GRASP depletion–mediated Golgi destruction decreases cell adhesion and migration via the reduction of a5ß1 integrin

Ahat, Erpan; Xiang, Yi; Zhang, Xiaoyan; Bekier, Michael E.; Wang, Yanzhuang

The discovery of the GRASP proteins provides us with a molecular tool to control Golgi stacking and thus to probe the functional significance of Golgi structure formation. We have inhibited Golgi stacking by microinjecting GRASP65 antibodies (Wang et al., 2003 (Wang et al., , 2008 , by depleting both GRASPs in cells via knockdown (Tang et al., 2010; Xiang and Wang, 2010) , and by GRASP knockout (KO) (Bekier et al., 2017) . These caused accelerated trafficking of several marker proteins, including CD8, Vesicular stomatitis virus G-protein, cathepsin D, and integrins (Wang et al., 2008; Xiang et al., 2013; Bekier et al., 2017) . Furthermore, GRASP depletion significantly decreased both global N-linked glycosylation and glycan complexity and changed the glycolipid composition at the cell surface (Xiang et al., 2013; Bekier et al., 2017) . A plausible explanation is that stacking reduces the accessibility of coat proteins to Golgi membranes, which decreases the rate of vesicle budding and transport but ensures accurate glycosylation and sorting. The Golgi harbors various glycosyltransferases and glycosidases in different subcompartments, but unlike the ER, which contains a high concentration of folding chaperones that retain improperly modified cargoes (Helenius and Aebi, 2001; Hebert and Molinari, 2007; Pearse and Hebert, 2010) , the Golgi lacks a rigorous system to control the fidelity of its biosynthetic processes. An ordered structure and a controlled cargo flow through the Golgi are likely required to carry out precise, sequential modifications as cargo proteins pass between cisternae (Zhang and Wang, 2015; Huang and Wang, 2017) . In addition, GRASP depletion also caused missorting of the lysosomal enzyme cathepsin D to the extracellular space (Xiang et al., 2013) , suggesting that stacking may ensure that sorting occurs only when cargo molecules reach the TGN. These findings demonstrate that Golgi stack formation is fundamentally important for Golgi function (Zhang and Wang, 2016) .