Multi-color live-cell fluorescence imaging of primary ciliary membrane assembly and dynamics.

The ciliary membrane is continuous with the plasma membrane but has distinct lipid and protein composition, which is key to defining the function of the primary cilium. Ciliary membranes dynamically assemble and disassemble in association with the cell cycle and directly transmit signals and molecules through budding membranes. Various imaging approaches have greatly advanced the understanding of the ciliary membrane function. In particular, fluorescence live-cell imaging has revealed important insights into the dynamics of ciliary membrane assembly by monitoring the changes of fluorescent-tagged ciliary proteins. Protein dynamics can be tracked simultaneously using multi-color live cell imaging by coupling ciliary-associated factors with different colored fluorescent tags. Ciliary membrane and membrane associated-proteins such as Smoothened, 5-HTr6, SSTR3, Rab8a, and Arl13b have been used to track ciliary membranes and centriole proteins like Centrin1/2, CEP164, and CEP83 are often used to mark the ciliary basal body. Here, we describe a method for studying ciliogenesis membrane dynamics using spinning disk confocal live-cell imaging.

Ciliogenesis membrane dynamics and organization.

Ciliogenesis is a complex multistep process used to describe assembly of cilia and flagella. These organelles play essential roles in motility and signaling on the surface of cells. Cilia are built at the distal ends of centrioles through the formation of an axoneme that is surrounded by the ciliary membrane. As is the case in the biogenesis of other cellular organelles, regulators of membrane trafficking play essential roles in ciliogenesis, albeit with a unique feature that membranes are organized around microtubule-based structures. Membrane association with the distal end of the centriole is a critical initiating step for ciliogenesis. Studies of this process in different cell types suggests that a singular mechanism may not be utilized to initiate cilium assembly. In this review, we focus on recent insights into cilium biogenesis and the roles membrane trafficking regulators play in described ciliogenesis mechanisms with relevance to human disease.

Intracellular connections between basal bodies promote the coordinated behavior of motile cilia.

Hydrodynamic flow produced by multiciliated cells is critical for fluid circulation and cell motility. Hundreds of cilia beat with metachronal synchrony for fluid flow. Cilia-driven fluid flow produces extracellular hydrodynamic forces that cause neighboring cilia to beat in a synchronized manner. However, hydrodynamic coupling between neighboring cilia is not the sole mechanism that drives cilia synchrony. Cilia are nucleated by basal bodies (BBs) that link to each other and to the cell's cortex via BB-associated appendages. The intracellular BB and cortical network is hypothesized to synchronize ciliary beating by transmitting cilia coordination cues. The extent of intracellular ciliary connections and the nature of these stimuli remain unclear. Moreover, how BB connections influence the dynamics of individual cilia has not been established. We show by focused ion beam scanning electron microscopy imaging that cilia are coupled both longitudinally and laterally in the ciliate Tetrahymena thermophila by the underlying BB and cortical cytoskeletal network. To visualize the behavior of individual cilia in live, immobilized Tetrahymena cells, we developed Delivered Iron Particle Ubiety Live Light (DIPULL) microscopy. Quantitative and computer analyses of ciliary dynamics reveal that BB connections control ciliary waveform and coordinate ciliary beating. Loss of BB connections reduces cilia-dependent fluid flow forces.

Male infertility-associated Ccdc108 regulates multiciliogenesis via the intraflagellar transport machinery.

Motile cilia on the cell surface generate movement and directional fluid flow that is crucial for various biological processes. Dysfunction of these cilia causes human diseases such as sinopulmonary disease and infertility. Here, we show that Ccdc108, a protein linked to male infertility, has an evolutionarily conserved requirement in motile multiciliation. Using Xenopus laevis embryos, Ccdc108 is shown to be required for the migration and docking of basal bodies to the apical membrane in epidermal multiciliated cells (MCCs). We demonstrate that Ccdc108 interacts with the IFT-B complex, and the ciliation requirement for Ift74 overlaps with Ccdc108 in MCCs. Both Ccdc108 and IFT-B proteins localize to migrating centrioles, basal bodies, and cilia in MCCs. Importantly, Ccdc108 governs the centriolar recruitment of IFT while IFT licenses the targeting of Ccdc108 to the cilium. Moreover, Ccdc108 is required for the centriolar recruitment of Drg1 and activated RhoA, factors that help establish the apical actin network in MCCs. Together, our studies indicate that Ccdc108 and IFT-B complex components cooperate in multiciliogenesis.

