A bifunctional ARHGEF17-AS1 locus encodes MSEP, a protein that supports mitotic spindle organization.

A novel ∼54-kDa protein, MSEP, encoded by the ARHGEF17-AS1 locus, is essential for mitotic spindle organization and localizes to spindle poles.

• Why it matters: Understanding how bifunctional loci produce both coding and noncoding outputs is crucial, as many encoded proteins remain uncharacterized, limiting insights into cell division mechanisms.
• What they did: Researchers identified MSEP through open reading frame analysis within ARHGEF17-AS1, examined its localization, interactions with TPX2, and regulation by cell-cycle kinases Aurora A and WEE1, using genetic and biochemical methods.
• The result: Depleting MSEP caused spindle abnormalities and reduced TPX2 and Aurora A at spindle poles, revealing that MSEP and ARHGEF17-AS1 RNA independently support spindle integrity through parallel pathways.