1. Droplet and fibril formation of the functional amyloid Orb2
- Author
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Samridhi Garg, Alexander S. Falk, Anoop Rawat, Connor Hurd, Kidist Ashami, Silvia A. Cervantes, and Ansgar B. Siemer
- Subjects
0301 basic medicine ,Cytoplasmic polyadenylation element ,RNA-binding protein ,Biochemistry ,HSU, HEPES–salt–urea ,LTM, long-term memory ,Fluorescence microscope ,Drosophila Proteins ,DIC, differential interference contrast ,Carbon Isotopes ,ThT, Thioflavin T ,biology ,rf, radio frequency ,Chemistry ,Editors' Pick ,liquid-liquid phase separation ,LLPS, liquid–liquid phase separation ,Drosophila melanogaster ,HTTex1, huntingtin exon-1 ,Differential interference contrast microscopy ,Thioflavin T ,solid-state NMR ,CP, cross-polarization ,Research Article ,Binding domain ,Amyloid ,Protein family ,fluorescence recovery after photobleaching ,Fibril ,Fluorescence ,CPEB ,03 medical and health sciences ,long-term memory ,Protein Domains ,ROI, regions of interest ,Animals ,Amino Acid Sequence ,Benzothiazoles ,Molecular Biology ,mRNA Cleavage and Polyadenylation Factors ,EM, electron microscopy ,030102 biochemistry & molecular biology ,RBD, RNA-binding domain ,CPEB, cytoplasmic polyadenylation element binding ,Osmolar Concentration ,FRAP, fluorescence recovery after photobleaching ,RNA ,Fluorescence recovery after photobleaching ,Cell Biology ,RRM, RNA recognition motif ,030104 developmental biology ,biology.protein ,Biophysics ,FUS, fused in sarcoma ,Transcription Factors - Abstract
The functional amyloid Orb2 belongs to the cytoplasmic polyadenylation element binding (CPEB) protein family and plays an important role in long-term memory formation in Drosophila. The Orb2 domain structure combines RNA recognition motifs with low complexity sequences similar to many RNA binding proteins shown to form protein droplets via liquid-liquid phase separation (LLPS) in vivo and in vitro. This similarity suggests that Orb2 might also undergo LLPS. However, cellular Orb2 puncta have very little internal protein mobility and Orb2 forms fibrils in Drosophila brains that are functionally active indicating that LLPS might not play a role for Orb2. In the present work, we reconcile these two views on Orb2 droplet formation. We show that soluble Orb2 can indeed phase separate into protein droplets. However, these droplets have either no or only an extremely short-lived liquid phase and appear maturated right after formation. For Orb2 fragments that lack the C-terminal RNA binding domain (RBD), droplet formation is a prerequisite for fibril formation of an otherwise stable monomeric Orb2 solution. Solid-state NMR shows that these fibrils have additional well ordered static domains beside the Gln/His-rich fibril core. Further, we find that full-length Orb2B, which is by far the major component of Orb2 fibrils in vivo, does not transition into cross-β fibrils but remains in the droplet phase. Together, our data suggest that phase separation might play a role in initiating the formation of functional Orb2 fibrils.
- Published
- 2021
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