CEP97 phosphorylation by Dyrk1a is critical for centriole separation during multiciliogenesis.

Proper cilia formation in multiciliated cells (MCCs) is necessary for appropriate embryonic development and homeostasis. Multicilia share many structural characteristics with monocilia and primary cilia, but there are still significant gaps in our understanding of the regulation of multiciliogenesis. Using the Xenopus embryo, we show that CEP97, which is known as a negative regulator of primary cilia formation, interacts with dual specificity tyrosine phosphorylation regulated kinase 1A (Dyrk1a) to modulate multiciliogenesis. We show that Dyrk1a phosphorylates CEP97, which in turn promotes the recruitment of Polo-like kinase 1 (Plk1), which is a critical regulator of MCC maturation that functions to enhance centriole disengagement in cooperation with the enzyme Separase. Knockdown of either CEP97 or Dyrk1a disrupts cilia formation and centriole disengagement in MCCs, but this defect is rescued by overexpression of Separase. Thus, our study reveals that Dyrk1a and CEP97 coordinate with Plk1 to promote Separase function to properly form multicilia in vertebrate MCCs.

A 584 bp deletion in CTRB2 inhibits chymotrypsin B2 activity and secretion and confers risk of pancreatic cancer.

Genome-wide association studies (GWASs) have discovered 20 risk loci in the human genome where germline variants associate with risk of pancreatic ductal adenocarcinoma (PDAC) in populations of European ancestry. Here, we fine-mapped one such locus on chr16q23.1 (rs72802365, p = 2.51 × 10-17, OR = 1.36, 95% CI = 1.31-1.40) and identified colocalization (PP = 0.87) with aberrant exon 5-7 CTRB2 splicing in pancreatic tissues (pGTEx = 1.40 × 10-69, ßGTEx = 1.99; pLTG = 1.02 × 10-30, ßLTG = 1.99). Imputation of a 584 bp structural variant overlapping exon 6 of CTRB2 into the GWAS datasets resulted in a highly significant association with pancreatic cancer risk (p = 2.83 × 10-16, OR = 1.36, 95% CI = 1.31-1.42), indicating that it may underlie this signal. Exon skipping attributable to the deletion (risk) allele introduces a premature stop codon in exon 7 of CTRB2, yielding a truncated chymotrypsinogen B2 protein that lacks chymotrypsin activity, is poorly secreted, and accumulates intracellularly in the endoplasmic reticulum (ER). We propose that intracellular accumulation of a nonfunctional chymotrypsinogen B2 protein leads to ER stress and pancreatic inflammation, which may explain the increased pancreatic cancer risk in carriers of CTRB2 exon 6 deletion alleles.

The deubiquitinating enzyme USP37 enhances CHK1 activity to promote the cellular response to replication stress.

The deubiquitinating enzyme USP37 is known to contribute to timely onset of S phase and progression of mitosis. However, it is not clear if USP37 is required beyond S-phase entry despite expression and activity of USP37 peaking within S phase. We have utilized flow cytometry and microscopy to analyze populations of replicating cells labeled with thymidine analogs and monitored mitotic entry in synchronized cells to determine that USP37-depleted cells exhibited altered S-phase kinetics. Further analysis revealed that cells depleted of USP37 harbored increased levels of the replication stress and DNA damage markers γH2AX and 53BP1 in response to perturbed replication. Depletion of USP37 also reduced cellular proliferation and led to increased sensitivity to agents that induce replication stress. Underlying the increased sensitivity, we found that the checkpoint kinase 1 is destabilized in the absence of USP37, attenuating its function. We further demonstrated that USP37 deubiquitinates checkpoint kinase 1, promoting its stability. Together, our results establish that USP37 is required beyond S-phase entry to promote the efficiency and fidelity of replication. These data further define the role of USP37 in the regulation of cell proliferation and contribute to an evolving understanding of USP37 as a multifaceted regulator of genome stability.

CLEM Characterization of Rab8 and Associated Membrane Trafficking Regulators at Primary Cilium Structures.

