The nuclear cap-binding complex interacts with the U4/U6·U5 tri-snRNP and promotes spliceosome assembly in mammalian cells

The nuclear cap-binding complex (CBC) binds to the 7-methyl guanosine cap present on every RNA polymerase II transcript. CBC has been implicated in many aspects of RNA biogenesis; in addition to roles in miRNA biogenesis, nonsense-mediated decay, 3'-end formation, and snRNA export from the nucleus, CBC promotes pre-mRNA splicing. An unresolved question is how CBC participates in splicing. To investigate CBC's role in splicing, we used mass spectrometry to identify proteins that copurify with mammalian CBC. Numerous components of spliceosomal snRNPs were specifically detected. Among these, three U4/U6·U5 snRNP proteins (hBrr2, hPrp4, and hPrp31) copurified with CBC in an RNA-independent fashion, suggesting that a significant fraction of CBC forms a complex with the U4/U6·U5 snRNP and that the activity of CBC might be associated with snRNP recruitment to pre-mRNA. To test this possibility, CBC was depleted from HeLa cells by RNAi. Chromatin immunoprecipitation and live-cell imaging assays revealed decreased cotranscriptional accumulation of U4/U6·U5 snRNPs on active transcription units, consistent with a requirement for CBC in cotranscriptional spliceosome assembly. Surprisingly, recruitment of U1 and U2 snRNPs was also affected, indicating that RNA-mediated interactions between CBC and snRNPs contribute to splicing. On the other hand, CBC depletion did not impair snRNP biogenesis, ruling out the possibility that decreased snRNP recruitment was due to changes in nuclear snRNP concentration. Taken together, the data support a model whereby CBC promotes pre-mRNA splicing through a network of interactions with and among spliceosomal snRNPs during cotranscriptional spliceosome assembly.

Identification of a binding site for ASF/SF2 on an RNA fragment derived from the hepatitis delta virus genome

The hepatitis delta virus (HDV) is a small (∼1700 nucleotides) RNA pathogen which encodes only one open reading frame. Consequently, HDV is dependent on host proteins to replicate its RNA genome. Recently, we reported that ASF/SF2 binds directly and specifically to an HDV-derived RNA fragment which has RNA polymerase II promoter activity. Here, we localized the binding site of ASF/SF2 on the HDV RNA fragment by performing binding experiments using purified recombinant ASF/SF2 combined with deletion analysis and site-directed mutagenesis. In addition, we investigated the requirement of ASF/SF2 for HDV RNA replication using RNAi-mediated knock-down of ASF/SF2 in 293 cells replicating HDV RNA. Overall, our results indicate that ASF/SF2 binds to a purine-rich region distant from both the previously published initiation site of HDV mRNA transcription and binding site of RNAP II, and suggest that this protein is not involved in HDV replication in the cellular system used.

Central and baroreflex control of heart rate during the wake-sleep cycle in rat

Spontaneous fluctuations in Heart Period (HP) and Mean Arterial Pressure (MAP) make it possible to evaluate baroreceptor-heart rate reflex sensitivity (BRS). 30-s sequences of HP and MAP beat-to-beat values were considered in the different wake-sleep states (Wake, W; Quiet Sleep, QS; Active Sleep, AS) in rats to assess whether 1) BRS changes between states and 2) the different indexes supply consistent BRS measures. BRS indexes were calculated according to validated literature procedures as regression coefficients of HP vs. MAP 1) within all ramps of increasing or decreasing MAP of four beats or more, with HP and MAP changing in the same direction (baroreflex-mediated fluctuations, BRSp), 2) within all such ramps irrespective of the relative direction of HP and MAP changes (baroreflex + non-baroreflex, i.e. non-homeostatic centrally driven, fluctuations, BRSA). HP vs. MAP regression coefficient along the entire 30-s sequence (bHPMAP) was also calculated.

RESULTS

BRSp did not change among states, BRSA decreased from QS to W to AS, bHPMAP decreased from QS to W and became negative in AS.

CONCLUSIONS

1) as indicated by BRSp, baroreflex sensitivity is state independent, 2) BRSp to BRS(A) to bHPMAP are increasingly affected by non-baroreflex fluctuations, BRSp being most apt to measure BRS, 3) non-homeostatic MAP and HP fluctuations increase from QS to W and prevail in AS. These potentially harmful fluctuations are normally buffered by baroreflexes: in the case of baroreflex impairment, circulatory risk may arise in conditions like AS, when they prevail.