Role of the Second Cysteine-rich Domain and Pro275 in Protein Kinase D2 Interaction with ADP-Ribosylation Factor 1, Trans-Golgi Network Recruitment,and Protein Transport

Protein kinase D (PKD) isoenzymes regulate the formation of transport carriers from the trans-Golgi network (TGN) that are en route to the plasma membrane. The PKD C1a domain is required for the localization of PKDs at the TGN. However, the precise mechanism of how PKDs are recruited to the TGN is still elusive. Here, we report that ADP-ribosylation factor (ARF1), a small GTPase of the Ras superfamily and a key regulator of secretory traffic, specifically interacts with PKD isoenzymes. ARF1, but not ARF6, binds directly to the second cysteine-rich domain (C1b) of PKD2, and precisely to Pro275 within this domain. Pro275 in PKD2 is not only crucial for the PKD2-ARF1 interaction but also for PKD2 recruitment to and PKD2 function at the TGN, namely, protein transport to the plasma membrane. Our data suggest a novel model in which ARF1 recruits PKD2 to the TGN by binding to Pro275 in its C1b domain followed by anchoring of PKD2 in the TGN membranes via binding of its C1a domain to diacylglycerol. Both processes are critical for PKD2-mediated protein transport.     

Molecular cloning and characterization of the human protein kinase D2. A novel member of the protein kinase D family of serine threonine kinases

We have isolated the full-length cDNA of a novel human serine threonine protein kinase gene. The deduced protein sequence contains two cysteine-rich motifs at the N terminus, a pleckstrin homology domain, and a catalytic domain containing all the characteristic sequence motifs of serine protein kinases. It exhibits the strongest homology to the serine threonine protein kinases PKD/PKCmicro and PKCnu, particularly in the duplex zinc finger-like cysteine-rich motif, in the pleckstrin homology domain and in the protein kinase domain. In contrast, it shows only a low degree of sequence similarity to other members of the PKC family. Therefore, the new protein has been termed protein kinase D2 (PKD2). The mRNA of PKD2 is widely expressed in human and murine tissues. It encodes a protein with a molecular mass of 105 kDa in SDS-polyacrylamide gel electrophoresis, which is expressed in various human cell lines, including HL60 cells, which do not express PKCmicro. In vivo phorbol ester binding studies demonstrated a concentration-dependent binding of [(3)H]phorbol 12,13-dibutyrate to PKD2. The addition of phorbol 12,13-dibutyrate in the presence of dioleoylphosphatidylserine stimulated the autophosphorylation of PKD2 in a synergistic fashion. Phorbol esters also stimulated autophosphorylation of PKD2 in intact cells. PKD2 activated by phorbol esters efficiently phosphorylated the exogenous substrate histone H1. In addition, we could identify the C-terminal Ser(876) residue as an in vivo phosphorylation site within PKD2. Phosphorylation of Ser(876) of PKD2 correlated with the activation status of the kinase. Finally, gastrin was found to be a physiological activator of PKD2 in human AGS-B cells stably transfected with the CCK(B)/gastrin receptor. Thus, PKD2 is a novel phorbol ester- and growth factor-stimulated protein kinase.     

Dynamic re-distribution of protein kinase D (PKD) as revealed by a GFP-PKD fusion protein: dissociation from PKD activation.

Protein kinase D (PKD)/protein kinase Cmicro (PKCmicro, a serine/threonine protein kinase with distinct structural and enzymological properties, is rapidly activated in intact cells via PKC. The amino-terminal region of PKD contains a cysteine-rich domain (CRD) that directly binds phorbol esters with a high affinity. Here, we show that treatment of transfected RBL 2H3 cells with phorbol 12,13-dibutyrate (PDB) induces a striking CRD-dependent translocation of PKD from the cytosol to the plasma membrane, as shown by real time visualization of a functional green fluorescent protein (GFP)-PKD fusion protein. A single amino acid substitution in the second cysteine-rich motif of PKD (P287G) prevented PDB-induced membrane translocation but did not affect PKD activation. Our results indicate that PKD translocation and activation are distinct processes that operate in parallel to regulate the activity and localization of this enzyme in intact cells.     

