Model of the post–Golgi trafficking pathway followed by newly synthesized α-granule cargo. After leaving the Golgi complex, α-granule cargo traffics to recycling endosomes that contain Rab11a and VAMP3. The dotted arrows indicate the potential involvement of Rab5a compartments. The VPS33B-VPS16B complex also associates with recycling endosomes and is required for normal trafficking of cargo and α-granule biogenesis. Then, the α-granule proteins traffic to MVBs, possibly defining the transition between MVB type I and MVB type II, which contain higher levels of α-granule proteins and electron-dense lumen. The type II MVBs then mature to α-granules.