| Abstract: | Most secretory cargo proteins in eukaryotes are synthesized in the endoplasmic reticulum and actively exported in membrane-bound vesicles that are formed by the cytosolic coat protein complex II (COPII). COPII proteins are assisted by a variety of cargo-specific adaptor proteins required for the concentration and export of secretory proteins from the endoplasmic reticulum (ER). Adaptor proteins are key regulators of cargo export, and defects in their function may result in disease phenotypes in mammals. Here we report the role of 14-3-3 proteins as a cytosolic adaptor in mediating SAC1 transport in COPII-coated vesicles. Sac1 is a phosphatidyl inositol-4 phosphate (PI4P) lipid phosphatase that undergoes serum dependent translocation between the endoplasmic reticulum and Golgi complex and controls cellular PI4P lipid levels. We developed a cell-free COPII vesicle budding reaction to examine SAC1 exit from the ER that requires COPII and at least one additional cytosolic factor, the 14-3-3 protein. Recombinant 14-3-3 protein stimulates the packaging of SAC1 into COPII vesicles and the sorting subunit of COPII, Sec24, interacts with 14-3-3. We identified a minimal sorting motif of SAC1 that is important for 14-3-3 binding and which controls SAC1 export from the ER. This LS motif is part of a 7-aa stretch, RLSNTSP, which is similar to the consensus 14-3-3 binding sequence. Homology models, based on the SAC1 structure from yeast, predict this region to be in the exposed exterior of the protein. Our data suggest a model in which the 14-3-3 protein mediates SAC1 traffic from the ER through direct interaction with a sorting signal and COPII.Most of the transmembrane secretory cargo proteins from the endoplasmic reticulum (ER) are selectively exported in cytosolic coat protein complex II (COPII) vesicles via direct interaction of their export motif with the COPII coat. The COPII coat core machinery consists of five cytosolic proteins: Sar1, Sec23, Sec24, Sec13, and Sec31 (secretory pathway proteins) (1). Sec24 is considered to be the primary subunit responsible for binding to membrane cargo proteins at the ER and concentrating them into the forming vesicle (2). Some of these cargo proteins require the assistance of cytosolic or membrane-spanning accessory adaptor proteins for their incorporation into COPII vesicles. Several adaptor proteins have been identified to assist the COPII machinery in yeast (3–5); however, fewer have been characterized in higher eukaryotes. In metazoans, ERGIC-53 mediates the export of blood clotting factors, Cathepsin Z and C and α-1 antitrypsin (6), and SCAP [sterol-regulatory elementary binding protein (SREBP) cleavage activating protein] mediates the regulated transport of SREBP protein from the ER to the Golgi in cells that are sterol-deficient (7). Most COPII adaptor proteins are membrane-embedded, but at least one example of a cytosolic accessory protein, 14-3-3, has been proposed to control the anterograde trafficking of many of cell surface receptor proteins, possibly at the level of the ER (8). 14-3-3s are small (30 kDa), acidic, and ubiquitously expressed eukaryotic proteins that are conserved from yeast to mammals and modulate various cellular processes by interacting with a variety of target proteins (9, 10). These include cell cycle regulation, signaling by MAP kinases, apoptosis, and transfer of signaling molecules between the nucleus and cytosol (11–14). Yeast cell viability depends on the expression of at least one of the two 14-3-3 isoforms (Bmh1 and Bmh2) (15). There are seven different isoforms in mammals (β, γ, δ, ε, η, σ, θ), some of which show differential tissue localization (14). Because of their redundant roles in cellular processes, depleting cellular levels of 14-3-3 to study a particular process poses a challenge. It is thought that their role in trafficking is to interfere with the ER retention/retrieval motif of target membrane proteins, and thus promote the transport of these cargos to the cell surface (16). For some proteins (e.g., KCNK3 and MHC class II, GPR15) (17–19), recruitment of 14-3-3 requires phosphorylation of a residue involved in 14-3-3 binding, whereas in other proteins (e.g., Kir6.2) 14-3-3 recognizes the correct assembly of multimeric proteins (20, 21).In this paper we examine the role of 14-3-3 proteins as an adaptor for COPII vesicular transport of SAC1 (suppressor of actin mutations 1-like protein). SAC1 is a phosphatidyl inositol-4 (PI4) lipid phosphatase that belongs to a family of enzymes with a CX5R(T/S) Sac catalytic domain, which is conserved from yeast to metazoans. Sac proteins control several cellular processes, including phosphoinositide homeostasis, membrane trafficking, and cytoskeleton organization. SAC1 is a 587-aa transmembrane protein with both N- and C-terminal domains exposed to the cytosol. Deletion of SAC1 in yeast and mammalian cells leads to changes in Golgi morphology and function and a SAC1 mouse knockout is embryonically lethal. Recently, SAC1 has been identified as Drosophila vesicle-associated protein binding partner and down-regulation of Drosophila vesicle-associated protein or SAC1 in Drosophila leads to the pathogenesis associated with amyotrophic lateral sclerosis (22).It has been reported previously that SAC1 is localized to the Golgi membranes only when cells are starved for nutrients or growth factors, but remains in the ER under normal growth conditions (23, 24). Given the role for PI(4)P in vesicle traffic from the trans Golgi network, starvation conditions that lodge SAC1 and thus deplete the local supply of PI(4)P in the Golgi may suppress anterograde traffic in cells that must cease net cell growth. The regulation of SAC1 traffic may be crucial to the control of cell growth and anterograde membrane traffic.The retrieval of mammalian SAC1 from the Golgi to the ER in the presence of growth factors or mitogens is controlled by COPI-mediated retrograde transport and requires the p38 MAPK pathway (23). Although the regulation of SAC1 retrieval from the Golgi has been reported, little is known about the control of SAC1 export from the ER under conditions of serum starvation. Recently, the N-terminal cytoplasmic domain of SAC1 was reported to contribute to Golgi localization in mammalian cells (25). We have established a cell-free reconstitution system that recapitulates the biogenesis and ER export of SAC1 and identified 14-3-3 proteins as an important factor in the packaging of SAC1 into COPII transport vesicles. Given the role of 14-3-3 proteins in various signaling pathways and the fact that SAC1 transport is affected by the p38 MAPK pathway, an understanding of the molecular role of 14-3-3 proteins in vesicular traffic could provide a mechanistic link between signaling and membrane assembly (23). |
