Mechanism of action of the flippase Drs2p in modulating GTP hydrolysis of Arl1p

Lipid flippases ATP9A and ATP9B form a complex and contribute to the exocytic pathway from the Golgi

Type IV P-type ATPases (P4-ATPases) serve as lipid flippases, translocating membrane lipids from the exoplasmic (or luminal) leaflet to the cytoplasmic leaflet of lipid bilayers. In mammals, these P4-ATPases are localized to distinct subcellular compartments. ATP8A1 and ATP9A, members of the P4-ATPase family, are involved in endosome-mediated membrane trafficking, although the roles of P4-ATPases in the exocytic pathway remain to be clarified. ATP9A and ATP9B are located in the TGN, with ATP9A also present in endosomal compartments. This study revealed the overlapping roles of ATP9A and ATP9B in transporting VSVG from the Golgi to the plasma membrane within the exocytic pathway. Furthermore, we demonstrated that the flippase activities of ATP9A and ATP9B were crucial for the transport process. Notably, we discovered the formation of homomeric and/or heteromeric complexes between ATP9A and ATP9B. Therefore, ATP9A and ATP9B play a role in the exocytic pathway from the Golgi to the plasma membrane, forming either homomeric or heteromeric complexes.

COPI mediates recycling of an exocytic SNARE by recognition of a ubiquitin sorting signal

The COPI coat forms transport vesicles from the Golgi complex and plays a poorly defined role in endocytic trafficking. Here we show that COPI binds K63-linked polyubiquitin and this interaction is crucial for trafficking of a ubiquitinated yeast SNARE (Snc1). Snc1 is a v-SNARE that drives fusion of exocytic vesicles with the plasma membrane, and then recycles through the endocytic pathway to the Golgi for reuse in exocytosis. Removal of ubiquitin from Snc1, or deletion of a β'-COP subunit propeller domain that binds K63-linked polyubiquitin, disrupts Snc1 recycling causing aberrant accumulation in internal compartments. Moreover, replacement of the β'-COP propeller domain with unrelated ubiquitin-binding domains restores Snc1 recycling. These results indicate that ubiquitination, a modification well known to target membrane proteins to the lysosome or vacuole for degradation, can also function as recycling signal to sort a SNARE into COPI vesicles in a non-degradative pathway.

The Arl3 and Arl1 GTPases co-operate with Cog8 to regulate selective autophagy via Atg9 trafficking

The Arl3-Arl1 GTPase cascade plays important roles in vesicle trafficking at the late Golgi and endosomes. Subunits of the conserved oligomeric Golgi (COG) complex, a tethering factor, are important for endosome-to-Golgi transport and contribute to the efficient functioning of the cytoplasm-to-vacuole targeting (Cvt) pathway, a well-known selective autophagy pathway. According to our findings, the Arl3-Arl1 GTPase cascade co-operates with Cog8 to regulate the Cvt pathway via Atg9 trafficking. arl3cog8Δ and arl1cog8Δ exhibit profound defects in aminopeptidase I maturation in rich medium. In addition, the Arl3-Arl1 cascade acts on the Cvt pathway via dynamic nucleotide binding. Furthermore, Atg9 accumulates at the late Golgi in arl3cog8Δ and arl1cog8Δ cells under normal growth conditions but not under starvation conditions. Thus, our results offer insight into the requirement for multiple components in the Golgi-endosome system to determine Atg9 trafficking at the Golgi, thereby regulating selective autophagy.

Multiple activities of Arl1 GTPase in the trans-Golgi network

ADP-ribosylation factors (Arfs) and ADP-ribosylation factor-like proteins (Arls) are highly conserved small GTPases that function as main regulators of vesicular trafficking and cytoskeletal reorganization. Arl1, the first identified member of the large Arl family, is an important regulator of Golgi complex structure and function in organisms ranging from yeast to mammals. Together with its effectors, Arl1 has been shown to be involved in several cellular processes, including endosomal trans-Golgi network and secretory trafficking, lipid droplet and salivary granule formation, innate immunity and neuronal development, stress tolerance, as well as the response of the unfolded protein. In this Commentary, we provide a comprehensive summary of the Arl1-dependent cellular functions and a detailed characterization of several Arl1 effectors. We propose that involvement of Arl1 in these diverse cellular functions reflects the fact that Arl1 is activated at several late-Golgi sites, corresponding to specific molecular complexes that respond to and integrate multiple signals. We also provide insight into how the GTP-GDP cycle of Arl1 is regulated, and highlight a newly discovered mechanism that controls the sophisticated regulation of Arl1 activity at the Golgi complex.

On the molecular mechanism of flippase- and scramblase-mediated phospholipid transport

Phospholipid flippases are key regulators of transbilayer lipid asymmetry in eukaryotic cell membranes, critical to many trafficking and signaling pathways. P4-ATPases, in particular, are responsible for the uphill transport of phospholipids from the exoplasmic to the cytosolic leaflet of the plasma membrane, as well as membranes of the late secretory/endocytic pathways, thereby establishing transbilayer asymmetry. Recent studies combining cell biology and biochemical approaches have improved our understanding of the path taken by lipids through P4-ATPases. Additionally, identification of several protein families catalyzing phospholipid 'scrambling', i.e. disruption of phospholipid asymmetry through energy-independent bi-directional phospholipid transport, as well as the recent report of the structure of such a scramblase, opens the way to a deeper characterization of their mechanism of action. Here, we discuss the molecular nature of the mechanism by which lipids may 'flip' across membranes, with an emphasis on active lipid transport catalyzed by P4-ATPases. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.