RT Journal Article
SR Electronic
T1 Liquid-liquid phase separation facilitates the biogenesis of secretory storage granules
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 2021.12.22.472607
DO 10.1101/2021.12.22.472607
A1 Anup Parchure
A1 Meng Tian
A1 Cierra K Boyer
A1 Shelby C Bearrows
A1 Kristen E Rohli
A1 Jianchao Zhang
A1 Bulat R Ramazanov
A1 Yanzhuang Wang
A1 Samuel B Stephens
A1 Julia von Blume
YR 2021
UL http://biorxiv.org/content/early/2021/12/23/2021.12.22.472607.abstract
AB Insulin is a key regulator of human metabolism, and its dysfunction leads to diseases such as type 2 diabetes. It remains unknown how proinsulin is targeted from the trans-Golgi network (TGN) to secretory storage granules as no cargo receptor has been identified. Chromogranin proteins (CGs) are central regulators of granule biosynthesis, and it was proposed that their aggregation is critical for this process. However, the molecular mechanism by which these molecules facilitate sorting at the TGN is poorly understood. Here, we show that CGs undergo liquid–liquid phase separation (LLPS) at low pH independently of divalent cations, such as calcium. Liquid CG condensates, but not aggregates, recruit and sort proinsulin and other granule-destined cargo molecules towards secretory granules. Cargo selectivity is independent of sequence or structural elements but is based on the size and concentration of the client molecules at the TGN. Finally, electrostatic interactions and the N-terminal intrinsically disordered domain of chromogranin B facilitate LLPS and are critical for granule formation. We propose that phase-separated CGs act as a “cargo sponge” within the TGN lumen, gathering soluble client proteins into the condensate independently of specific sequence or structural elements, facilitating receptor-independent sorting. These findings challenge the canonical TGN sorting models and provide insights into granule biosynthesis in insulin-secreting β-cells.One sentence summary Liquid Chromogranin condensates recruit cargo molecules at the TGN for their delivery to secretory storage granules.Competing Interest StatementThe authors have declared no competing interest.