Phospholipase D, usually called PLD, is an enzyme that sits on cell membranes and cleaves a common membrane lipid to produce phosphatidic acid, or PA. That single reaction turns out to be one of the most versatile tools cells have for moving material around. PA reshapes membranes, recruits coat proteins to budding vesicles, and helps fuse transport carriers with their targets. Because of this, PLD activity shows up at nearly every stage of intracellular transport, from packaging cargo at the Golgi apparatus to pulling receptors off the cell surface to releasing neurotransmitters at synapses. The story of PLD transport is really the story of how one lipid signal coordinates dozens of different trafficking events across the cell.
How Phosphatidic Acid Reshapes Membranes
When PLD cuts its substrate, it leaves behind PA embedded in the membrane. PA is a cone-shaped lipid, meaning it is wider at one end than the other. When enough PA accumulates on one side of a membrane, the lipid layer curves inward, creating the kind of bending that precedes vesicle budding or membrane fusion. But curvature is only one of at least four ways PA influences transport. It also serves as a raw material for other lipids involved in fission and fusion, it physically binds to proteins that drive membrane remodeling, and it activates enzymes whose own products push trafficking forward.1PubMed. Phosphatidic acid in membrane rearrangements One of those downstream products is diacylglycerol, or DAG, which itself promotes vesicle fission. Enzymes called lipid phosphate phosphatases convert PA to DAG, so the balance between these two lipids acts as a kind of rheostat for how aggressively a membrane buds or fuses.2PubMed Central. Phosphatidate degradation: phosphatidate phosphatases (lipins) and lipid phosphate phosphatases
Two Isoforms in Two Places
Mammalian cells express two classical PLD isoforms, PLD1 and PLD2, and they do not occupy the same real estate. PLD1 sits mainly on internal compartments: the Golgi apparatus, perinuclear vesicles, and late endosomes or lysosomes. PLD2, by contrast, parks itself on the plasma membrane, the cell’s outer boundary.3PubMed Central. Phospholipase D2 localizes to the plasma membrane and regulates angiotensin II receptor endocytosis This split has been confirmed in multiple cell types, including macrophages, where PLD1 marks a late endosomal and lysosomal compartment while PLD2 stays at the cell surface.4PubMed. Dynamics and function of phospholipase D and phosphatidic acid during phagocytosis
This geographic division of labor is central to understanding PLD transport. Because PLD2 is at the plasma membrane, it is ideally positioned to regulate events like receptor internalization and the initial steps of endocytosis. PLD1, deeper inside the cell, is better placed to control vesicle formation in the Golgi, fusion events at late endosomes, and the release of vesicles at presynaptic terminals. Both isoforms generate PA, but where they generate it determines which trafficking step gets pushed forward.
Golgi Trafficking and the Secretory Pathway
The Golgi apparatus is the cell’s central shipping hub, processing and dispatching proteins and lipids to their destinations. PLD has long been implicated in vesicle formation here, partly because it is activated by a family of small regulatory proteins called ADP-ribosylation factors, or ARFs. ARFs are themselves master regulators of vesicle coat assembly, so PLD sits at the intersection of two major trafficking signals.5PubMed. Phospholipase D as an effector for ADP-ribosylation factor in the regulation of vesicular traffic PLD1 activity at the Golgi helps coat proteins polymerize on membranes, a step required for budding new transport vesicles. ARF-activated PLD1 also works alongside another small GTPase called RalA, forming a signaling complex implicated in membrane trafficking.6PubMed. Functional association between Arf and RalA in active phospholipase D complex
The PA that PLD1 generates at the Golgi, along with the DAG it is converted into, participates in both the fission events that pinch off new vesicles and the fusion events that deliver cargo to the next compartment. In this way, PLD does not just initiate vesicle formation; it helps complete the delivery cycle.7PubMed Central. Phospholipase D in the Golgi apparatus
