CBs) are major sub-nuclear structures in most eucaryotic cells. In human adenovirus 5 (Ad5) infection, CBs are reorganized into microfoci in the late phase of infection. Here we show that many CB protein components (p80-coilin, SMN-1, and WRAP53) remained stable throughout most of the infectious cycle of Ad type 5 (Ad5) in human A549 epithelial cells, even when CBs were reorganized into microfoci. Reduction of p80-coilin expression by RNA interference resulted in significant reductions in the levels of early (E1A, E2A-DBP), intermediate (pIX and IVa2), and late (L1-IIIa, L2-penton base, L3-hexon, L4-100K, and L5-fiber) proteins in Ad5-infected A549 cells. Depletion of p80-coilin did not significantly alter the total cellular levels of the corresponding Ad cytoplasmic mRNAs (with the exception of E1A 12S and 13S and pIX mRNA) or the production of the Ad5 pre-mRNAs tested (E1A, E2A-DBP, IVa2, or late pre-mRNAs containing the tripartite leader). However, the ratio of viral cytoplasmic to nuclear-spliced Ad RNAs was reduced in p80-coilin-depleted, Ad5-infected cells compared to control-infected cells. Immunofluorescent staining of Ad5-infected cells revealed co-localization of p80-coilin with areas of immunoreactivity defined by a polyclonal antibody that recognized the L4-22K and L4-33K proteins in a fraction of microfoci. Immunoprecipitation analysis showed that only the L4-22K protein formed a stable complex with p80-coilin in Ad5-infected cells and in cells co-transfected with plasmids encoding p80-coilin and either the L4-22K or L4-33K protein. p80-coilin therefore plays an important role in Ad replication by complex formation with L4-22K and facilitating nuclear export of Ad mRNAs.