Immunofluorescence microchamber technique for characterizing isolated organelles

Rat Organic Anion Transport Protein 1A1 Interacts Directly With Organic Anion Transport Protein 1A4 Facilitating Its Maturation and Trafficking to the Hepatocyte Plasma Membrane

Organic anion transport proteins (OATPs) on the basolateral surface of hepatocytes mediate uptake of a number of drugs and endogenous compounds. Previous studies showed that rat OATP1A1 (rOATP1A1) has a postsynaptic density protein, drosophila disc large tumor suppressor, zonula occludens-1 protein (PDZ) consensus binding motif at its C-terminus and binds to PDZ domain containing 1 (PDZK1), which is required for its cell-surface localization. PDZK1 associates with rOATP1A1-containing endocytic vesicles within cells, mediating recruitment of motor proteins required for microtubule-based trafficking to the plasma membrane. rOATP1A4 also traffics to the plasma membrane, although it lacks a PDZ binding consensus sequence. The current study was designed to test the hypothesis that trafficking of rOATP1A4 to the plasma membrane requires its direct interaction with rOATP1A1 resulting in a complex that traffics through the cell in common subcellular vesicles in which the cytosolic tail of rOATP1A1 is bound to PDZK1. We found that 74% of rOATP1A4-containing rat liver endocytic vesicles (n = 12,044) also contained rOATP1A1. Studies in transfected HEK293 cells showed surface localization of rOATP1A1 only when coexpressed with PDZK1 whereas rOATP1A4 required coexpression with rOATP1A1 and PDZK1. Studies in stably transfected HeLa cells that constitutively expressed PDZK1 showed that coexpression of rOATP1A4 with rOATP1A1 resulted in more rapid appearance of rOATP1A4 on the plasma membrane and faster maturation to its fully glycosylated form. Similar results were observed on immunofluorescence analysis of single cells. Immunoprecipitation of rat liver or transfected HeLa cell lysates with rOATP1A1 antibody specifically co-immunoprecipitated rOATP1A4 as determined by western blotting. Conclusion: These studies indicate that optimal rOATP1A4 trafficking to the cell surface is dependent upon coexpression and interaction with rOATP1A1. As rOATP1A1 binds to the chaperone protein, PDZK1, rOATP1A4 functionally hitchhikes through the cell with this complex.

Feeding-fasting dependent recruitment of membrane microdomain proteins to lipid droplets purified from the liver

Lipid droplets (LDs) are cellular stores of neutral fat that facilitate lipid and protein trafficking in response to metabolic cues. Unlike other vesicles, the phospholipid membrane on the LD is a monolayer. Interestingly, this monolayer membrane has free cholesterol, and may therefore contain lipid microdomains that serve as a platform for assembling proteins involved in signal transduction, cell polarity, pathogen entry etc. In support of this, cell culture studies have detected microdomain-associated "raftophilic" proteins on LDs. However, the physiological significance of this observation has been unclear. Here we show that two proteins (Flotillin-1 and SNAP23) that bind to membrane microdomains associate differently with LDs purified from rat liver depending on the feeding/fasting state of the animal. Flotillin-1 increases on LDs in the fed state, possibly because LDs interact with the endoplasmic reticulum (ER), facilitating supply of flotillin-1 from ER to LDs. Interestingly, this increase in flotillin-1 is correlated with an increase in free cholesterol on the LDs in fed state. In opposite behaviour to flotillin-1, SNAP23 increases on LDs in the fasted state and this appears to mediate LD-mitochondria interactions. Such LD-mitochondria interactions may provide fatty acids to mitochondria for promoting beta-oxidation in hepatocytes in response to fasting. Our work brings out physiologically relevant aspects of lipid droplet biology that are different from, and may not be entirely possible to replicate and study in cell culture.