Pulling Receptors Off the Cell Surface
Endocytosis, the process by which cells internalize material from their surface, depends on PA in surprisingly nuanced ways. One well-studied route is clathrin-mediated endocytosis, where the cell builds a protein cage around a patch of membrane and pulls it inward. Researchers found that the role PA plays here is regulatory rather than obligatory. Reducing PA levels by inhibiting an enzyme called diacylglycerol kinase slowed the internalization of epidermal growth factor receptors and decreased the rate at which new clathrin-coated pits formed. Inhibiting PLD, on the other hand, unexpectedly raised cellular PA levels and had the opposite effect on receptor internalization. Transferrin receptors, which follow a constitutive (always-on) internalization pathway, were unaffected by either treatment, suggesting PA specifically tunes the internalization of receptors that are activated by a signal.8PubMed Central. Phosphatidic acid plays a regulatory role in clathrin-mediated endocytosis
PLD2’s position at the plasma membrane makes it particularly important for receptor internalization. Studies of G-protein coupled receptors, a huge family of signaling receptors, showed that PLD2 mediates both agonist-driven and constitutive endocytosis. When researchers blocked PLD2 activity, the internalization of opioid receptors and cannabinoid receptors was impaired, pointing to a broadly important role for PLD2 in pulling these receptors off the surface for recycling or degradation.9PubMed. Role of phospholipase D2 in the agonist-induced and constitutive endocytosis of G-protein coupled receptors
Phagocytosis in Immune Cells
Macrophages, the immune system’s professional cleanup crew, use phagocytosis to engulf bacteria, dead cells, and other debris. This process involves dramatic membrane remodeling, so it is no surprise that PLD is involved. Both PLD1 and PLD2 are activated during phagocytosis, but they contribute in distinct ways. In human macrophages, blocking either isoform alone with a dominant-negative construct reduced phagocytosis by roughly 55 to 65 percent. Blocking both together cut phagocytosis by about 91 percent, and the effects were additive rather than redundant. Only PLD1 was found on the phagosome membrane itself, suggesting it acts locally at the site of engulfment while PLD2 contributes from the plasma membrane.10PubMed. Phospholipases D1 and D2 coordinately regulate macrophage phagocytosis
Mouse knockout studies confirmed and extended these findings. Macrophages from mice lacking PLD1 showed about a 34 percent decrease in bead phagocytosis, while those lacking PLD2 showed a steeper drop of about 57 percent. Treating wild-type macrophages with a chemical inhibitor that blocks both isoforms reduced phagocytosis by about 40 percent. The fact that blocking both chemically produced a milder deficit than genetic loss of PLD2 alone hints that the two isoforms perform functions that are neither fully redundant nor simply additive; some basal phagocytosis persists even without any PLD activity at all.11PLoS ONE. Deficiencies of the Lipid-Signaling Enzymes Phospholipase D1 and D2 Alter Cytoskeletal Organization, Macrophage Phagocytosis, and Cytokine-Stimulated Neutrophil Recruitment
Neurotransmitter Release
At the synapse, communication between neurons requires vesicles loaded with neurotransmitter to fuse with the presynaptic membrane and dump their contents into the gap between cells. PLD1 appears to play a major role in controlling how ready those vesicles are to fuse. Researchers have found evidence that PLD1 regulates the “fusogenic status” of presynaptic release sites, meaning it helps set the stage so that when an electrical signal arrives, vesicles can merge with the membrane quickly enough for fast neurotransmission.12PubMed. A role for phospholipase D1 in neurotransmitter release PA’s cone shape probably contributes here too, since the kind of negative curvature it generates is known to lower the energy barrier for membrane fusion.
