Improved efficiency of protein aggregate removal using ubiquitin with a signal peptide

Lipocalin-2 (LCN2) is a protein secreted by activated astrocytes, and its signal peptide (SP) is essential for secretion and recruitment to the autophagic pathway. SP is a short sequence present at the N-terminus of secreted proteins, such as LCN2, which facilitates transport to the endoplasmic reticulum (ER). Although SP is cleaved during the initial stages of translation in the ER, it influences the subsequent pathways of mature proteins produced in the ER lumen. ER-generated proteins are secreted or recruited to the autophagic pathway. To explore this further, we sought to determine the functional role of SP from a novel perspective. In this study, we fused LCN2 SP to the N-terminus of ubiquitin (Ub), an intracellular protein used for the proteasomal degradation of misfolded proteins and autophagic degradation of protein aggregates. We demonstrated that SP enabled the secretion of free Ub and facilitated the targeting of Ub conjugates to the autophagic pathway. We also found that SP affected intracellular Ub conjugate levels by regulating their degradation via the autophagic pathway. Furthermore, the ER-generated Ub (Ub^E) showed increased participation in polyubiquitinating protein aggregates generated under proteotoxic stress conditions, promoting the formation of perinuclear aggresome-like structures, and recruitment to the autophagosome. It is highly likely that Ub^E shares a common route with protein aggregates before being recruited to autophagosomes. Thus, this study suggests that Ub^E confers an altered trafficking pathway compared with endogenous Ub, thereby facilitating protein aggregate clearance without altering Ub’s intrinsic biochemical activity.