New Insights Into the Golgi Stacking Proteins

Abstract

The Golgi stacking proteins, GRASP55 and GRASP65, are best known for their roles in Golgi structure formation. These peripheral Golgi proteins form trans-oligomers that hold the flat cisternal membranes into stacks. Depletion of both GRASP proteins in cells disrupts the Golgi stack structure, increases protein trafficking, but impairs accurate glycosylation, and sorting. Golgi unstacking by GRASPs depletion also reduces cell adhesion and migration in an integrin-dependent manner. In addition to Golgi structure formation and regulation of cellular activities, GRASPs, in particular GRASP55, have recently drawn attention in their roles in autophagy, and unconventional secretion. In autophagy, GRASP55 senses the energy level by O-GlcNAcylation, which regulates GRASP55 translocation from the Golgi to the autophagosome-lysosome interface, where it interacts with LC3 and LAMP2 to facilitate autophagosome-lysosome fusion. This newly discovered function of GRASP55 in autophagy may help explain its role in the stress-induced, autophagosome-dependent unconventional secretion. In this review, we summarize the emerging functions of the GRASP proteins, focusing on their roles in cell adhesion and migration, autophagy, unconventional secretion, as well as on novel GRASP-interacting proteins.

Keywords: GRASP55; GRASP65; Golgi; O-GlcNAcylation; autophagy; stacking; unconventional secretion.

FIGURE 1

GRASP55 (A) and GRASP65 (B) domain structures and interacting proteins. GRASP55 and GRASP65 are peripheral membrane proteins that are attached to the Golgi membranes via myristoylation (myr). Both GRASPs share similar domain structures, with a conserved GRASP domain at the N-terminus consisted of PDZ1 and PDZ2 subdomains, and a C-terminal serine/proline-rich (SPR) domain. The GRASP domain forms dimers and trans-oligomers that tether adjacent membranes. The SPR domain contains multiple phosphorylation sites whose phosphorylation in mitosis impairs GRASP oligomerization and leads to Golgi fragmentation (Wang et al., 2003, 2005; Tang et al., 2012). GRASP55 and GRASP65 interacts with Golgin-45 and GM130, respectively, which are essential for their roles in Golgi structure formation and function (Barr et al., 1997; Short et al., 2001). GRASP55 also regulates autophagy by interacting with Beclin-1, LC3 and LAMP2, and controls CFTR, TGFα (pro form) and TGFβ1 secretion (Kuo et al., 2000; Gee et al., 2011; Nüchel et al., 2018; Zhang et al., 2018, 2019). GRASP65 interacts with Mena and Dja1, which are essential for Golgi structure formation (Tang et al., 2016; Li et al., 2019b). More GRASP-interacting proteins can be found on Table 1, only those with known binding sites on GRASPs are shown here. Indicated sites are based on rat GRASP sequences.

FIGURE 2

GRASP55 regulates autophagy and unconventional secretion. (A) The conventional ER-Golgi-plasma membrane (PM) secretory pathway. (B) GRASP55 facilitates autophagosome (AP)-lysosome (Lys) fusion. Upon glucose starvation or amino acid starvation, GRASP55 is translocated to the AP-Lys interface to promote autolysosome formation by bridging LC3 and LAMP2, and by facilitating the UVRAG PI3K complex formation (not shown) (Zhang et al., 2018, 2019). Undigested contents in the autolysosomes and lysosomes may be secreted by lysosome exocytosis (not shown) (Reddy et al., 2001; Samie and Xu, 2014). (C) GRASP55 is required for CFTR trafficking that bypasses the Golgi. Mutant CFTR is transported to the PM in a Golgi-independent manner under ER stress and inhibition of the conventional secretory pathway. During this process phosphorylated and monomerized GRASP55 binds CFTR at the ER membrane and regulates its sequestration (Kim et al., 2016). So far no evidence has been provided that CFTR is indeed localized on the outer membrane of AP, and how CFTR is translocated from ER to AP remains elusive. GRASP55 may also regulate this pathway by facilitating direct fusion of CFTR containing APs with PM or Lys via trans-oligomer formation (not reported). (D) GRASP55 is involved in unconventional secretion of leaderless cytosolic proteins (i.e., Acb1 and IL-1β). IL-1β could be secreted via secretory autophagosomes which is GRASP55 dependent (Zhang et al., 2015). The function of GRASP55 here is unclear but GRASP55 may serve as an ER stress sensor or UPR regulator in this process (Chiritoiu et al., 2019). Yeast protein Acb1 secretion requires the formation of a new type of Grh1-positive membrane compartment called CUPS (compartment for unconventional protein secretion) whose identity is largely unknown (Bruns et al., 2011). CUPS derived vesicles (saccules) fuse with plasma membrane and releases Acb1 to outside of the cell (Curwin et al., 2016).