Role of the regulatory domain of protein kinase D2 in phorbol ester binding, catalytic activity, and nucleocytoplasmic shuttling

Protein kinase D2 (PKD2) belongs to the PKD family of serine/threonine kinases that is activated by phorbol esters and G protein-coupled receptors (GPCRs). Its C-terminal regulatory domain comprises two cysteine-rich domains (C1a/C1b) followed by a pleckstrin homology (PH) domain. Here, we examined the role of the regulatory domain in PKD2 phorbol ester binding, catalytic activity, and subcellular localization: The PH domain is a negative regulator of kinase activity. C1a/C1b, in particular C1b, is required for phorbol ester binding and gastrin-stimulated PKD2 activation, but it has no inhibitory effect on the catalytic activity. Gastrin triggers nuclear accumulation of PKD2 in living AGS-B cancer cells. C1a/C1b, not the PH domain, plays a complex role in the regulation of nucleocytoplasmic shuttling: We identified a nuclear localization sequence in the linker region between C1a and C1b and a nuclear export signal in the C1a domain. In conclusion, our results define the critical components of the PKD2 regulatory domain controlling phorbol ester binding, catalytic activity, and nucleocytoplasmic shuttling and reveal marked differences to the regulatory properties of this domain in PKD1. These findings could explain functional differences between PKD isoforms and point to a functional role of PKD2 in the nucleus upon activation by GPCRs.     

Regulatory domain determinants that control PKD1 activity

The canonical pathway for protein kinase D1 (PKD1) activation by growth factor receptors involves diacylglycerol binding to the C1 domain and protein kinase C-dependent phosphorylation at the activation loop. PKD1 then autophosphorylates at Ser(916), a modification frequently used as a surrogate marker of PKD1 activity. PKD1 also is cleaved by caspase-3 at a site in the C1-PH interdomain during apoptosis; the functional consequences of this cleavage event remain uncertain. This study shows that PKD1-Δ1-321 (an N-terminal deletion mutant lacking the C1 domain and flanking sequence that models the catalytic fragment that accumulates during apoptosis) and PKD1-CD (the isolated catalytic domain) display high basal Ser(916) autocatalytic activity and robust activity toward CREBtide (a peptide substrate) but little to no activation loop autophosphorylation and no associated activity toward protein substrates, such as cAMP-response element binding protein and cardiac troponin I. In contrast, PKD1-ΔPH (a PH domain deletion mutant) is recovered as a constitutively active enzyme, with high basal autocatalytic activity and high basal activity toward peptide and protein substrates. These results indicate that individual regions in the regulatory domain act in a distinct manner to control PKD1 activity. Finally, cell-based studies show that PKD1-Δ1-321 does not substitute for WT-PKD1 as an in vivo activator of cAMP-response element binding protein and ERK phosphorylation. Proteolytic events that remove the C1 domain (but not the autoinhibitory PH domain) limit maximal PKD1 activity toward physiologically relevant protein substrates and lead to a defect in PKD1-dependent cellular responses.     

Protein kinase D (PKD): A novel target for diacylglycerol and phorbol esters

A novel serine/threonine protein kinase regulated by phorbol esters and diacylglycerol (named PKD) has been identified. PKD contains a cysteine-rich repeat sequence homologous to that seen in the regulatory domain of protein kinase C (PKC). A bacterially expressed NH₂-terminal domain of PKD exhibited high affinity phorbol ester binding activity (K_d = 35 nM). Expression of PKD cDNA in COS cells conferred increased phorbol ester binding to intact cells. The catalytic domain of PKD contains all characteristic sequence motifs of serine protein kinases but shows only a low degree of sequence similarity to PKCs. The bacterially expressed catalytic domain of PKD efficiently phosphorylated the exogenous peptide substrate syntide-2 in serine but did not catalyse significant phosphorylation of a variety of other substrates utilised by PKCs and other major second messenger regulated kinases. PKD expressed in COS cells showed syntide-2 kinase activity that was stimulated by phorbol esters in the presence of phospholipids. We propose that PKD may be a novel component in the transduction of diacylglycerol and phorbol ester signals.     

