Native Salt Bridges Are a Key Regulator of Ubiquitin's Mechanical Stability

Although it is known that various intramolecular interactions determine protein mechanical stability, a detailed molecular-level understanding of the key regulators of protein mechanical stability is still lacking. Here, we present evidence for salt bridges in ubiquitin as important intramolecular interactions that can affect protein mechanical stability. Ubiquitin has two salt bridges: one relatively surface-exposed (SB1:K11-E34) and the other relatively buried (SB2:K27-D52). Ubiquitin is a reversible post-translational modifier and is stable mechanically (F_avg^u = 185 pN). On breaking SB1, the mechanical stability of ubiquitin is slightly enhanced (F_avg^u = 193 pN). In contrast, the mechanical stability significantly decreased upon breaking SB2 (F_avg^u = 158 pN). These results suggest that SB1 are SB2 are regulators of the mechanical stability of ubiquitin. Interestingly, the mechanical stability decreased further (F_avg^u = 145 pN) for the double salt bridge (DB) null variant. Monte Carlo simulations elucidate that the main regulating factor is the spontaneous unfolding rate constant (k_u⁰), being the highest for the DB null variant followed by the SB2 null variant, and it remains unaltered for the SB1 null variant, while the native-to-transition-state distance (x_u) remains unchanged. Our study provides mechanistic understanding on how two native salt bridges can independently regulate the mechanical stability in a protein, which has implications in designing protein-based robust biomaterials in the future.

OUP accepted manuscript

The mitochondrial kinase PTEN-induced kinase 1 (PINK1) and cytosolic ubiquitin ligase (E3) Parkin/PRKN are involved in mitochondrial quality control responses. PINK1 phosphorylates ubiquitin and the Parkin ubiquitin-like (Ubl) domain at serine 65 and promotes Parkin activation and translocation to damaged mitochondria. Upon Parkin activation, the Ubl domain is ubiquitinated at lysine (K) 27 and K48 residues. However, the contribution of K27/K48 ubiquitination toward Parkin activity remains unclear. In this study, ubiquitination of K56 (corresponding to K27 in the human), K77 (K48 in the human) or both was blocked by generating Drosophila Parkin (dParkin) mutants to examine the effects of Parkin Ubl domain ubiquitination on Parkin activation in Drosophila. The dParkin, in which K56 was replaced with arginine (dParkin K56R), rescued pupal lethality in flies by co-expression with PINK1, whereas dParkin K77R could not. The dParkin K56R exhibited reduced abilities of mitochondrial fragmentation and motility arrest, which are mediated by degrading Parkin E3 substrates Mitofusin and Miro, respectively. Pathogenic dParkin K56N, unlike dParkin K56R, destabilized the protein, suggesting that not only was dParkin K56N non-ubiquitin-modified at K56, but also the structure of the Ubl domain for activation was largely affected. Ubiquitin attached to K27 of the Ubl domain during PINK1-mediated Parkin activation was likely to be phosphorylated because human Parkin K27R weakened Parkin self-binding and activation in trans. Therefore, our findings suggest a new mechanism of Parkin activation, where an activation complex is formed through phospho-ubiquitin attachment on the K27 residue of the Parkin Ubl domain.

K27-linked noncanonic ubiquitination in immune regulation

Ubiquitination is a common form of posttranslational modification that has been implicated in regulating considerable immune signaling pathways. The functions of canonic K48- and K63-linked ubiquitination have been well studied. However, the roles of noncanonic ubiquitination remain largely unexplored and require further investigations. There is increasing evidence suggesting that K27-linked noncanonic ubiquitination turns out to be indispensable to both innate immune signaling and T cell signaling. In this review, we provide an overview of the latest findings related to K27-linked ubiquitination, and highlight the crucial roles of K27-linked ubiquitination in regulating antimicrobial response, cytokine signaling and response, as well as T cell activation and differentiation. We also propose interesting areas for better understanding how K27-linked ubiquitination regulates immunity.