Exosome Biogenesis
Cells also communicate over longer distances by releasing tiny membrane-enclosed packages called exosomes. These form inside compartments known as multivesicular bodies, where the limiting membrane buds inward to create small internal vesicles. When the multivesicular body fuses with the plasma membrane, those internal vesicles are released as exosomes. PLD2 and the small GTPase ARF6 control the budding of these intraluminal vesicles, and thus the quantity and cargo of the exosomes that eventually get secreted.13PubMed. Syntenin-ALIX exosome biogenesis and budding into multivesicular bodies are controlled by ARF6 and PLD2 More recent work has shown that PLD2-generated PA recruits a specific sorting factor called MVB12B to late endosomes. Losing PLD2 or losing MVB12B produces the same outcome: fewer exosomes released and altered loading of cargo into them.14bioRxiv. PLD2-phosphatidic acid recruit ESCRT-I to late endosomes for exosome biogenesis
Endosomal Sorting and Recycling
Once material has been internalized into endosomes, the cell has to decide what to recycle back to the surface, what to send to lysosomes for destruction, and what to route to the Golgi. PLD influences this sorting at multiple points. Inhibiting PLD or DAG kinase strongly increased the retrograde transport of a model cargo from endosomes back toward the Golgi, while recycling and degradation routes were largely unaffected. Multiple PLD and DAG kinase isoforms were found to participate, underscoring how broadly PA and DAG balance shapes sorting decisions.15PubMed Central. Diacylglycerol kinase and phospholipase D inhibitors alter the cellular lipidome and endosomal sorting towards the Golgi apparatus
A clear example comes from fly photoreceptors. The light-sensitive ion channel TRPL must be recycled back to the light-detecting membrane via the endoplasmic reticulum. PLD and the retromer complex, a protein machine that retrieves cargo from endosomes, are both required for this recycling. When PLD activity is lost, TRPL is degraded instead of recycled, and the photoreceptor loses sensitivity.16PubMed. Phospholipase D and retromer promote recycling of TRPL ion channel via the endoplasmic reticulum
Pathogens That Exploit PLD Transport
Viruses and bacteria have evolved ways to hijack PLD-driven trafficking for their own benefit. Influenza virus, for instance, benefits from PLD activity during entry. Because PLD2 sits at the plasma membrane, it facilitates the early steps of viral entry by promoting the membrane rearrangements the virus needs to get inside. PLD1, located deeper in the cell, is thought to assist later during viral assembly and budding of new particles at the Golgi.17Journal of Biological Chemistry. Phospholipase D Facilitates Efficient Entry of Influenza Virus, Allowing Escape from Innate Immune Inhibition Ebola virus similarly depends on PA for the association of its matrix protein VP40 with the plasma membrane. Inhibiting PA production reduced the budding of Ebola virus-like particles, suggesting that PA generated by PLD provides a docking platform for viral assembly at the cell surface.18PubMed Central. Role of phosphatidic acid lipids on plasma membrane association of the Ebola virus matrix protein VP40
On the bacterial side, Legionella pneumophila, the bacterium that causes Legionnaires’ disease, secretes an effector protein called LpdA that is itself a phospholipase D. By injecting its own PLD into the host cell, Legionella can manipulate the lipid composition of the vacuole it hides in, essentially redecorating its intracellular niche to avoid being destroyed.19PubMed Central. Legionella pneumophila Effector LpdA Is a Palmitoylated Phospholipase D Virulence Factor
PLD Transport in Cancer Metastasis
For a cancer cell to invade surrounding tissue and colonize distant organs, it needs to deliver specialized enzymes to its surface that can chew through the extracellular matrix. PLD2 has been shown to drive exactly this process in breast cancer. In a transgenic mouse model of breast cancer, knocking out PLD2 inhibited the formation of lung metastases. The mechanism traced back to vesicle transport: PLD2-generated PA binds directly to the tail of KIF5B, the heavy chain of the motor protein kinesin-1. This binding is required for kinesin-1 to attach to vesicles carrying the matrix-degrading enzyme MT1-MMP and haul them to the cell surface, where they form invasive structures called invadopodia. Without PLD2, the motor cannot grab the vesicles, MT1-MMP never reaches the surface, and invasion stalls.20Developmental Cell. Phospholipase D2 Drives Breast Cancer Metastasis through Direct Regulation of Kinesin-1 Mediated MT1-MMP Vesicular Transport This finding is worth noting because it provides a mechanistic rationale for targeting PLD2 therapeutically in metastatic disease.