Recruitment of arfaptins to the trans‐Golgi network by PI(4)P and their involvement in cargo export

The BAR (Bin/Amphiphysin/Rvs) domain proteins arfaptin1 and arfaptin2 are localized to the trans‐Golgi network (TGN) and, by virtue of their ability to sense and/or generate membrane curvature, could play an important role in the biogenesis of transport carriers. We report that arfaptins contain an amphipathic helix (AH) preceding the BAR domain, which is essential for their binding to phosphatidylinositol 4‐phosphate (PI(4)P)‐containing liposomes and the TGN of mammalian cells. The binding of arfaptin1, but not arfaptin2, to PI(4)P is regulated by protein kinase D (PKD) mediated phosphorylation at Ser100 within the AH. We also found that only arfaptin1 is required for the PKD‐dependent trafficking of chromogranin A by the regulated secretory pathway. Altogether, these findings reveal the importance of PI(4)P and PKD in the recruitment of arfaptins at the TGN and their requirement in the events leading to the biogenesis of secretory storage granules.     

A Golgi PKD Activity Reporter Reveals a Crucial Role of PKD in Nocodazole-Induced Golgi Dispersal

The protein kinase D (PKD) family comprises multifunctional serine/threonine-specific protein kinases with three mammalian isoforms: PKD1, PKD2 and PKD3. A prominent PKD function is the regulation of basolateral-targeted transport carrier fission from the trans -Golgi network (TGN). To visualize site-specific PKD activation at this organelle, we designed a molecular reporter consisting of a PKD-specific substrate sequence fused to enhanced green fluorescent protein (EGFP), specifically targeted to the TGN via the p230 GRIP domain. Quantitative analyses using a phosphospecific antibody and ratiometric fluorescence imaging revealed that Golgi-specific phosphorylation of the reporter was strictly dependent on stimulation of endogenous PKD or transient expression of active PKD constructs. Conversely, PKD-specific pharmacological inhibitors and siRNA-mediated PKD knockdown suppressed reporter phosphorylation. Using this reporter we investigated a potential role for PKD in the regulation of Golgi complex morphology. Interestingly, nocodazole-induced Golgi complex break-up and dispersal was associated with local PKD activation as measured by reporter phosphorylation and this was efficiently blocked by expression of a dominant-negative PKD mutant or PKD depletion. Our data thus identify a novel link between PKD activity and the microtubule cytoskeleton, whereby Golgi complex integrity is regulated.     

Regulation of 5-hydroxyeicosanoid dehydrogenase activity in monocytic cells

The requirement of DAG (diacylglycerol) to recruit PKD (protein kinase D) to the TGN (trans-Golgi network) for the targeting of transport carriers to the cell surface, has led us to a search for new components involved in this regulatory pathway. Previous findings reveal that the heterotrimeric Gbetagamma (GTP-binding protein betagamma subunits) act as PKD activators, leading to fission of transport vesicles at the TGN. We have recently shown that PKCeta (protein kinase Ceta) functions as an intermediate member in the vesicle generating pathway. DAG is capable of activating this kinase at the TGN, and at the same time is able to recruit PKD to this organelle in order to interact with PKCeta, allowing phosphorylation of PKD's activation loop. The most qualified candidates for the production of DAG at the TGN are PI-PLCs (phosphatidylinositol-specific phospholipases C), since some members of this family can be directly activated by Gbetagamma, utilizing PtdIns(4,5)P2 as a substrate, to produce the second messengers DAG and InsP3. In the present study we show that betagamma-dependent Golgi fragmentation, PKD1 activation and TGN to plasma membrane transport were affected by a specific PI-PLC inhibitor, U73122 [1-(6-{[17-3-methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl)-1H-pyrrole-2,5-dione]. In addition, a recently described PI-PLC activator, m-3M3FBS [2,4,6-trimethyl-N-(m-3-trifluoromethylphenyl)benzenesulfonamide], induced vesiculation of the Golgi apparatus as well as PKD1 phosphorylation at its activation loop. Finally, using siRNA (small interfering RNA) to block several PI-PLCs, we were able to identify PLCbeta3 as the sole member of this family involved in the regulation of the formation of transport carriers at the TGN. In conclusion, we demonstrate that fission of transport carriers at the TGN is dependent on PI-PLCs, specifically PLCbeta3, which is necessary to activate PKCeta and PKD in that Golgi compartment, via DAG production.     

