The Golgi matrix proteins GRASP65 and GRASP55 have recognized roles in maintaining the architecture of the Golgi complex, in mitotic progression and in unconventional protein secretion whereas, surprisingly, they have been shown to be dispensable for the transport of commonly used reporter cargo proteins along the secretory pathway. However, it is becoming increasingly clear that many trafficking machineries operate in a cargo-specific manner, thus we have investigated whether GRASPs may control the trafficking of selected classes of cargo. We have taken into consideration the C-terminal valine-bearing receptors CD8␣ and Frizzled4 that we show bind directly to the PSD95-DlgA-zo-1 (PDZ) domains of GRASP65 and GRASP55. We demonstrate that both GRASPs are needed sequentially for the efficient transport to and through the Golgi complex of these receptors, thus highlighting a novel role for the GRASPs in membrane trafficking. Our results open new perspectives for our understanding of the regulation of surface expression of a class of membrane proteins, and suggests the causal mechanisms of a dominant form of autosomal human familial exudative vitreoretinopathy that arises from the Frizzled4 mutation involving its C-terminal valine.GRASP65 and GRASP55 were identified in in vitro assays as factors that are required for the stacking of the Golgi cisternae (1, 2). This activity arose as the result of the tethering functions displayed by GRASP65 and GRASP55, through their interactions with their partner proteins GM130 and golgin-45, respectively (2-4). Several other studies have shown more recently that the GRASPs are involved in the maintenance of the structure of the Golgi ribbon in mammal cells during interphase, in controlling the fragmentation of the Golgi complex at the onset of mitosis (5-8), in establishing cell polarity in migrating cells (9), and in the consumption of COPII vesicles and the formation of the cis-Golgi in yeast (10).Although the role of the GRASPs in control of the Golgi complex structure (both in interphase and during mitosis) is well established, their involvement in cargo transport along the secretory pathway is still debated. Indeed, a direct role for GRASPs in cargo transport has so far been established only for the unconventional secretion routes in Dictyostilium and Drosophila (11, 12), whereas they have been shown not to be directly involved in the trafficking of commonly studied reporter cargo proteins along the "conventional" secretory pathway (e.g. the temperature-sensitive (ts-045) mutant of the G protein of vesicular stomatitis virus (VSVG) 2 ) and secretory horseradish peroxidase (5, 6, 13, 14). GRASPs can engage different types of interactions including the ones mediated by their PDZ domains, through which the GRASPs cannot only homodimerize, thus participating in cisternal stacking (15) but can also bind the C-terminal valine motifs (C-TVM) of membrane proteins such as protransforming growth factor ␣ and p24a (16,17).Interestingly a C-terminal valine can behave as "transport sign...