Abstract
The Golgi complex is the central sorting and processing station of the secretory pathway, ensuring that cargo proteins, which are synthesized in the endoplasmic reticulum, are properly glycosylated and packaged into carriers for transport to their final destinations. Two recent studies highlight the fact that properties of membrane lipids play key roles in Golgi structural organization and trafficking. The Antonny laboratory has demonstrated the mechanism by which a Golgi tether containing a membrane-curvature-sensing domain at one end can link highly curved and flat membranes together in a reversible manner. In this way, a strong interaction that binds membranes together in an oriented fashion can easily be disrupted as the properties of the membranes change. The Lippincott-Schwartz laboratory has developed a new model for intra-Golgi trafficking, called the rapid-partitioning model, which incorporates lipid trafficking as an integral part. Simulations reveal that the sorting of lipids into processing and export domains that are connected to each Golgi cisterna, and bidirectional trafficking throughout the Golgi to allow proteins to associate with their preferred lipid environment, is sufficient to drive protein transport through the secretory pathway. Although only a proof in principle, this model for the first time invokes lipid sorting as the driving force in intra-Golgi trafficking, and provides a framework for future experimental work.
Highlights
Because of its structural complexity and highly dynamic nature, understanding the mechanisms that regulate trafficking through the Golgi complex has been elusive
When brefeldin A (BFA) is washed out, the Golgi reforms and returns to its steady-state structure. These results indicate that a constant input of energy through cycles of GTP binding and hydrolysis on ADP ribosylation factor 1 (Arf1) are required to maintain Golgi structure, a hallmark of a selforganizing system
These data raise the important issue of how Arf1 maintains Golgi structure. This task is assured by the effectors that bind to active Arf1-GTP, including coat complexes [such as coatomer complex protein I (COPI); Golgi-localized, γ-ear-containing, Arfbinding proteins (GGA)-clathrin; and adaptor protein 1 (AP-1)clathrin], lipid-modifying enzymes [such as phosphotidylinositol 4-kinase (PI4K) and lipid-transfer proteins] and membrane tethers
Summary
Because of its structural complexity and highly dynamic nature, understanding the mechanisms that regulate trafficking through the Golgi complex has been elusive. These data raise the important issue of how Arf maintains Golgi structure This task is assured by the effectors that bind to active Arf1-GTP, including coat complexes [such as coatomer complex protein I (COPI); Golgi-localized, γ-ear-containing, Arfbinding proteins (GGA)-clathrin; and adaptor protein 1 (AP-1)clathrin], lipid-modifying enzymes [such as phosphotidylinositol 4-kinase (PI4K) and lipid-transfer proteins] and membrane tethers. The work of Drin et al is a beautiful illustration of a selforganization module that ensures the oriented tethering of membranes until the properties of these membranes change This concept provides an ideal solution to the problem of how the Golgi complex maintains its structure in the face of a massive flow of membrane through the organelle.
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