Heterotrimeric kinesin-2, together with kinesin-1, steers vesicular acetylcholinesterase movements toward the synapse

Acetylcholinesterase (AChE), which is implicated in the pathophysiology of neurological disorders, is distributed along the axon and enriched at the presynaptic basal lamina. It hydrolyses the neurotransmitter acetylcholine, which inhibits synaptic transmission. Aberrant AChE activity and ectopic axonal accumulation of the enzyme are associated with neurodegenerative disorders, such as Alzheimer's disease. The molecular mechanism that underlies AChE transport is still unclear. Here, we show that expression of Drosophila AChE tagged with photoactivatable green fluorescent protein and m-Cherry (GPAC) in cholinergic neurons compensates for the RNA interference-mediated knockdown of endogenous AChE activity. GPAC-AChE, which is enriched in the neuropil region of the brain, moves in the apparently vesicular form in axons with an anterograde bias in Drosophila larvae. Two anterograde motors, kinesin-1 and -2, propel distinct aspects of GPAC-AChE movements. Total loss of kinesin-2 reduces the density of anterograde traffic and increases bidirectional movements of GPAC-AChE vesicles without altering their speed. A partial loss of kinesin-1 reduces both the density and speed of anterograde GPAC-AChE traffic and enhances the pool of stationary vesicles. Together, these results suggest that combining activity of a relatively weak kinesin-2 with that of a stronger kinesin-1 motor could steer AChE-containing vesicles toward synapse, and provides a molecular basis for the observed subcellular distribution of the enzyme.-Kulkarni, A., Khan, Y., Ray, K. Heterotrimeric kinesin-2, together with kinesin-1, steers vesicular acetylcholinesterase movements toward the synapse.

The Na(+)-Taurocholate Cotransporting Polypeptide Traffics with the Epidermal Growth Factor Receptor

Na(+)-taurocholate cotransporting polypeptide (ntcp) mediates bile acid transport, also serving as the hepatitis B virus receptor. It traffics in vesicles along microtubules, requiring activity of protein kinase C (PKC)ζ for motility. We have now found that the epidermal growth factor receptor (EGFR) is the target of PKCζ activity and that EGFR and ntcp colocalize in vesicles. ntcp-containing vesicles that are not associated with EGFR have reduced microtubule-based motility, consistent with intracellular accumulation and reduced surface expression of ntcp in cells following EGFR knockdown.

Organic anion uptake by hepatocytes

Many of the compounds taken up by the liver are organic anions that circulate tightly bound to protein carriers such as albumin. The fenestrated sinusoidal endothelium of the liver permits these compounds to have access to hepatocytes. Studies to characterize hepatic uptake of organic anions through kinetic analyses, suggested that it was carrier-mediated. Attempts to identify specific transporters by biochemical approaches were largely unsuccessful and were replaced by studies that utilized expression cloning. These studies led to identification of the organic anion transport proteins (oatps), a family of 12 transmembrane domain glycoproteins that have broad and often overlapping substrate specificities. The oatps mediate Na(+)-independent organic anion uptake. Other studies identified a seven transmembrane domain glycoprotein, Na(+)/taurocholate transporting protein (ntcp) as mediating Na(+)-dependent uptake of bile acids as well as other organic anions. Although mutations or deficiencies of specific members of the oatp family have been associated with transport abnormalities, there have been no such reports for ntcp, and its physiologic role remains to be determined, although expression of ntcp in vitro recapitulates the characteristics of Na(+)-dependent bile acid transport that is seen in vivo. Both ntcp and oatps traffic between the cell surface and intracellular vesicular pools. These vesicles move through the cell on microtubules, using the microtubule based motors dynein and kinesins. Factors that regulate this motility are under study and may provide a unique mechanism that can alter the plasma membrane content of these transporters and consequently their accessibility to circulating ligands.

Rab1a regulates sorting of early endocytic vesicles

We previously reported that Rab1a is associated with asialoorosomucoid (ASOR)-containing early endocytic vesicles, where it is required for their microtubule-based motility. In Rab1a knockdown (KD) cell lines, ASOR failed to segregate from its receptor and, consequently, did not reach lysosomes for degradation, indicating a defect in early endosome sorting. Although Rab1 is required for Golgi/endoplasmic reticulum trafficking, this process was unaffected, likely due to retained expression of Rab1b in these cells. The present study shows that Rab1a has a more general role in endocytic vesicle processing that extends to EGF and transferrin (Tfn) trafficking. Compared with results in control Huh7 cells, EGF accumulated in aggregates within Rab1a KD cells, failing to reach lysosomal compartments. Tfn, a prototypical example of recycling cargo, accumulated in a Rab11-mediated slow-recycling compartment in Rab1a KD cells, in contrast to control cells, which sort Tfn into a fast-recycling Rab4 compartment. These data indicate that Rab1a is an important regulator of early endosome sorting for multiple cargo species. The effectors and accessory proteins recruited by Rab1a to early endocytic vesicles include the minus-end-directed kinesin motor KifC1, while others remain to be discovered.

