Architecture of RabL2-associated complexes at the ciliary base: A structural modeling perspective: Deciphering the structural organization of ciliary RabL2 complexes.

Cilia are slender, micrometer-long organelles present on the surface of eukaryotic cells. They function in signaling and locomotion and are constructed by intraflagellar transport (IFT). The assembly of IFT complexes into so-called IFT trains to initiate ciliary entry at the base of the cilium remains a matter of debate. Here, we use structural modeling to provide an architectural framework for how RabL2 is anchored at the ciliary base via CEP19 before being handed over to IFT trains for ciliary entry. Our models suggest that the N-terminal domain of CEP43 forms a homo-dimer to anchor at the subdistal appendages of cilia through a direct interaction with CEP350. A long linker region separates the N-terminal domain of CEP43 from the C-terminal domain, which captures CEP19 above the subdistal appendages and close to the distal appendages. Furthermore, we present a structural model for how RabL2-CEP19 associates with the IFT-B complex, providing insight into how RabL2 is handed over from CEP19 to the IFT complex. Interestingly, RabL2 association with the IFT-B complex appears to induce a significant conformational change in the IFT complex via a kink in the coiled-coils of the IFT81/74 proteins, which may prime the IFT machinery for entry into cilia.

The IFT81-IFT74 complex acts as an unconventional RabL2 GTPase-activating protein during intraflagellar transport.

Cilia are important cellular organelles for signaling and motility and are constructed via intraflagellar transport (IFT). RabL2 is a small GTPase that localizes to the basal body of cilia via an interaction with the centriolar protein CEP19 before downstream association with the IFT machinery, which is followed by initiation of IFT. We reconstituted and purified RabL2 with CEP19 or IFT proteins to show that a reconstituted pentameric IFT complex containing IFT81/74 enhances the GTP hydrolysis rate of RabL2. The binding site on IFT81/74 that promotes GTP hydrolysis in RabL2 was mapped to a 70-amino-acid-long coiled-coil region of IFT81/74. We present structural models for RabL2-containing IFT complexes that we validate in vitro and in cellulo and demonstrate that Chlamydomonas IFT81/74 enhances GTP hydrolysis of human RabL2, suggesting an ancient evolutionarily conserved activity. Our results provide an architectural understanding of how RabL2 is incorporated into the IFT complex and a molecular rationale for why RabL2 dissociates from anterograde IFT trains soon after departure from the ciliary base.

RABL2 promotes the outward transition zone passage of signaling proteins in cilia via ARL3.

Certain transmembrane and membrane-tethered signaling proteins export from cilia as BBSome cargoes via the outward BBSome transition zone (TZ) diffusion pathway, indispensable for maintaining their ciliary dynamics to enable cells to sense and transduce extracellular stimuli inside the cell. Murine Rab-like 2 (Rabl2) GTPase resembles Chlamydomonas Arf-like 3 (ARL3) GTPase in promoting outward TZ passage of the signaling protein cargo-laden BBSome. During this process, ARL3 binds to and recruits the retrograde IFT train-dissociated BBSome as its effector to diffuse through the TZ for ciliary retrieval, while how RABL2 and ARL3 cross talk in this event remains uncertain. Here, we report that Chlamydomonas RABL2 in a GTP-bound form (RABL2GTP) cycles through cilia via IFT as an IFT-B1 cargo, dissociates from retrograde IFT trains at a ciliary region right above the TZ, and converts to RABL2GDP for activating ARL3GDP as an ARL3 guanine nucleotide exchange factor. This confers ARL3GTP to detach from the ciliary membrane and become available for binding and recruiting the phospholipase D (PLD)-laden BBSome, autonomous of retrograde IFT association, to diffuse through the TZ for ciliary retrieval. Afterward, RABL2GDP exits cilia by being bound to the ARL3GTP/BBSome entity as a BBSome cargo. Our data identify ciliary signaling proteins exported from cilia via the RABL2-ARL3 cascade-mediated outward BBSome TZ diffusion pathway. According to this model, hedgehog signaling defect-induced Bardet-Biedl syndrome caused by RABL2 mutations in humans could be well explained in a mutation-specific manner, providing us with a mechanistic understanding behind the outward BBSome TZ passage required for proper ciliary signaling.

SNHG15 aids SARS-CoV-2 entry via RABL2A.

Angiotensin-converting enzyme 2 (ACE2) and several proteins have been identified as entry factors for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, whether long noncoding RNAs are involved in SARS-CoV-2 entry remains unknown. In this study, we investigated the role of small nucleolar RNA host gene 15 (SNHG15) in SARS-CoV-2 entry using a SARS-CoV-2 spike pseudotyped lentivirus with a luciferase reporter. Overexpression of SNHG15 promoted but SNHG15 knockdown limited SARS-CoV-2 entry in a dose- and time-dependent manner. SNHG15 interacted with Rab-like protein 2A (RABL2A). Overexpression and knockdown of RABL2A produced similar effects on SARS-CoV-2 entry as those of SNHG15. Furthermore, RABL2A knockdown abolished the SNHG15-mediated increase in SARS-CoV-2 entry. In conclusion, SNHG15 is a critical regulatory factor that aids SARS-CoV-2 entry through RABL2A.