Correlative light and electron microscopy (CLEM) enables determination of high-resolution structural information for proteins of interest within their biological context through the combination of electron and fluorescence microscopies. Numerous electron microscopy (EM) studies of primary cilia have provided ultrastructural details about these antennal-like organelles that extend from the surface of the cell. The core structure of the cilium includes a microtubule-based axoneme, a basal body derived from the mother centriole, and the ciliary membrane, which is connected to the plasma membrane. The small GTPase Rab8 localizes to the ciliary membrane and is important for ciliogenesis, and Rab11 transports the Rab8 guanine nucleotide exchange factor (GEF) Rabin8 to the mother centriole to activate Rab8-dependent ciliary membrane growth. Some primary cilia have a ciliary pocket membrane (CPM) which is observed as an involution from the plasma membrane to the base of the cilia membrane. The Rab11- and Rab8-assocaited membrane trafficking regulator Eps15 Homology Domain-containing protein 1 (EHD1) and EHD3 also function in early stages of ciliogenesis; however, they localize to the CPM. These ciliary localizations of Rab8 and EHD1 can be resolved using CLEM with conventional fluorescence microscopy and transmission electron microscopy (TEM) imaging. Here, we describe in detail the protocol for this CLEM method applicable for ciliary proteins and proteins in other cellular organelles.

Recent advances in understanding assembly of the primary cilium membrane.

Ciliogenesis describes the assembly of cilia in interphase cells. Several hundred proteins have been linked to ciliogenesis, which proceeds through a highly coordinated multistage process at the distal end of centrioles requiring membranes. In this short review, we focus on recently reported insights into the biogenesis of the primary cilium membrane and its association with other ciliogenic processes in the intracellular ciliogenesis pathway.

RAB19 Directs Cortical Remodeling and Membrane Growth for Primary Ciliogenesis.

Primary cilia are sensory organelles that utilize the compartmentalization of membrane and cytoplasm to communicate signaling events, and yet, how the formation of a cilium is coordinated with reorganization of the cortical membrane and cytoskeleton is unclear. Using polarized epithelia, we find that cortical actin clearing and apical membrane partitioning occur where the centrosome resides at the cell surface prior to ciliation. RAB19, a previously uncharacterized RAB, associates with the RAB-GAP TBC1D4 and the HOPS-tethering complex to coordinate cortical clearing and ciliary membrane growth, which is essential for ciliogenesis. This RAB19-directed pathway is not exclusive to polarized epithelia, as RAB19 loss in nonpolarized cell types blocks ciliogenesis with a docked ciliary vesicle. Remarkably, inhibiting actomyosin contractility can substitute for the function of the RAB19 complex and restore ciliogenesis in knockout cells. Together, this work provides a mechanistic understanding behind a cytoskeletal clearing and membrane partitioning step required for ciliogenesis.

Live-cell fluorescence imaging of ciliary dynamics.

The cilium was one of the first organelles observed through a microscope. Motile cilia appear as oscillating cell appendages and have long been recognized to function in cell motility. In contrast, the far more widespread non-motile cilia, termed primary cilia, were thought to be vestigial and largely ignored following their initial description over a century ago. Only in the last two decades has the critical function of primary cilia been elucidated. Primary cilia play essential roles in signal transduction, chemical sensation, mechanosensation and light detection. Various microscopy approaches have been important for characterizing the structure, dynamics and function of the cilia. In this review, we discuss the application of live-cell imaging technologies and their contribution to our current understanding of ciliary processes.

The C7orf43/TRAPPC14 component links the TRAPPII complex to Rabin8 for preciliary vesicle tethering at the mother centriole during ciliogenesis.