Synthesis and phorbol ester-binding studies of the individual cysteine-rich motifs of protein kinase D

To investigate the phorbol ester-binding properties of the individual cysteine-rich motifs of protein kinase D (PKD), the 52-mer peptides containing each cysteine-rich motif of PKD (PKD-C1A, PKD-C1B) have been synthesized. The [3H]phorbol-12,13-dibutyrate (PDBu) binding to PKD-C1A was affected drastically by incubation temperature while that to PKD-C1B was not. Scatchard analysis of [3H]PDBu binding to both PKD C1 peptides gave dissociation constants of 2.5 +/- 0.4 and 2.7 +/- 0.8 nM for PKD-C1A and PKD-C1B, respectively, indicating that the two cysteine-rich motifs of PKD are functionally equivalent like those of PKCgamma.     

G-PKDrep-live, a genetically encoded FRET reporter to measure PKD activity at the trans-Golgi-network

The serine/threonine protein kinase D (PKD) is recruited to the trans-Golgi-network (TGN) by interaction with diacylglycerol (DAG) and Arf1 and promotes the fission of vesicles containing cargo destined for the plasma membrane. PKD activation is mediated by PKC(-induced phosphorylation. However, signaling pathways that activate PKD specifically at the TGN are only poorly characterized. Recently we created G-PKDrep, a genetically encoded fluorescent reporter for PKD activity at the TGN in fixed cells. To establish a reporter useful for monitoring Golgi-specific PKD activity in living cells we now refined G-PKDrep to generate G-PKDrep-live. Specifically, phosphorylation of G-PKDrep-live expressed in mammalian cells results in changes of fluorescence resonance energy transfer (FRET), and allows for indirect imaging of PKD activity. In a proof-of-principle experiment using phorbolester treatment, we demonstrate the reporter's capability to track rapid activation of PKD at the TGN. Furthermore, activation-induced FRET changes are reversed by treatment with PKD-specific pharmacological inhibitors. Thus, the newly developed reporter G-PKDrep-live is a suitable tool to visualize dynamic changes in PKD activity at the TGN in living cells. See accompanying commentary by Gautam DOI: 10.1002/biot.201100424.     

Tyrosine phosphorylation of protein kinase D in the pleckstrin homology domain leads to activation

Protein kinase D (PKD) is a member of the AGC family of Ser/Thr kinases and is distantly related to protein kinase C (PKC). Formerly known as PKCmu, PKD contains protein domains not found in conventional PKC isoforms. A functional pleckstrin homology (PH) domain is critical for the regulation of PKD activity. Here we report that PKD is tyrosine-phosphorylated within the PH domain, leading to activation. This phosphorylation is mediated by a pathway that consists of the Src and Abl tyrosine kinases and occurs in response to stimulation with pervanadate and oxidative stress. Mutational analysis revealed three tyrosine phosphorylation sites (Tyr(432), Tyr(463), and Tyr(502)), which are regulated by the Src-Abl pathway, and phosphorylation of only one of these (Tyr(463)) leads to PKD activation. By using a phospho-specific antibody, we show that Abl directly phosphorylates PKD at Tyr(463) in vitro, and in cells phosphorylation of this site is sufficient to mediate full activation of PKD. Mutation of the other two sites, Tyr(432) and Tyr(502), had no significant influence on PKD activity. These data reveal a tyrosine phosphorylation-dependent activation mechanism for PKD and suggest that this event contributes to the release of the autoinhibitory PKD PH domain leading to kinase activation and downstream responses.     

Specific translocation of protein kinase Calpha to the plasma membrane requires both Ca2+ and PIP2 recognition by its C2 domain

The C2 domain of protein kinase Calpha (PKCalpha) controls the translocation of this kinase from the cytoplasm to the plasma membrane during cytoplasmic Ca2+ signals. The present study uses intracellular coimaging of fluorescent fusion proteins and an in vitro FRET membrane-binding assay to further investigate the nature of this translocation. We find that Ca2+-activated PKCalpha and its isolated C2 domain localize exclusively to the plasma membrane in vivo and that a plasma membrane lipid, phosphatidylinositol-4,5-bisphosphate (PIP2), dramatically enhances the Ca2+-triggered binding of the C2 domain to membranes in vitro. Similarly, a hybrid construct substituting the PKCalpha Ca2+-binding loops (CBLs) and PIP2 binding site (beta-strands 3-4) into a different C2 domain exhibits native Ca2+-triggered targeting to plasma membrane and recognizes PIP2. Conversely, a hybrid containing the CBLs but lacking the PIP2 site translocates primarily to trans-Golgi network (TGN) and fails to recognize PIP2. Similarly, PKCalpha C2 domains possessing mutations in the PIP2 site target primarily to TGN and fail to recognize PIP2. Overall, these findings demonstrate that the CBLs are essential for Ca2+-triggered membrane binding but are not sufficient for specific plasma membrane targeting. Instead, targeting specificity is provided by basic residues on beta-strands 3-4, which bind to plasma membrane PIP2.     

