The VPS33B-VPS16B protein complex facilitates Golgi ribbon formation and Golgi-endosome trafficking

Loss of expression of VIPAS39/VPS16B or the Sec1/Munc18 (SM) family protein VPS33B causes early lethal ARC syndrome in humans. These proteins have been shown to form a complex containing 2 opposed VPS33B molecules known as VPS33B-VPS16B. ARC patient-derived fibroblasts have dispersed Golgi elements and absence of ribbons, an abnormality also observed in VPS16B-knockout HEK293 cells. These observations indicate that VPS33B-VPS16B is involved in Golgi formation/stability. Immunofluorescence microscopy studies showed colocalization of VPS33B-VPS16B with several Golgi-associated proteins, and co-immunoprecipitation studies indicated interactions involving the Golgi tethering factor GRASP55/GORASP2. Interactions were also detected involving VPS33B-VPS16B and VPS51 and VPS53, components of the Golgi-associated retrograde protein (GARP) and endosome-associated recycling protein (EARP) complexes. Abnormalities were observed in retrograde trafficking of mannose-6-phosphate receptors, and of cathepsin B between Golgi and endosomes in VPS33B-VPS16B deficient cells, along with abnormally large late endosomes/lysosomes. These observations suggest that VPS33B-VPS16B has previously unreported roles in key aspects of Golgi apparatus formation/stability, Golgi-endosome trafficking, and perhaps lysosome dynamics. The VPS33B-VPS16B complex is required for Golgi ribbon formation and retrograde trafficking. It interacts with the Golgi tether GRASP55 and the GARP/EARP complexes. Primary patient fibroblasts lacking VPS33B-VPS16B have abnormally large endolysosomes.