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.