The primary cilium is a cellular sensor that detects light, chemicals, and movement and is important for morphogen and growth factor signaling. The small GTPase Rab11-Rab8 cascade is required for ciliogenesis. Rab11 traffics the guanine nucleotide exchange factor (GEF) Rabin8 to the centrosome to activate Rab8, needed for ciliary growth. Rabin8 also requires the transport particle protein complex (TRAPPC) proteins for centrosome recruitment during ciliogenesis. Here, using an MS-based approach for identifying Rabin8-interacting proteins, we identified C7orf43 (also known as microtubule-associated protein 11 (MAP11)) as being required for ciliation both in human cells and zebrafish embryos. We find that C7orf43 directly binds to Rabin8 and that C7orf43 knockdown diminishes Rabin8 preciliary centrosome accumulation. Interestingly, we found that C7orf43 co-sediments with TRAPPII complex subunits and directly interacts with TRAPPC proteins. Our findings establish that C7orf43 is a TRAPPII-specific complex component, referred to here as TRAPPC14. Additionally, we show that TRAPPC14 is dispensable for TRAPPII complex integrity but mediates Rabin8 association with the TRAPPII complex. Finally, we demonstrate that TRAPPC14 interacts with the distal appendage proteins Fas-binding factor 1 (FBF1) and centrosomal protein 83 (CEP83), which we show here are required for GFP-Rabin8 centrosomal accumulation, supporting a role for the TRAPPII complex in tethering preciliary vesicles to the mother centriole during ciliogenesis. In summary, our findings have revealed an uncharacterized TRAPPII-specific component, C7orf43/TRAPPC14, that regulates preciliary trafficking of Rabin8 and ciliogenesis and support previous findings that the TRAPPII complex functions as a membrane tether.

Akt Regulates a Rab11-Effector Switch Required for Ciliogenesis.

Serum starvation stimulates cilia growth in cultured cells, yet serum factors associated with ciliogenesis are unknown. Previously, we showed that starvation induces rapid Rab11-dependent vesicular trafficking of Rabin8, a Rab8 guanine-nucleotide exchange factor (GEF), to the mother centriole, leading to Rab8 activation and cilium growth. Here, we demonstrate that through the LPA receptor 1 (LPAR1), serum lysophosphatidic acid (LPA) inhibits Rab11a-Rabin8 interaction and ciliogenesis. LPA/LPAR1 regulates ciliogenesis initiation via downstream PI3K/Akt activation, independent of effects on cell cycle. Akt stabilizes Rab11a binding to its effector, WDR44, and a WDR44-pAkt-phosphomimetic mutant blocks ciliogenesis. WDR44 depletion promotes Rabin8 preciliary trafficking and ciliogenesis-initiating events at the mother centriole. Our work suggests disruption of Akt signaling causes a switch from Rab11-WDR44 to the ciliogenic Rab11-FIP3-Rabin8 complex. Finally, we demonstrate that Akt regulates downstream ciliogenesis processes associated with Rab8-dependent cilia growth. Together, this study uncovers a mechanism whereby serum mitogen signaling regulates Rabin8 preciliary trafficking and ciliogenesis initiation.

Genomic knockout of alms1 in zebrafish recapitulates Alström syndrome and provides insight into metabolic phenotypes.

Alström syndrome (OMIM #203800) is an autosomal recessive obesity ciliopathy caused by loss-of-function mutations in the ALMS1 gene. In addition to multi-organ dysfunction, such as cardiomyopathy, retinal degeneration and renal dysfunction, the disorder is characterized by high rates of obesity, insulin resistance and early-onset type 2 diabetes mellitus (T2DM). To investigate the underlying mechanisms of T2DM phenotypes, we generated a loss-of-function deletion of alms1 in the zebrafish. We demonstrate conservation of hallmark clinical characteristics alongside metabolic syndrome phenotypes, including a propensity for obesity and fatty livers, hyperinsulinemia and glucose response defects. Gene expression changes in ß-cells isolated from alms1-/- mutants revealed changes consistent with insulin hypersecretion and glucose sensing failure, which were corroborated in cultured murine ß-cells lacking Alms1. We also found evidence of defects in peripheral glucose uptake and concomitant hyperinsulinemia in the alms1-/- animals. We propose a model in which hyperinsulinemia is the primary and causative defect underlying generation of T2DM associated with alms1 deficiency. These observations support the alms1 loss-of-function zebrafish mutant as a monogenic model for mechanistic interrogation of T2DM phenotypes.

M-Ras/Shoc2 signaling modulates E-cadherin turnover and cell-cell adhesion during collective cell migration.

