Orchestrated Restructuring Events During Secretory Granule Maturation Mediate Intragranular Cargo Segregation

Regulated secretion is an essential process where proteins are packaged into membranous secretory vesicles. However, the details of cargo packaging and secretory granule maturation are largely unknown. Here, we demonstrate that multiple distinct proteins undergo intragranular restructuring during secretory granule maturation in vivo, resulting in spatial segregation of distinct protein components within the same granule. Furthermore, through a combination of genetics and multimodality imaging, we demonstrate the molecular identity of each distinct intragranular structure. We further identify temporally-regulated genes that are essential for the restructuring events, including those controlling pH (Vha16-1), Cl- ions (Clic and ClC-c) and Ca2+ ions (fwe). Finally, we show that altered cargo glycosylation influences dimensions of these structures, thereby affecting secretory granule morphology. This study elucidates key steps and factors involved in intragranular, rather than intergranular segregation of cargo through regulated restructuring events during secretory granule maturation. Understanding how multiple distinct proteins are efficiently packaged into and secreted from the same secretory granule may provide insight into diseases resulting from defects in secretion.