Oatp1a1 requires PDZK1 to traffic to the plasma membrane by selective recruitment of microtubule-based motor proteins

Previous studies identified a family of organic anion transport proteins (OATPs), many of which have C-terminal PDZ binding consensus sequences. In particular, the C-terminal four amino acids of Oatp1a1, a transporter on rat and mouse hepatocytes, comprise a consensus binding site for PDZK1. In PDZK1 knockout mice and in transfected cells where PDZK1 expression was knocked down, Oatp1a1 accumulates in intracellular vesicles. The present study tests the hypothesis that Oatp1a1 traffics to and from the cell surface in vesicles along microtubules, and that PDZK1 guides recruitment of specific motors to these vesicles. Oatp1a1-containing vesicles were prepared from wild-type and PDZK1 knockout mice. As seen by immunofluorescence, kinesin-1, a microtubule plus-end directed motor, was largely associated with vesicles from wild-type mouse liver, whereas dynein, a minus-end directed motor, was largely associated with vesicles from PDZK1 knockout mouse liver. Quantification of motility on directionally marked microtubules following addition of 50 µM ATP showed that wild-type vesicles moved equally toward the plus and minus ends whereas PDZK1 knockout vesicles moved predominantly toward the minus end, consistent with net movement toward the cell interior. These studies provide a novel mechanism by which PDZK1 regulates intracellular trafficking of Oatp1a1 by recruiting specific motors to Oatp1a1-containing vesicles. In the absence of PDZK1, Oatp1a1-containing vesicles cannot recruit kinesin-1 and associate with dynein as a predominant minus-end directed motor. Whether this is a result of direct interaction of the Oatp1a1 cytoplasmic domain with dynein or with a dynein-containing protein complex remains to be established.

In vitro motility of liver connexin vesicles along microtubules utilizes kinesin motors

Trafficking of the proteins that form gap junctions (connexins) from the site of synthesis to the junctional domain appears to require cytoskeletal delivery mechanisms. Although many cell types exhibit specific delivery of connexins to polarized cell sites, such as connexin32 (Cx32) gap junctions specifically localized to basolateral membrane domains of hepatocytes, the precise roles of actin- and tubulin-based systems remain unclear. We have observed fluorescently tagged Cx32 trafficking linearly at speeds averaging 0.25 μm/s in a polarized hepatocyte cell line (WIF-B9), which is abolished by 50 μM of the microtubule-disrupting agent nocodazole. To explore the involvement of cytoskeletal components in the delivery of connexins, we have used a preparation of isolated Cx32-containing vesicles from rat hepatocytes and assayed their ATP-driven motility along stabilized rhodamine-labeled microtubules in vitro. These assays revealed the presence of Cx32 and kinesin motor proteins in the same vesicles. The addition of 50 μM ATP stimulated vesicle motility along linear microtubule tracks with velocities of 0.4-0.5 μm/s, which was inhibited with 1 mM of the kinesin inhibitor AMP-PNP (adenylyl-imidodiphosphate) and by anti-kinesin antibody but only minimally affected by 5 μM vanadate, a dynein inhibitor, or by anti-dynein antibody. These studies provide evidence that Cx32 can be transported intracellularly along microtubules and presumably to junctional domains in cells and highlight an important role of kinesin motor proteins in microtubule-dependent motility of Cx32.

Proteomic analysis of endocytic vesicles: Rab1a regulates motility of early endocytic vesicles

Texas-Red-asialoorosomucoid (ASOR) fluorescence-sorted early and late endocytic vesicles from rat liver were subjected to proteomic analysis with the aim of identifying functionally important proteins. Several Rab GTPases, including Rab1a, were found. The present study immunolocalized Rab1a to early and late endocytic vesicles and examined its potential role in endocytosis. Huh7 cells with stable knockdown of Rab1a exhibited reduced endocytic processing of ASOR. This correlated with the finding that Rab1a antibody reduced microtubule-based motility of rat-liver-derived early but not late endocytic vesicles in vitro. The inhibitory effect of Rab1a antibody was observed to be specifically towards minus-end-directed motility. Total and minus-end-directed motility was also reduced in early endocytic vesicles prepared from Rab1a-knockdown cells. These results corresponded with virtual absence of the minus-end-directed kinesin Kifc1 from early endocytic vesicles in Rab1a knockdown cells and imply that Rab1a regulates minus-end-directed motility largely by recruiting Kifc1 to early endocytic vesicles.

