The Thoc1 Encoded Ribonucleoprotein Is a Substrate for the NEDD4-1 E3 Ubiquitin Protein Ligase

Ribonucleoprotein (RNP) complexes form around nascent RNA during transcription to facilitate proper transcriptional elongation, RNA processing, and nuclear export. RNPs are highly heterogeneous, and different types of RNPs tend to package functionally related transcripts. These observations have inspired the hypothesis that RNP mediated mechanisms help specify coordinated gene expression. This hypothesis is supported by the observation that mutations in RNP components can cause defects in specific developmental pathways. How RNP biogenesis itself is regulated, however, is not well understood. The evolutionarily conserved THO RNP complex functions early during transcription to package nascent transcripts and facilitate subsequent RNP biogenesis. THO deficiency compromises transcriptional elongation as well as RNP mediated events like 3' end formation and nuclear export for some transcripts. Using molecularly manipulated cells and in vitro reconstituted biochemical reactions, we demonstrate that the essential THO protein component encoded by the Thoc1 gene is poly-ubiquitinated by the NEDD4-1 E3 ubiquitin ligase. Poly-ubiquitinated pThoc1 is degraded by the proteasome. These results indicate THO activity is regulated by the ubiquitin-proteasome pathway, and that this regulation is evolutionarily conserved between yeast and mammals. Manipulation of NEDD4-1 levels has modest effects on Thoc1 protein levels under steady state conditions, but destabilization of Thoc1 protein upon treatment with a transcriptional elongation inhibitor is dependent on NEDD4-1. This suggests NEDD4-1 functions in conjunction with other post-translational mechanisms to regulate Thoc1 protein and THO activity.