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Talk by Julia von Blume

Title: "Mechanism of secretory granule biogenesis"
Occasion: SFB Special Seminar
Host: Christian Ungermann
Start: 28.11.2022 - 14:00
Location: CellNanOs 38/201

About the speaker: Prof. Dr Julia von Blume researches cell biology at the Yale School of Medicine New Haven, USA.

Abstract of the talk: Insulin secretion regulates blood glucose levels and is thus a critical metabolic regulator in mammals. Insulin is made by pancreatic b-cells and stored in their secretory granules (SGs). Responding to nutrient stimuli, mature SGs are mobilized to fuse with the plasma membrane and deliver insulin to the bloodstream. The mechanism of how proinsulin and its processing enzymes are sorted and targeted from the trans-Golgi network (TGN) to SGs remains largely mysterious. Remarkably, no cargo receptor for proinsulin has been identified in the last three decades. Chromogranin proteins (CGs) are central regulators of granule biosynthesis in neuroendocrine cell types. Previous studies have suggested that CG aggregation in the TGN is critical for the sorting and packaging of proinsulin and its processing enzymes into immature secretory granules. Nevertheless, the molecular mechanism of how CGs facilitate sorting at the TGN is poorly understood. Here, we show that CGs organize in droplet-like structures in the lumen of the TGN of live INS-1 (Insulin 1) cells. Furthermore, purified CGs undergo liquid-liquid phase separation (LLPS) at low pH independently of divalent cations, such as calcium. Similarly, in INS-1 cells, the elevation of the TGN pH leads to a redistribution of CGs from a droplet shape into a diffuse pattern. Intriguingly, liquid CG condensates, but not aggregates, recruit and sort granule-destined cargo molecules. Strikingly, our work shows that, cargo selectivity of CGs is independent of conserved sequence but is based on the size and concentration of the client molecules at the TGN. Finally, we show that electrostatic interactions and the N-terminal intrinsically disordered domain of chromogranin B facilitates LLPS and are critical for granule formation in b-cells. In conclusion, we propose that the TGN provides the milieu in which soluble CGs convert into a “cargo sponge,” gathering soluble client proteins into the condensate. Client molecules can partition into the condensate driven by weak intermolecular interactions, a feature of IDR-containing proteins, thus, facilitating receptor-independent sorting. These findings challenge the canonical TGN sorting models in insulin-secreting β-cells and potentially many other cell types. They are therefore of central relevance for cell biology and diabetes research.