GRASP55, a second mammalian GRASP protein involved in the stacking of Golgi cisternae in a cell-free system

We have identified a 55 kDa protein, named GRASP55 (Golgi reassembly stacki ng protein of 55 kDa), as a component of the Golgi stacking machinery. GRAS P55 is homologous to GRASP65, an N-ethylmaleimide-sensitive membrane protei n required for the stacking of Golgi cisternae in a cell-free system. GRASP 65 exists in a complex with the vesicle docking protein receptor GM130 to w hich it binds directly, and the membrane tethering protein p115, which also functions in the stacking of Golgi cisternae, GRASP55 binding to GM130, co uld not be detected using biochemical methods, although a weak interaction was detected with the yeast two-hybrid system. Cryo-electron microscopy rev ealed that GRASP65, like GM130, is present on the cis-Golgi, while GRASP55 is on the medial-Golgi. Recombinant GRASP55 and antibodies to the protein b lack the stacking of Golgi cisternae, which is similar to the observations made for GRASP65, These results demonstrate that GRASP55 and GRASP65 functi on in the stacking of Golgi cisternae.