UL36p is required for efficient transport of membrane-associated herpes simplex virus type 1 along microtubules

Microtubule-mediated anterograde transport is essential for the transport of herpes simplex virus type 1 (HSV-1) along axons, yet little is known regarding the mechanism and the machinery required for this process. Previously, we were able to reconstitute anterograde transport of HSV-1 on microtubules in an in vitro microchamber assay. Here we report that the large tegument protein UL36p is essential for this trafficking. Using a fluorescently labeled UL36 null HSV-1 strain, KDeltaUL36GFP, we found that it is possible to isolate a membrane-associated population of this virus. Although these viral particles contained normal amounts of tegument proteins VP16, vhs, and VP22, they displayed a 3-log decrease in infectivity and showed a different morphology compared to UL36p-containing virions. Membrane-associated KDeltaUL36GFP also displayed a slightly decreased binding to microtubules in our microchamber assay and a two-thirds decrease in the frequency of motility. This decrease in binding and motility was restored when UL36p was supplied in trans by a complementing cell line. These findings suggest that UL36p is necessary for HSV-1 anterograde transport.

Single vesicle analysis of endocytic fission on microtubules in vitro

Following endocytosis, internalized molecules are found within intracellular vesicles and tubules that move along the cytoskeleton and undergo fission, as demonstrated here using primary cultured rat hepatocytes. Although the use of depolymerizing drugs has shown that the cytoskeleton is not required to segregate endocytic protein, many studies suggest that the cytoskeleton is involved in the segregation of protein in normal cells. To investigate whether cytoskeletal-based movement results in the segregation of protein, we tracked the contents of vesicles during in vitro microscopy assays. These studies showed that the addition of ATP causes fission of endocytic contents along microtubules, resulting in the segregation of proteins that are targeted for different cellular compartments. The plasma membrane proteins, sodium (Na+) taurocholate cotransporting polypeptide (ntcp) and transferrin receptor, segregated from asialoorosomucoid (ASOR), an endocytic ligand that is targeted for degradation. Epidermal growth factor receptor, which is degraded, and the asialoglycoprotein receptor, which remains partially bound to ASOR, segregated less efficiently from ASOR. Vesicles containing ntcp and transferrin receptor had reduced fission in the absence of ASOR, suggesting that fission is regulated to allow proteins to segregate. A single round of fission resulted in 6.5-fold purification of ntcp from ASOR, and 25% of the resulting vesicles were completely depleted of the endocytic ligand.

In vitro motility system to study the role of motor proteins in receptor-ligand sorting

This chapter presents fluorescence microscope assays that can be used to study microtubule (MT)-based movement and receptor-ligand sorting in vitro. The strategy is to isolate endosomes in a concentrated active form and store them in frozen aliquots for single use. Glass microchambers are then constructed and coated with fluorescent MTs, and the endosomes are thawed and bound to the MTs. Proteins of interest are then detected and quantified by immunofluorescence. For motility experiments, time-lapse movies are captured using multichannel fluorescence microscopy, and motility is initiated by the addition of ATP. Movies are later categorized and quantified for MT-based motility and other associated events such as endocytic fission. These techniques were developed to assess the role of MTs and MT motor proteins in endocytic processing within liver cells, and we have streamlined a rapid procedure for isolating abundant, highly motile endosomes from rat liver. Cultured cells and other organelles can also be examined, and many important biological questions concerning intracellular traffic and organelle composition can be studied by creative adaptation of the protocols that are presented.

Kif5B and Kifc1 interact and are required for motility and fission of early endocytic vesicles in mouse liver

Early endocytic vesicles loaded with Texas Red asialoorosomucoid were prepared from mouse liver. These vesicles bound to microtubules in vitro, and upon ATP addition, they moved bidirectionally, frequently undergoing fission into two daughter vesicles. There was no effect of vanadate (inhibitor of dynein) on motility, whereas 5'-adenylylimido-diphosphate (kinesin inhibitor) was highly inhibitory. Studies with specific antibodies confirmed that dynein was not associated with these vesicles and that Kif5B and the minus-end kinesin Kifc1 mediated their plus- and minus-end motility, respectively. More than 90% of vesicles associated with Kifc1 also contained Kif5B, and inhibition of Kifc1 with antibody resulted in enhancement of plus-end-directed motility. There was reduced vesicle fission when either Kifc1 or Kif5B activity was inhibited by antibody, indicating that the opposing forces resulting from activity of both motors are required for fission to occur. Immunoprecipitation of native Kif5B by FLAG antibody after expression of FLAG-Kifc1 in 293T cells indicates that these two motors can interact with each other. Whether they interact directly or through a complex of potential regulatory proteins will need to be clarified in future studies. However, the present study shows that coordinated activity of these kinesins is essential for motility and processing of early endocytic vesicles.

