"Mechanisms of UBQLN2-mediated Phase Transitions associated with Protein Quality Control and Neurodegenerative Diseases" with Carlos Castañeda, Syracuse University (Online Seminar)

Title: Mechanisms of UBQLN2-mediated Phase Transitions associated with Protein Quality Control and Neurodegenerative Diseases

Speaker: Carlos Castañeda, Syracuse University

Hosted by: David Fushman

Abstract: 

Liquid-liquid phase separation (LLPS) has recently emerged as a possible mechanism that enables ubiquitin (Ub)-binding shuttle proteins to form biomolecular condensates to facilitate the degradation of ubiquitinated substrates. Shuttle protein LLPS is modulated by multivalent interactions among their various domains as well as heterotypic interactions with polyubiquitin (polyUb) chains. We recently showed that the Ub-binding shuttle protein UBQLN2 colocalizes with stress granules in cells, undergoes LLPS in vitro and specific interactions with monoUb drive disassembly of UBQLN2-rich droplets. UBQLN2 LLPS behavior is modulated by multivalent interactions involving its folded domains as well as the intrinsically-disordered STI1-II and proline-rich (Pxx) regions. For example, ALS-linked mutations in the Pxx region disrupt UBQLN2 LLPS and promote liquid-to-solid phase transitions. We hypothesize that monoUb binding to the UBQLN2 UBA reduces the number of multivalent “stickers” that are important for promoting UBQLN2 LLPS. However, the effects of polyUb chains on UBQLN2 LLPS is not known. In this presentation, we show that UBQLN2 exhibits reentrant phase behavior in the presence of long polyUb chains, which stabilize and destabilize UBQLN2 LLPS at low and high polyUb to UBQLN2 ratios, respectively. Intriguingly, different types of polyUb chains affect UBQLN2 LLPS differently. The compact K48-linked polyUb chains moderately enhance UBQLN2 LLPS whereas the extended K63-linked chains substantially stabilize UBQLN2 phase separation over a wide range of polyUb to UBQLN2 ratios. These differences can stem from the preference of UBQLN2 for one type of polyUb chain over another, the conformational change in UBQLN2 upon binding to different chains, and polyUb chain conformations whereby extended chains might serve as better multivalent crosslinkers to drive UBQLN2 LLPS. These results have significant implications for the cellular functions of UBQLN2, which could modulate stress granule disassembly and/or dynamics through interactions with different polyUb chains. Moreover, UBQLN2 is involved in both proteasomal degradation and autophagy, which are typically, but not exclusively, signaled by K48- and K63-linked chains, respectively, as well as the oligomeric states of shuttle proteins. These observations provide mechanistic insights to how UBQLN2 could differentiate between proteasomal degradation and autophagy pathways via LLPS.

Seminars start at 4:00 pm and refreshments served at 3:45 pm. All seminars are held in the Conference Room (1116) of the Institute for Physical Science and Technology (IPST) Building, unless otherwise noted.