Hsp90α forms condensate engaging client proteins with RG motif repeats.

Hsp90α, a pivotal canonical chaperone, is renowned for its broad interaction with numerous protein clients to maintain protein homeostasis, chromatin remodeling, and cell growth. Recent studies indicate its role in modifying various components of membraneless organelles (MLOs) such as stress granules and processing bodies, suggesting its participation in the regulation of protein condensates. In this study, we found that Hsp90α possesses an inherent ability to form dynamic condensates in vitro. Utilizing LC-MS/MS, we further pinpointed proteins in cell lysates that preferentially integrate into Hsp90α condensates. Significantly, we observed a prevalence of RG motif repeats in client proteins of Hsp90α condensates, many of which are linked to various MLOs. Moreover, each of the three domains of Hsp90α was found to undergo phase separation, with numerous solvent-exposed negatively charged residues on these domains being crucial for driving Hsp90α condensation through multivalent weak electrostatic interactions. Additionally, various clients like TDP-43 and hnRNPA1, along with poly-GR and PR dipeptide repeats, exhibit varied impacts on the dynamic behavior of Hsp90α condensates. Our study spotlights various client proteins associated with Hsp90α condensates, illustrating its intricate adaptive nature in interacting with diverse clients and its functional adaptability across multiple MLOs.

SecMS analysis of selenoproteins with selenocysteine insertion sequence and beyond.

Selenocysteine (Sec, U) is the 21st amino acid, and proteins with selenocysteine are defined as selenoproteins. The currently known selenoproteins are all featured by the presence of selenocysteine insertion sequence (SECIS) on their mRNA, and SECIS plays an essential role in the selenocysteine insertion mechanism. However, due to the extremely low occurrences of selenoproteins in a whole proteome (e.g., only 25 selenoproteins in the human proteome) and the low sequence conservation of SECIS, analysis of selenoproteins and discovery of new selenoproteins exclusively on SECIS are intrinsically challenging. To this end, the selenocysteine-specific mass spectrometry (SecMS) and SECIS-independent selenoprotein (SIS) database are developed, showing abilities to profile whole selenoproteomes sensitively and to discover potential new selenoproteins. Here, we detail the SecMS strategy and propose it will advance the exploration for new selenoproteins and functional studies of selenoproteins.

Quantitative analysis of phosphoproteome in necroptosis reveals a role of TRIM28 phosphorylation in promoting necroptosis-induced cytokine production.

Necroptosis is a form of regulated necrotic cell death that promotes inflammation. In cells undergoing necroptosis, activated RIPK1 kinase mediates the formation of RIPK1/RIPK3/MLKL complex to promote MLKL oligomerization and execution of necroptosis. RIPK1 kinase activity also promotes cell-autonomous activation of proinflammatory cytokine production in necroptosis. However, the signaling pathways downstream of RIPK1 kinase in necroptosis and how RIPK1 kinase activation controls inflammatory response induced by necroptosis are still largely unknown. Here, we quantitatively measured the temporal dynamics of over 7000 confident phosphorylation-sites during necroptosis using mass spectrometry. Our study defined a RIPK1-dependent phosphorylation pattern in late necroptosis that is associated with a proinflammatory component marked by p-S473 TRIM28. We show that the activation of p38 MAPK mediated by oligomerized MLKL promotes the phosphorylation of S473 TRIM28, which in turn mediates inflammation during late necroptosis. Taken together, our study illustrates a mechanism by which p38 MAPK may be activated by oligomerized MLKL to promote inflammation in necroptosis.