Collective cell migration is required for normal embryonic development and contributes to various biological processes, including wound healing and cancer cell invasion. The M-Ras GTPase and its effector, the Shoc2 scaffold, are proteins mutated in the developmental RASopathy Noonan syndrome, and, here, we report that activated M-Ras recruits Shoc2 to cell surface junctions where M-Ras/Shoc2 signaling contributes to the dynamic regulation of cell-cell junction turnover required for collective cell migration. MCF10A cells expressing the dominant-inhibitory M-RasS27N variant or those lacking Shoc2 exhibited reduced junction turnover and were unable to migrate effectively as a group. Through further depletion/reconstitution studies, we found that M-Ras/Shoc2 signaling contributes to junction turnover by modulating the E-cadherin/p120-catenin interaction and, in turn, the junctional expression of E-cadherin. The regulatory effect of the M-Ras/Shoc2 complex was mediated at least in part through the phosphoregulation of p120-catenin and required downstream ERK cascade activation. Strikingly, cells rescued with the Noonan-associated, myristoylated-Shoc2 mutant (Myr-Shoc2) displayed a gain-of-function (GOF) phenotype, with the cells exhibiting increased junction turnover and reduced E-cadherin/p120-catenin binding and migrating as a faster but less cohesive group. Consistent with these results, Noonan-associated C-Raf mutants that bypass the need for M-Ras/Shoc2 signaling exhibited a similar GOF phenotype when expressed in Shoc2-depleted MCF10A cells. Finally, expression of the Noonan-associated Myr-Shoc2 or C-Raf mutants, but not their WT counterparts, induced gastrulation defects indicative of aberrant cell migration in zebrafish embryos, further demonstrating the function of the M-Ras/Shoc2/ERK cascade signaling axis in the dynamic control of coordinated cell movement.

Publisher Correction: Investigation of F-BAR domain PACSIN proteins uncovers membrane tubulation function in cilia assembly and transport.

In the original version of this Article, the fifth sentence of the abstract incorrectly read 'Remarkably, we show that PACSIN1 and EHD1 assemble membrane t7ubules from the developing intracellular cilium that attach to the plasma membrane, creating an extracellular membrane channel (EMC) to the outside of the cell.', and should have read 'Remarkably, we show that PACSIN1 and EHD1 assemble membrane tubules from the developing intracellular cilium that attach to the plasma membrane, creating an extracellular membrane channel (EMC) to the outside of the cell.'. This has been corrected in both the PDF and HTML versions of the Article.

Investigation of F-BAR domain PACSIN proteins uncovers membrane tubulation function in cilia assembly and transport.

The intracellular ciliogenesis pathway requires membrane trafficking, fusion, and reorganization. Here, we demonstrate in human cells and zebrafish that the F-BAR domain containing proteins PACSIN1 and -2 play an essential role in ciliogenesis, similar to their binding partner and membrane reorganizer EHD1. In mature cilia, PACSINs and EHDs are dynamically localized to the ciliary pocket membrane (CPM) and transported away from this structure on membrane tubules along with proteins that exit the cilium. PACSINs function early in ciliogenesis at the ciliary vesicle (CV) stage to promote mother centriole to basal body transition. Remarkably, we show that PACSIN1 and EHD1 assemble membrane t7ubules from the developing intracellular cilium that attach to the plasma membrane, creating an extracellular membrane channel (EMC) to the outside of the cell. Together, our work uncovers a function for F-BAR proteins and membrane tubulation in ciliogenesis and explains how the intracellular cilium emerges from the cell.

Oncogenic RAS-Induced Perinuclear Signaling Complexes Requiring KSR1 Regulate Signal Transmission to Downstream Targets.

The precise characteristics that distinguish normal and oncogenic RAS signaling remain obscure. Here, we show that oncogenic RAS and BRAF induce perinuclear relocalization of several RAS pathway proteins, including the kinases CK2 and p-ERK1/2 and the signaling scaffold KSR1. This spatial reorganization requires endocytosis, the kinase activities of MEK-ERK and CK2, and the presence of KSR1. CK2α colocalizes with KSR1 and Rab11, a marker of recycling endosomes, whereas p-ERK associates predominantly with a distinct KSR1-positive endosomal population. Notably, these perinuclear signaling complexes (PSC) are present in tumor cell lines, mouse lung tumors, and mouse embryonic fibroblasts undergoing RAS-induced senescence. PSCs are also transiently induced by growth factors (GF) in nontransformed cells with delayed kinetics (4-6 hours), establishing a novel late phase of GF signaling that appears to be constitutively activated in tumor cells. PSCs provide an essential platform for RAS-induced phosphorylation and activation of the prosenescence transcription factor C/EBPß in primary MEFs undergoing senescence. Conversely, in tumor cells, C/EBPß activation is suppressed by 3'UTR-mediated localization of Cebpb transcripts to a peripheral cytoplasmic domain distinct from the PSC region. Collectively, our findings indicate that sustained PSC formation is a critical feature of oncogenic RAS/BRAF signaling in cancer cells that controls signal transmission to downstream targets by regulating selective access of effector kinases to substrates such as C/EBPß.Significance: In addressing the long-standing question of the difference between normal and oncogenic RAS pathway signaling, this study shows that oncogenic RAS specifically triggers constitutive endocytosis-dependent movement of effector kinases to a perinuclear region, thereby creating connections to unique downstream targets such as the core prosenescence and the inflammatory regulatory transcription factor C/EBPß. Cancer Res; 78(4); 891-908. ©2017 AACR.

