The conserved ubiquitin-like protein Hub1 plays a critical role in splicing in human cells.

Kaja Kowalska,Stefan Jentsch,Tad A Holak,Grzegorz M Popowicz,Shravan Kumar Mishra,Tim Ammon

doi:10.1093/jmcb/mju026

Abstract

Different from canonical ubiquitin-like proteins, Hub1 does not form covalent conjugates with substrates but binds proteins non-covalently. In Saccharomyces cerevisiae, Hub1 associates with spliceosomes and mediates alternative splicing of SRC1, without affecting pre-mRNA splicing generally. Human Hub1 is highly similar to its yeast homolog, but its cellular function remains largely unexplored. Here, we show that human Hub1 binds to the spliceosomal protein Snu66 as in yeast; however, unlike its S. cerevisiae homolog, human Hub1 is essential for viability. Prolonged in vivo depletion of human Hub1 leads to various cellular defects, including splicing speckle abnormalities, partial nuclear retention of mRNAs, mitotic catastrophe, and consequently cell death by apoptosis. Early consequences of Hub1 depletion are severe splicing defects, however, only for specific splice sites leading to exon skipping and intron retention. Thus, the ubiquitin-like protein Hub1 is not a canonical spliceosomal factor needed generally for splicing, but rather a modulator of spliceosome performance and facilitator of alternative splicing.

Highlights

Ubiquitin family proteinsare central regulators of cellular functions (Hochstrasser, 2000)
To address whether human Hub1 is involved in splicing, we first asked whether expression of human Hub1 could complement the phenotypes of the Hub1 deletion mutants of S. cerevisiae and S. pombe
Since S. cerevisiae strains with a deletion of the Hub1-encoding gene are viable and cells exhibit no discernable growth defects, we rather assayed in a genetic background in which Hub1 becomes essential for viability (Mishra et al, 2011)

Summary

Introduction

Are central regulators of cellular functions (Hochstrasser, 2000). Canonical members of this protein family are enzymatically and reversibly conjugated to other proteins, thereby functioning as covalent protein ‘modifiers’. Structurally very similar to ubiquitin (McNally et al, 2003; Ramelot et al, 2003), the highly conserved protein Hub does not function as a covalent modifier but binds proteins only non-covalently (Luders et al, 2003; Yashiroda and Tanaka, 2004; Mishra et al, 2011). In Saccharomyces cerevisiae, Hub binds tightly to the spliceosomal protein Snu, a protein of the U4/U6.U5 small nuclear ribonucleic particle (tri-snRNP) (Wilkinson et al, 2004; Mishra et al, 2011).

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Conclusion