Rab-like small GTPases in the regulation of ciliary Bardet-Biedl syndrome (BBS) complex transport.

Primary cilia, microtubule-based hair-like structures protruding from most cells, contain membranes enriched in signaling molecules and function as sensory and regulatory organelles critical for development and tissue homeostasis. Intraflagellar transport (IFT), cilia-specific bidirectional transport, is required for the assembly, maintenance, and function of cilia. BBSome, the coat complex, acts as the adaptor between the IFT complex and membrane proteins and is therefore essential for establishing the specific compartmentalization of signaling molecules in the cilia. Recent findings have revealed that three ciliary Rab-like small GTPases, IFT27, IFT22, and Rabl2, play critical regulatory roles in ciliary BBSome transport. In this review, we provide an overview of these three Rab-like small GTPases and their relationship with BBSome.

RABL2A-CCDC34 Axis Promotes Sorafenib Resistance in Hepatocellular Carcinoma.

In this study, we examined the regulatory role of CCDC34 in the sorafenib sensitivity of hepatocellular carcinoma (HCC) and its functional partners. Wide-type Huh7 and Hep3B and induced sorafenib-resistant (SR) Huh7/SR and Hep3B/SR cells were used as in vitro cell models. Immunofluorescent staining and coimmunoprecipitation were performed to check protein-protein interaction. Cell Counting Kit-8 (CCK-8), terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL), PI/Annexin V staining, and western blot analysis were performed to assess cell response to sorafenib. The results showed that CCDC34 upregulation in HCC was associated with poor survival. Huh7/SR and Hep3B/SR cells had significantly higher CCDC34 expression than the parental cell lines. RABL2A expression was significantly upregulated in SR HCC cells and interacted with CCDC34 in its GTP-bound state in Huh7/SR and Hep3B/SR cells. RABL2A depletion sensitized Huh7/SR and Hep3B/SR cells to sorafenib. RABL2A Q80L mutant (GTP-bound state locked), but not S35N mutant (GDP-bound state locked) overexpression increased sorafenib IC50 of Huh7 and Hep3B cells. CCDC34 depletion nearly abrogated the protective effects of RABL2A Q80L overexpression both in vitro and in vivo. RABL2A Q80L overexpression significantly increased the expression of p-p38 and p-JNK, the effects of which were significantly attenuated by CCDC34 depletion. In summary, we infer that the RABL2A-CCDC34 axis plays an important role in mediating p38/MAPK and JNK/MAPK signaling, thereby contributing to acquired sorafenib resistance in HCC.

HTR6 and SSTR3 targeting to primary cilia.

Primary cilia are hair-like projections of the cell membrane supported by an inner microtubule scaffold, the axoneme, which polymerizes out of a membrane-docked centriole at the ciliary base. By working as specialized signaling compartments, primary cilia provide an optimal environment for many G protein-coupled receptors (GPCRs) and their effectors to efficiently transmit their signals to the rest of the cell. For this to occur, however, all necessary receptors and signal transducers must first accumulate at the ciliary membrane. Serotonin receptor 6 (HTR6) and Somatostatin receptor 3 (SSTR3) are two GPCRs whose signaling in brain neuronal cilia affects cognition and is implicated in psychiatric, neurodegenerative, and oncologic diseases. Over a decade ago, the third intracellular loops (IC3s) of HTR6 and SSTR3 were shown to contain ciliary localization sequences (CLSs) that, when grafted onto non-ciliary GPCRs, could drive their ciliary accumulation. Nevertheless, these CLSs were dispensable for ciliary targeting of HTR6 and SSTR3, suggesting the presence of additional CLSs, which we have recently identified in their C-terminal tails. Herein, we review the discovery and mapping of these CLSs, as well as the state of the art regarding how these CLSs may orchestrate ciliary accumulation of these GPCRs by controlling when and where they interact with the ciliary entry and exit machinery via adaptors such as TULP3, RABL2 and the BBSome.

RABL2 positively controls localization of GPCRs in mammalian primary cilia.