In Vitro Modeling Using Ciliopathy-Patient-Derived Cells Reveals Distinct Cilia Dysfunctions Caused by CEP290 Mutations.

Mutations in CEP290, a transition zone protein in primary cilia, cause diverse ciliopathies, including Leber congenital amaurosis (LCA) and Joubert-syndrome and related disorders (JSRD). We examined cilia biogenesis and function in cells derived from CEP290-LCA and CEP290-JSRD patients. CEP290 protein was reduced in LCA fibroblasts with no detectable impact on cilia; however, optic cups derived from induced pluripotent stem cells (iPSCs) of CEP290-LCA patients displayed less developed photoreceptor cilia. Lack of CEP290 in JSRD fibroblasts resulted in abnormal cilia and decreased ciliogenesis. We observed selectively reduced localization of ADCY3 and ARL13B. Notably, Hedgehog signaling was augmented in CEP290-JSRD because of enhanced ciliary transport of Smoothened and GPR161. These results demonstrate a direct correlation between the extent of ciliogenesis defects in fibroblasts and photoreceptors with phenotypic severity in JSRD and LCA, respectively, and strengthen the role of CEP290 as a selective ciliary gatekeeper for transport of signaling molecules in and out of the cilium.

Dynamics of HIV-1 RNA Near the Plasma Membrane during Virus Assembly.

UNLABELLED: To increase our understanding of the events that lead to HIV-1 genome packaging, we examined the dynamics of viral RNA and Gag-RNA interactions near the plasma membrane by using total internal reflection fluorescence microscopy. We labeled HIV-1 RNA with a photoconvertible Eos protein via an RNA-binding protein that recognizes stem-loop sequences engineered into the viral genome. Near-UV light exposure causes an irreversible structural change in Eos and alters its emitted fluorescence from green to red. We studied the dynamics of HIV-1 RNA by photoconverting Eos near the plasma membrane, and we monitored the population of photoconverted red-Eos-labeled RNA signals over time. We found that in the absence of Gag, most of the HIV-1 RNAs stayed near the plasma membrane transiently, for a few minutes. The presence of Gag significantly increased the time that RNAs stayed near the plasma membrane: most of the RNAs were still detected after 30 min. We then quantified the proportion of HIV-1 RNAs near the plasma membrane that were packaged into assembling viral complexes. By tagging Gag with blue fluorescent protein, we observed that only a portion, ∼13 to 34%, of the HIV-1 RNAs that reached the membrane were recruited into assembling particles in an hour, and the frequency of HIV-1 RNA packaging varied with the Gag expression level. Our studies reveal the HIV-1 RNA dynamics on the plasma membrane and the efficiency of RNA recruitment and provide insights into the events leading to the generation of infectious HIV-1 virions. IMPORTANCE: Nascent HIV-1 particles assemble on plasma membranes. During the assembly process, HIV-1 RNA genomes must be encapsidated into viral complexes to generate infectious particles. To gain insights into the RNA packaging and virus assembly mechanisms, we labeled and monitored the HIV-1 RNA signals near the plasma membrane. Our results showed that most of the HIV-1 RNAs stayed near the plasma membrane for only a few minutes in the absence of Gag, whereas most HIV-1 RNAs stayed at the plasma membrane for 15 to 60 min in the presence of Gag. Our results also demonstrated that only a small proportion of the HIV-1 RNAs, approximately 1/10 to 1/3 of the RNAs that reached the plasma membrane, was incorporated into viral protein complexes. These studies determined the dynamics of HIV-1 RNA on the plasma membrane and obtained temporal information on RNA-Gag interactions that lead to RNA encapsidation.