PKCeta is required for beta1gamma2/beta3gamma2- and PKD-mediated transport to the cell surface and the organization of the Golgi apparatus

Protein kinase D (PKD) binds to a pool of diacylglycerol (DAG) in the TGN and undergoes a process of activation that involves heterotrimeric GTP-binding protein subunits betagamma to regulate membrane fission. This fission reaction is used to generate transport carriers at the TGN that are en route to the cell surface. We now report that PKD is activated specifically by G protein subunit beta1gamma2 and beta3gamma2 via the Golgi apparatus-associated PKCeta. Compromising the kinase activity of PKCeta-inhibited protein transport from TGN to the cell surface. Expression of constitutively activated PKCeta caused Golgi fragmentation, which was inhibited by a kinase inactive form of PKD. Our findings reveal that betagamma, PKCeta, and PKD act in series to generate transport carriers from the TGN and their overactivation results in complete vesiculation of the Golgi apparatus.     

Mechanism of persistent protein kinase D1 translocation and activation

The specificity of many signal transduction pathways relies on the spatiotemporal features of each signaling step. G protein-coupled receptor-mediated activation of protein kinases leads to diverse cellular effects. Upon receptor activation, PKD1 and several C-type protein kinases (PKCs), translocate to the plasma membrane and become catalytically active. Here we show that, unlike PKCs, PKD1 remains active at the membrane for hours. The two DAG binding C1 domains of PKD1 have distinct functional roles in targeting and maintaining PKD1 at the plasma membrane. C1A achieves fast, maximal, and reversible translocation, while C1B translocates partially, but persistently, to the plasma membrane. The persistent localization requires the C1B domain of PKD1, which binds Galphaq. We incorporate the kinetics of PKD1 translocation into a three-state model that suggests how PKD1 binding to DAG and Galphaq uniquely encodes frequency-dependent PKD1 signaling.     

Rapid protein kinase D translocation in response to G protein-coupled receptor activation. Dependence on protein kinase C.

Protein kinase D (PKD)/protein kinase C (PKC) mu is a serine/threonine protein kinase that can be activated by physiological stimuli like growth factors, antigen-receptor engagement and G protein-coupled receptor (GPCR) agonists via a phosphorylation-dependent mechanism that requires PKC activity. In order to investigate the dynamic mechanisms associated with GPCR signaling, the intracellular translocation of a green fluorescent protein-tagged PKD was analyzed by real-time visualization in fibroblasts and epithelial cells stimulated with bombesin, a GPCR agonist. We found that bombesin induced a rapidly reversible plasma membrane translocation of green fluorescent protein-tagged PKD, an event that can be divided into two distinct mechanistic steps. The first step, which is exclusively mediated by the cysteine-rich domain in the N terminus of PKD, involved its translocation from the cytosol to the plasma membrane. The second step, i.e. the rapid reverse translocation of PKD from the plasma membrane to the cytosol, required its catalytic domain and surprisingly PKC activity. These findings provide evidence for a novel mechanism by which PKC coordinates the translocation and activation of PKD in response to bombesin-induced GPCR activation.     

Protein kinase D specifically mediates apoptosis signal-regulating kinase 1-JNK signaling induced by H2O2 but not tumor necrosis factor

