Inchingolo, M. A. | Search bioRxiv

bioRxiv2026

The microprotein SEP53BP1: its bizarre mode of translational expression and intracellular behaviour.

Microproteins are proteins of <100 amino acids. They represent a major, and until recently, overlooked fraction of the human proteome. However, it has now been demonstrated that many of these proteins play key roles in cellular physiology. Our group reported the expression of a microprotein expressed from an ioORF within the 53BP1 CDS arising as a result of delayed translational reinitiation mediated by a small uORF within the 5 TL. We named this microprotein SEP53BP1. We have sought to expand these studies with the ultimate aim of establishing a function for this microprotein. Although this remains elusive, we report findings providing new insights into the elements regulating its translation and demonstrate that the SEP53BP1 sequence serves as a Golgi targeting tag. Lastly, despite the fact that subunits of the proteasome feature prominently on interactome studies we were unable to demonstrate an impact of microprotein over-expression on the activities of both the proteasome and immunoproteasome.

molecular biology↗

bioRxiv2021

A small protein, derived from an alternative 53BP1 promoter transcript and expressed via translational reinitiation on an internal overlapping ORF, modulates proteasome activity.

The complexity of the metazoan proteome is significantly increased by the expression of small proteins (<100 aas) derived from smORFs within lncRNAs, uORFs, 3 UTRs and, more rarely, reading frames overlapping the CDS. These smORF encoded proteins (SEPs) can have diverse roles, ranging from the regulation of cellular physiological to essential developmental functions. We report the characterisation of a new member of this protein family, SEP53BP1, derived from a small internal ORF that overlaps the CDS that encodes 53BP1. Its expression is coupled to the utilisation of an alternative, cell-type specific, promoter coupled to translational reinitiation events mediated by a uORF in the alternative 5 TL of the mRNA. The uORF-mediated initiation at the internal AUG53BP1 is conserved in metazoan species ranging from human to zebrafish. As such, it couples SEP53BP1 expression to the integrated stress response (ISR). We demonstrate that one function of this protein is to interact with, and stimulate, the activity of the 26S proteasome. As such, it opens the door to new approaches in the treatment of clinical conditions that arise due to the accumulation of toxic intracellular protein aggregates

molecular biology↗