VAMP4 regulates the insulin levels by facilitating the membrane fusion of iISGs and resorted vesicles with lysosomes. Working model illustrates that VAMP4 acts as a key regulator for the control of insulin quantity and quality. Briefly, VAMP4 KO mice exhibit hyperresponsiveness to glucose due to increased blood insulin levels in response to glucose challenge. VAMP4-deficient pancreatic β cells show an accumulation of intracellular insulin, an increase in insulin release, and an elevated proportion of immature proinsulin. Mechanistically, in β cells, VAMP4 is packaged into iISGs at TGNs, and partial VAMP4-positive iISGs directly fuse with lysosomes via crinophagy. VAMP4 can also be resorted to vesicles and removed from iISGs by membrane remodeling during granule maturation, and VAMP4-positive resorted vesicles subsequently fuse with lysosomes. VAMP4 forms a SNARE complex with STX7, STX8, and VTI1B on lysosomes and promotes the membrane fusion of iISGs and resorted vesicles with lysosomes, which ensures the breakdown of excess (pro)insulin and obsolete materials to maintain intracellular insulin homeostasis. VAMP4 deficiency not only blocks the fusion of iISGs with lysosomes, causing the accumulation of numerous iISGs and some blocked iISGs swarming to the maturation and secretion pathway, but also blocks the fusion of VAMP4-positive resorted vesicles with lysosomes, leading to undesired cargo material accumulation. RV, resorted vesicle.