PLD3 and Neurodegenerative Disease
Beyond the classical PLD1 and PLD2, a less-studied family member called PLD3 has emerged as a player in endosomal protein sorting with potential links to Alzheimer’s disease. PLD3 is a membrane protein found in endosomes, where it colocalizes with amyloid precursor protein, or APP. Losing PLD3 function impairs endosomal tubulation, disrupts the trafficking of several membrane proteins, and reduces the association of a sorting receptor with APP, all of which can affect how APP is processed into the amyloid-beta fragments that accumulate in Alzheimer’s brains.21PubMed Central. Analysis of novel endosome-to-Golgi retrieval genes reveals a role for PLD3 in regulating endosomal protein sorting and amyloid precursor protein processing
PLD1 also intersects with Alzheimer’s biology, but through a different mechanism. Overexpressing PLD1 dramatically increased the amount of presenilin 1, a core component of the enzyme that cleaves APP, on the cell surface. A catalytically dead PLD1 mutant had no such effect, confirming that PA production is required. PLD1 and presenilin 1 physically interact, suggesting PLD1 actively shuttles presenilin to the plasma membrane.22Journal of Biological Chemistry. Intracellular Trafficking of Presenilin 1 Is Regulated by β-Amyloid Precursor Protein and Phospholipase D1 Mislocalization of presenilin could shift where APP cleavage happens inside the cell, potentially influencing whether toxic fragments are produced.
PLD Transport in Plants
PLD-driven transport is not exclusive to animals. In plants, where PLD gene families are especially large, the enzyme plays roles ranging from hormone transport to stress responses. One striking example involves auxin, the growth hormone that must be moved directionally from cell to cell to orient root and shoot growth. A specific isoform called phospholipase D zeta2 drives the vesicular secretion of auxin for its polar transport in the transition zone of the root tip.23PubMed Central. Phospholipase Dzeta2 drives vesicular secretion of auxin for its polar cell-cell transport in the transition zone of the root apex Because auxin gradients determine everything from root bending to fruit development, PLD activity in this context shapes the architecture of the entire plant.
PA also feeds back onto the cytoskeleton in plants. Recent work has identified a positive feedback loop between actin turnover and PLD activity, with PA binding directly to actin capping protein and a microtubule-associated protein called MAP65-1. These interactions position PLD as a coordinator of both membrane trafficking and the cytoskeletal tracks that vesicles travel along.24Trends in Plant Science. Phosphatidic acid and phospholipase D in the regulation of cytoskeletal dynamics
How PLD Knows Where to Work
A question that runs through all of these trafficking roles is how PLD ends up at the right membrane at the right time. Part of the answer lies in a structural feature called the PX domain, which acts as a membrane address label. The PX domain of PLD1 binds to specific phosphoinositide lipids and also to PA itself, creating a self-reinforcing loop: PLD1 makes PA, and PA helps recruit more PLD1 to the same spot.25PubMed. Mechanism of membrane binding of the phospholipase D1 PX domain This kind of positive feedback is common in membrane biology and helps explain how a lipid signal that starts small can rapidly concentrate at a site of vesicle budding or fusion.
Pharmacological Inhibitors and Surprises
For decades, researchers relied on an alcohol called 1-butanol to block PLD signaling in cells. Butanol works by diverting PLD’s enzymatic activity away from PA production, but it is a blunt tool with off-target effects. The development of FIPI, a small-molecule inhibitor that blocks PA production with subnanomolar potency, allowed much cleaner experiments. What surprised researchers was that several cellular processes previously attributed to PLD based on butanol experiments were unaffected by FIPI, suggesting that some earlier conclusions about PLD’s transport roles may need revisiting.26PubMed Central. 5-Fluoro-2-indolyl des-chlorohalopemide (FIPI), a phospholipase D pharmacological inhibitor that alters cell spreading and inhibits chemotaxis FIPI did block cell spreading and chemotaxis, confirming PLD’s role in those processes, but the discrepancies with butanol highlight a recurring theme in this field: teasing apart what PLD actually does from what upstream or parallel pathways contribute is still a work in progress.