Although both tumor necrosis factor (TNF) and H2O2 induce activation of c-Jun N-terminal kinase (JNK) kinase cascades, it is not known whether they utilize distinct intracellular signaling pathways. In this study, we first examined a variety of pharmacological inhibitors on TNF and H2O2-induced JNK activation. Go6983 or staurosporine, which inhibits protein kinase C isoforms had no effects on TNF or H2O2-induced JNK activation. However, Go6976 and calphostin, which can inhibit protein kinase C as well as protein kinase D (PKD), blocked H2O2- but not TNF-induced JNK activation, suggesting that PKD may be specifically involved in H2O2-induced JNK activation. Consistently, H2O2, but not TNF, induced phosphorylation of PKD and translocation of PKD from endothelial cell membrane to cytoplasm where it associates with the JNK upstream activator, apoptosis signal-regulating kinase 1 (ASK1). The association is mediated through the pleckstrin homology domain of PKD and the C-terminal domain of ASK1. Inhibition of PKD by Go6976 or by small interfering RNA of PKD blocked H2O2-induced ASK1-JNK activation and endothelial cell apoptosis. Interestingly, H2O2 induced 14-3-3 binding to PKD via the phospho-Ser-205/208 and phospho-Ser-219/223 and H2O2-induced 14-3-3 binding of PKD was specifically blocked by Go6976 but not by Go6983. More significantly, the 14-3-3-binding defective forms of PKD failed to associate with ASK1 and to activate JNK signaling, highlighting the importance of 14-3-3 binding of PKD in H2O2-induced activation of ASK1-JNK cascade. Thus, our data have identified PKD as a critical mediator in H2O2- but not TNF-induced ASK1-JNK signaling.     

Protein Kinase D2 Assembles a Multiprotein Complex at the Trans-Golgi Network to Regulate Matrix Metalloproteinase Secretion

Vesicle formation and fission are tightly regulated at the trans-Golgi network (TGN) during constitutive secretion. Two major protein families regulate these processes: members of the adenosyl-ribosylation factor family of small G-proteins (ARFs) and the protein kinase D (PKD) family of serine/threonine kinases. The functional relationship between these two key regulators of protein transport from the TGN so far is elusive. We here demonstrate the assembly of a novel functional protein complex at the TGN and its key members: cytosolic PKD2 binds ARF-like GTPase (ARL1) and shuttles ARL1 to the TGN. ARL1, in turn, localizes Arfaptin2 to the TGN. At the TGN, where PKD2 interacts with active ARF1, PKD2, and ARL1 are required for the assembly of a complex comprising of ARF1 and Arfaptin2 leading to secretion of matrix metalloproteinase-2 and -7. In conclusion, our data indicate that PKD2 is a core factor in the formation of this multiprotein complex at the TGN that controls constitutive secretion of matrix metalloproteinase cargo.     

Regulation of secretory transport by protein kinase D-mediated phosphorylation of the ceramide transfer protein

Protein kinase D (PKD) has been identified as a crucial regulator of secretory transport at the trans-Golgi network (TGN). Recruitment and activation of PKD at the TGN is mediated by the lipid diacylglycerol, a pool of which is generated by sphingomyelin synthase from ceramide and phosphatidylcholine. The nonvesicular transfer of ceramide from the endoplasmic reticulum to the Golgi complex is mediated by the lipid transfer protein CERT (ceramide transport). In this study, we identify CERT as a novel in vivo PKD substrate. Phosphorylation on serine 132 by PKD decreases the affinity of CERT toward its lipid target phosphatidylinositol 4-phosphate at Golgi membranes and reduces ceramide transfer activity, identifying PKD as a regulator of lipid homeostasis. We also show that CERT, in turn, is critical for PKD activation and PKD-dependent protein cargo transport to the plasma membrane. Thus, the interdependence of PKD and CERT is key to the maintenance of Golgi membrane integrity and secretory transport.     

PKD Regulates Membrane Fission to Generate TGN to Cell Surface Transport Carriers

The serine/threonine protein kinase D (PKD) is recruited to the trans-Golgi network (TGN) by binding diacylglycerol (DAG) and the ARF1 GTPase. PKD, at the TGN, promotes the production of phosphatidylinositol-4 phosphate (PI4P) by activating the lipid kinase phophatidylinositol 4-kinase IIIß (PI4KIIIß). PI4P recruits proteins such as oxysterol-binding protein 1 (OSBP) and ceramide transport protein (CERT) that control sphingolipid and sterol levels at the TGN. CERT mediated transport of ceramide to the TGN, we suggest, is used for increasing the local production and concentration of DAG. Once the crucial concentration of DAG is achieved, OSBP and CERT dissociate from the TGN on phosphorylation by PKD and DAG is sequentially converted into phosphatidic acid (PA) and lyso-PA (LPA). Therefore, the net effect of the activated PKD at the TGN is the sequential production of the modified lipids DAG, PA, and LPA that are necessary for membrane fission to generate cell surface specific transport carriers.