In yeast, the TREX complex contains the THO transcription elongation complex, which functions in direct cotranscriptional recruitment of the mRNA export proteins Sub2 and Yra1 to nascent transcripts. Here we report the identification of the human THO complex and show that it associates with spliced mRNA, but not with unspliced pre-mRNA in vitro. Transcription is not required for this recruitment. We also show that the human THO complex colocalizes with splicing factors in nuclear speckle domains in vivo. Considering that splicing occurs cotranscriptionally in humans, our data indicate that recruitment of the human TREX complex to spliced mRNA is not directly coupled to transcription, but is instead coupled to transcription indirectly through splicing.

Transcription and splicing are functionally coupled, resulting in highly efficient splicing of RNA polymerase II (RNAP II) transcripts. The mechanism involved in this coupling is not known. To identify potential coupling factors, we carried out a comprehensive proteomic analysis of immunopurified human RNAP II, identifying >100 specifically associated proteins. Among these are the SR protein family of splicing factors and all of the components of U1 snRNP, but no other snRNPs or splicing factors. We show that SR proteins function in coupling transcription to splicing and provide evidence that the mechanism involves cotranscriptional recruitment of SR proteins to RNAP II transcripts. We propose that the exclusive association of U1 snRNP/SR proteins with RNAP II positions these splicing factors, which are known to function early in spliceosome assembly, close to the nascent pre-mRNA. Thus, these factors readily out-compete inhibitory hnRNP proteins, resulting in efficient spliceosome assembly on nascent RNAP II transcripts.

Summary Phosphoinositide-3-kinase (PI3K)-α inhibitors have shown clinical activity in squamous carcinoma (SCC) of head and neck (H&N) bearing PIK3CA mutations or amplification. Studying models of therapeutic resistance we have observed that SCCs cells that become refractory to PI3Kα inhibition maintain PI3K-independent activation of the mammalian target of rapamycin (mTOR). This persistent mTOR activation is mediated by the tyrosine kinase receptor AXL. AXL is overexpressed in resistant tumors from both laboratory models and patients treated with the PI3Kα inhibitor BYL719. AXL dimerizes with and phosphorylates epidermal growth factor receptor (EGFR), resulting in activation of phospholipase Cγ (PLCγ)- protein kinase C (PKC), which in turn activates mTOR. Combined treatment with PI3Kα and either EGFR, AXL, or PKC inhibitors reverts this resistance.

Summary Mutations in the RNA binding protein FUS cause ALS, a fatal adult motor neuron disease. Decreased expression of SMN causes the fatal childhood motor neuron disorder SMA. The SMN complex localizes in both the cytoplasm and nuclear Gems, and loss of Gems is a cellular hallmark of SMA patient fibroblasts. Here, we report that FUS associates with the SMN complex, an interaction mediated by U1 snRNP and by direct interactions between FUS and SMN. Functionally, we show that FUS is required for Gem formation in HeLa cells, and expression of FUS containing a severe ALS-causing mutation (R495X) also results in Gem loss. Strikingly, a reduction in Gems is observed in ALS patient fibroblasts expressing either mutant FUS or TDP-43, another ALS-causing protein that interacts with FUS. The physical and functional interactions between SMN, FUS, TDP-43, and Gems indicate that ALS and SMA share a biochemical pathway, adding strong new support to the view that these motor neuron diseases are related.

In the current model for spliceosome assembly, U1 snRNP binds to the 5' splice site in the E complex followed by ATP-dependent binding of U2 snRNP to the branchpoint sequence (BPS) in the A complex. Here we report the characterization of highly purified, functional E complex. We provide evidence that this complex contains functional U2 snRNP and that this snRNP is required for E complex assembly. The BPS is not required for U2 snRNP binding in the E complex. These data suggest a model for spliceosome assembly in which U1 and U2 snRNPs first associate with the spliceosome in the E complex and then an ATP-dependent step results in highly stable U2 snRNP binding to the BPS in the A complex.