PKCzeta is required for microtubule-based motility of vesicles containing the ntcp transporter

Intracellular trafficking regulates the abundance and therefore activity of transporters present at the plasma membrane. The transporter, Na+-taurocholate co-transporting polypeptide (ntcp), is increased at the plasma membrane upon treatment of cells with cAMP, for which microtubules (MTs) are required and the PI3K pathway and PKCzeta have been implicated. However, trafficking of ntcp on MTs has not been demonstrated directly and the regulation and intracellular localization of ntcp is not well understood. Here, we utilize in vitro and whole-cell immunofluorescence microscopy assays to demonstrate that ntcp is present on intracellular vesicles that bind MTs and move bidirectionally, using kinesin-1 and dynein. These vesicles co-localize with markers for recycling endosomes and early but not late endosomes. They frequently undergo fission, providing a mechanism for the exclusion of ntcp from late endosomes. PI(3,4,5)P3 activates PKCzeta and enhances motility of the ntcp vesicles and overcomes the partial inhibition produced by a PI3-kinase inhibitor. Specific inhibition of PKCzeta blocks the motility of ntcp-containing vesicles but has no effect on late vesicles as shown both in vitro and in living cells transfected with ntcp-GFP. These data indicate that PKCzeta is required specifically for the intracellular movement of vesicles that contain the ntcp transporter.

Reconstitution of herpes simplex virus microtubule-dependent trafficking in vitro

Microtubule-mediated anterograde transport of herpes simplex virus (HSV) from the neuronal cell body to the axon terminal is crucial for the spread and transmission of the virus. It is therefore of central importance to identify the cellular and viral factors responsible for this trafficking event. In previous studies, we isolated HSV-containing cytoplasmic organelles from infected cells and showed that they represent the first and only destination for HSV capsids after they emerge from the nucleus. In the present study, we tested whether these cytoplasmic compartments were capable of microtubule-dependent traffic. Organelles containing green fluorescent protein-labeled HSV capsids were isolated and found to be able to bind rhodamine-labeled microtubules polymerized in vitro. Following the addition of ATP, the HSV-associated organelles trafficked along the microtubules, as visualized by time lapse microscopy in an imaging microchamber. The velocity and processivity of trafficking resembled those seen for neurotropic herpesvirus traffic in living axons. The use of motor-specific inhibitors indicated that traffic was predominantly kinesin mediated, consistent with the reconstitution of anterograde traffic. Immunocytochemical studies revealed that the majority of HSV-containing organelles attached to the microtubules contained the trans-Golgi network marker TGN46. This simple, minimal reconstitution of microtubule-mediated anterograde traffic should facilitate and complement molecular analysis of HSV egress in vivo.

Optical properties of Alexa 488 and Cy5 immobilized on a glass surface

The absorption and emission spectra were measured for Cy5 and Alexa 488 fluorophores confined on a glass surface. The data were obtained using fluorometry and spectroscopic ellipsometry. Red shifts of the surface-immobilized fluorophore absorption spectra relative to the fluorophore spectra in aqueous solution were observed using both methods. We interpret these red shifts in terms of a change in the polarizability and polarity of the effective solvent. A formula is given that can be used to estimate expected shifts in absorption and emission maxima for surface-immobilized fluorophores. Spectroscopic ellipsometry measurements provide identification of the fluorophores confined on a glass surface. These results suggest that the design of microarray detection systems should be based on the optical properties of fluorophores attached to the surface and not on the optical properties of fluorophores in solution.