The primary cilium, a solitary protrusion from most mammalian cells, functions as a cell sensor by receiving extracellular signals through receptors and channels accumulated in the organelle. Certain G-protein-coupled receptors (GPCRs) specifically localize to the membrane compartment of primary cilia. To gain insight into the mechanisms that regulate ciliary GPCR sorting, we investigated the atypical small GTPase RAB-like 2 (RABL2; herein referring to the near-identical human paralogs RABL2A and RABL2B). RABL2 recruitment to the mother centriole is dependent on the distal appendage proteins CEP164 and CEP83. We found that silencing of RABL2 causes mis-targeting of ciliary GPCRs, GPR161 and HTR6, whereas overexpression of RABL2 resulted in accumulation of these receptors in the organelle. Ablation of CEP19 and the intraflagellar transport B (IFT-B) complex, which interact with RABL2, also leads to mis-localization of GPR161. RABL2 controls localization of GPR161 independently of TULP3, which promotes entry of ciliary GPCRs. We further demonstrate that RABL2 physically associates with ciliary GPCRs. Taken together, these studies suggest that RABL2 plays an important role in trafficking of ciliary GPCRs at the ciliary base in mammalian cells.

The CEP19-RABL2 GTPase Complex Binds IFT-B to Initiate Intraflagellar Transport at the Ciliary Base.

Highly conserved intraflagellar transport (IFT) protein complexes direct both the assembly of primary cilia and the trafficking of signaling molecules. IFT complexes initially accumulate at the base of the cilium and periodically enter the cilium, suggesting an as-yet-unidentified mechanism that triggers ciliary entry of IFT complexes. Using affinity-purification and mass spectrometry of interactors of the centrosomal and ciliopathy protein, CEP19, we identify CEP350, FOP, and the RABL2B GTPase as proteins organizing the first known mechanism directing ciliary entry of IFT complexes. We discover that CEP19 is recruited to the ciliary base by the centriolar CEP350/FOP complex and then specifically captures GTP-bound RABL2B, which is activated via its intrinsic nucleotide exchange. Activated RABL2B then captures and releases its single effector, the intraflagellar transport B holocomplex, from the large pool of pre-docked IFT-B complexes, and thus initiates ciliary entry of IFT.

The impact of RABL2B gene (rs144944885) on human male infertility in patients with oligoasthenoteratozoospermia and immotile short tail sperm defects.

PURPOSE: Male infertility is a multifactorial disorder with impressively genetic basis; besides, sperm abnormalities are the cause of numerous cases of male infertility. In this study, we evaluated the genetic variants in exons 4 and 5 and their intron-exon boundaries in RABL2B gene in infertile men with oligoasthenoteratozoospermia (OAT) and immotile short tail sperm (ISTS) defects to define if there is any association between these variants and human male infertility. METHODS: To this purpose, DNA was extracted from peripheral blood and after PCR reaction and sequencing, the results of sequenced segments were analyzed. In the present study, 30 infertile men with ISTS defect and 30 oligoasthenoteratozoospermic infertile men were recruited. All men were of Iranian origin and it took 3 years to collect patient's samples with ISTS defect. RESULTS: As a result, the 50776482 delC intronic variant (rs144944885) was identified in five patients with oligoasthenoteratozoospermia defect and one patient with ISTS defect in heterozygote form. This variant was not identified in controls. The allelic frequency of the 50776482 delC variant was significantly statistically higher in oligoasthenoteratozoospermic infertile men (p < 0.05). Bioinformatics studies suggested that the 50776482 delC allele would modify the splicing of RABL2B pre-mRNA. In addition, we identified a new genetic variant in RABL2B gene. CONCLUSIONS: According to the present study, 50776482 delC allele in the RABL2B gene could be a risk factor in Iranian infertile men with oligoasthenoteratozoospermia defect, but more genetic studies are required to understand the accurate role of this variant in pathogenesis of human male infertility.

Genetic variants in the RABL2A gene in fertile and oligoasthenospermic infertile men.

OBJECTIVE: To define RABL2A localization in human sperm and to assess any potential association between RABL2A variants and male infertility associated with oligoasthenospermia. DESIGN: Genetic association study. SETTING: Public university. PATIENT(S): Australian men: 110 oligoasthenospermic infertile and 105 proven fertile. INTERVENTION(S): Human semen samples processed by immunostaining with high-throughput-sequencing platform to screen the entire protein-coding and flanking exon/intron regions of the RABL2A gene. MAIN OUTCOME MEASURE(S): Presence of RABL2A in human sperm and frequencies of RABL2A genetic variants in fertile and infertile men. RESULT(S): RABL2A localization in sperm was highly conserved between mouse and human, being localized to the tail. Direct DNA sequencing revealed 23 RABL2A genetic variants, including 16 intronic, 6 untranslated region (UTR), and one exonic missense variants. Of these, eight variants have not been previously reported. Although the majority of these variants showed no significant association with fertility status, allelic frequency of the intronic variant 114391996 delC was significantly increased in oligoasthenospermic men. Bioinformatics analysis suggested that the 114391996 delC allele would alter the splicing of RABL2A pre-mRNA. CONCLUSION(S): Our data suggest the 114391996 delC allele in the RABL2A gene may act as a risk factor for oligoasthenospermic infertility in Australian men.