Assay of Rab4-dependent trafficking on microtubules

We present an in vitro method to measure how Rab4 and other regulatory proteins affect microtubule-based organelle motility. The protocols utilize small-volume, disposable "microchambers" designed for epifluorescence, confocal, or other microscope platforms and into which microtubules, organelles, and primary and fluorescent secondary antibodies are added. Our work has focused on the isolation and use of endocytic vesicles from rat liver, and we present these protocols. However, the techniques can be adapted for other organelles or cell types. Multiple fluorescent probes, rapid image capture, and immunofluorescence under non-fixation conditions allow for measurements of the location and intensity changes of endogenous proteins upon addition of ATP or upon addition of other proteins or regulatory factors. We review measurements of microtubule-based motility as well as measurements for protein localization and protein segregation in vitro.

Microtubule-dependent movement of late endocytic vesicles in vitro: requirements for Dynein and Kinesin

Our previous studies demonstrated that fluorescent early endocytic vesicles prepared from rat liver after injection of Texas red asialoorosomucoid contain asialoglycoprotein and its receptor and move and undergo fission along microtubules using kinesin I and KIFC2, with Rab4 regulating KIFC2 activity (J. Cell Sci. 116, 2749, 2003). In the current study, procedures to prepare fluorescent late endocytic vesicles were devised. In addition, flow cytometry was utilized to prepare highly purified fluorescent endocytic vesicles, permitting validation of microscopy-based experiments as well as direct biochemical analysis. These studies revealed that late vesicles bound to and moved along microtubules, but in contrast to early vesicles, did not undergo fission. As compared with early vesicles, late vesicles had reduced association with receptor, Rab4, and kinesin I but were highly associated with dynein, Rab7, dynactin, and KIF3A. Dynein and KIF3A antibodies inhibited late vesicle motility, whereas kinesin I and KIFC2 antibodies had no effect. Dynamitin antibodies prevented the association of late vesicles with microtubules. These results indicate that acquisition and exchange of specific motor and regulatory proteins characterizes and may regulate the transition of early to late endocytic vesicles. Flow cytometric purification should ultimately facilitate detailed proteomic analysis and mapping of endocytic vesicle-associated proteins.

Roles of the cytoskeleton and motor proteins in endocytic sorting

After internalization, endocytic material is actively transported through the cytoplasm, predominantly by microtubule motor proteins. Microtubule-based endocytic transport facilitates sorting of endocytic contents, vesicle fusion and fission, delivery to lysosomes, cytosolic dispersal, as well as nuclear uptake and cytosolic egress of pathogens. Endosomes, like most organelles, move bidirectionally through the cytosol and regulate their cellular location by controlling the activity of motor proteins, and potentially by controlling microtubule and actin polymerization. Control of motor protein activity is manifest by increased microtubule "run lengths", and the binding of motor proteins to organelles can be regulated by motor protein receptors. A mechanistic understanding of how organelles control motor protein activity to allow for endocytic sorting presents an exciting avenue for future research.

Regulation of early endocytic vesicle motility and fission in a reconstituted system

We previously established conditions to reconstitute kinesin-dependent early endocytic vesicle motility and fission on microtubules in vitro. The present study examined the question whether motility and fission are regulated in this system. Screening for proteins by immunofluorescence microscopy revealed that the small G protein, Rab4, was associated with 80% of hepatocyte-derived early endocytic vesicles that contain the ligand asialoorosomucoid (ASOR). By contrast, other markers for early endocytic vesicles including clathrin, Rab5 and EEA1 were present in the preparation but did not colocalize with the ASOR vesicles. Guanine nucleotides exchanged into the Rab4 present on the vesicles as shown by solubilization of Rab4 by Rab-GDI; solubilization was inhibited by incubation with GTP-gamma-S and promoted by GDP. Pre-incubation of vesicles with GDP increased the number of vesicles moving on microtubules and markedly increased vesicle fission. This increase in motility from GDP was shown to be towards the minus end of microtubules, possibly through activation of the minus-end-directed kinesin, KIFC2. Pre-incubation of vesicles with GTP-gamma-S, by contrast, repressed motility. Addition of exogenous GST-Rab4- GTP-gamma-S led to a further repression of motility and fission. Repression was not seen with addition of GST-Rab4-GDP. Treatment of vesicles with Rab4 antibody also repressed motility, and repression was not seen when vesicles were pre-incubated with GDP. Based on these results we hypothesize that endogenous Rab4-GTP suppresses motility of ASOR-containing vesicles in hepatocytes and that conversion of Rab4-GTP to Rab4-GDP serves as a molecular switch that activates minus-end kinesin-based motility, facilitating early endosome fission and consequent receptor-ligand segregation.