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The pandemic of coronavirus disease 2019 (COVID-19) caused by the SARS-coronavirus 2(SARS-CoV-2) virus has become the greatest global public health crisis in recent years,and the COVID-19 epidemic is still continuing. However, due to the lack of effectivetherapeutic drugs, the treatment of corona viruses is facing huge challenges. In thiscontext, countries with a tradition of using herbal medicine such as China have beenwidely using herbal medicine for prevention and nonspecific treatment of corona virusesand achieved good responses. In this review, we will introduce the application of herbalmedicine in the treatment of corona virus patients in China and other countries, andreview the progress of related molecular mechanisms and antiviral activity ingredients ofherbal medicine, in order to provide a reference for herbal medicine in the treatment ofcorona viruses. We found that herbal medicines are used in the prevention and fightagainst COVID-19 in countries on all continents. In China, herbal medicine has beenreported to relieve some of the clinical symptoms of mild patients and shorten the length of hospital stay. However, as most herbal medicines for the clinical treatment of COVID-19still lack rigorous clinical trials, the clinical and economic value of herbal medicines in theprevention and treatment of COVID-19 has not been fully evaluated. Future work basedon large-scale randomized, double-blind clinical trials to evaluate herbal medicines andtheir active ingredients in the treatment of new COVID-19 will be very meaningful.

Certain ciliary transmembrane and membrane-tethered signaling proteins migrate from the ciliary tip to base via retrograde intraflagellar transport (IFT), essential for maintaining their ciliary dynamics to enable cells to sense and transduce extracellular stimuli inside the cell. During this process, the BBSome functions as an adaptor between retrograde IFT trains and these signaling protein cargoes. The Arf-like 13 (ARL13) small GTPase resembles ARL6/BBS3 in facilitating these signaling cargoes to couple with the BBSome at the ciliary tip prior to loading onto retrograde IFT trains for transporting towards the ciliary base, while the molecular basis for how this intricate coupling event happens remains elusive. Here, we report thatChlamydomonasARL13 only in a GTP-bound form (ARL13GTP) anchors to the membrane for diffusing into cilia. Upon entering cilia, ARL13 undergoes GTPase cycle for shuttling between the ciliary membrane (ARL13GTP) and matrix (ARL13GDP). To achieve this goal, the ciliary membrane-anchored BBS3GTPbinds and activates the ciliary matrix-residing ARL13GDPas an ARL13 guanine nucleotide exchange factor. At the ciliary tip, ARL13GTPbinds and recruits the ciliary matrix-residing and post-remodeled BBSome as an ARL13 effector to anchor to the ciliary membrane. This makes the BBSome spatiotemporally become available for the ciliary membrane-tethered phospholipase D (PLD) to couple with. Afterward, ARL13GTPhydrolyzes GTP for releasing the PLD-laden BBSome to load onto retrograde IFT trains. According to this model, hedgehog signaling defects associated withARL13bandBBS3mutations in humans could be satisfactorily explained, providing us a mechanistic understanding behind BBSome-cargo coupling required for proper ciliary signaling.Significance statementCertain signaling proteins export in a BBSome-dependent manner from cilia via retrograde IFT. As IFT cargoes, their coupling with the BBSome is mediated by ARL13 and BBS3. Here, we report that ARL13 in a GTP-bound state binds the membrane for diffusing into cilia followed by running GTPase cycle with BBS3 acting as an ARL13-specific guanine nucleotide exchange factor. ARL13GTPrecruits the IFT-shed BBSome as its effector to anchor to the membrane of the ciliary tip, making it spatiotemporally available for the ciliary membrane-tethered PLD to bind. Afterward, ARL13 GTP hydrolysis releases the PLD-laden BBSome for retrograde IFT train loading. ARL13 and BBS3 facilitate the cargo-BBSome coupling in such a way providing a regulatory mechanism for signaling protein export from cilia.

Highly conserved intraflagellar transport (IFT) trains and certain small GTPases coordinate to direct ciliation and to maintain the ciliary dynamics of signaling molecules via the IFT cargo adaptor BBSome. Unlike murine Rab-like 2 (RABL2) GTPase that enters cilia to drive outward transition zone (TZ) passage of the BBSome, human orthologue fails to enter cilia but resides at the ciliary base, indispensable for ciliation. However, mechanisms underlying how RABL2 regulates ciliation and BBSome barrier passage remain elusive. Here, we show that Chlamydomonas RABL2 regulates basal body targeting of the IFT-B1 subcomplex component of IFT trains as a RABL2-specific effector, mediating ciliation via controlling IFT-B1 basal body amount available for assembling anterograde IFT trains. RABL2GTP binds IFT-B1 to perform IFT; sheds from retrograde IFT trains at the proximal ciliary region right above the TZ; and converts to RABL2GDP rapidly. Next, RABL2GDP activates the ciliary membrane anchored Arf-like 3 (ARL3) GTPase (ARL3GDP) as a ARL3-specific guanine nucleotide exchange factor. Upon detaching from the ciliary membrane, the active ARL3GTP recruits its BBSome effector, autonomous of retrograde IFT train association, to move cross the TZ for ciliary retrieval. This ensures proper BBSome ciliary turnover for maintaining phototactic response of Chlamydomonas cells. For finishing RABL2 ciliary cycle, RABL2GDP passes the TZ for ciliary retrieval by loading onto the ARL3GTP/BBSome as a BBSome cargo. Our data thus propose that RABL2 mediates ciliation and BBSome ciliary retrieval simultaneously but via distinct molecular pathways.Significance statementIntraflagellar transport (IFT) and its cargo adaptor BBSome are indispensable for ciliation and ciliary singling. Rab-like 2 (RABL2) GTPase mediates ciliation and outward transition zone (TZ) passage of BBSomes with mechanisms yet to be determined. Here, we report that RABL2 decides ciliation by controlling the basal body amount of its effector IFT-B1 available for the assembly of anterograde IFT trains. RABL2GTP cycles through cilia as an IFT-B1 cargo; sheds from IFT at the ciliary base; and undergoes nucleotide exchange for activating ARL3 as an ARL3-specfic guanine nucleotide exchange factor. ARLGTP recruits IFT-shed BBSomes to pass the TZ for ciliary retrieval. RABL2GDP exists cilia via ARL3GTP/BBSome as a BBSome cargo. Therefore, RABL2 functions both outside and inside cilia for initiating IFT and BBSome ciliary retrieval, respectively.

Phospholipase D (PLD) interacts with the BBSome for loading onto retrograde intraflagellar transport (IFT) trains to exit cilia in Chlamydomonas reinhardtii. PLD ciliary retention and depletion of Rab-like 4 (RABL4) GTPase IFT27 cause the same non-phototactic phenotype but not impair IFT and ciliation. Here, we show that the IFT-B1 subunit IFT27 binds its partner IFT25 to form the heterodimeric IFT25/27 in an IFT27 nucleotide state-independent manner. IFT25/27, IFT-A, and IFT-B are irrelevant for maintaining the stability of one another. GTP-loading onto IFT27 enhances the IFT25/27 affinity for IFT-B1 in cytoplasm, while GDP-loaded IFT27 does not prevent IFT25/27 from entering and cycling through cilia by integrating into IFT-B1. Upon at the ciliary tip, IFT25/27 cycles on and off IFT-B1 and this process is irrelevant with the nucleotide state of IFT27. During BBSome remodeling at the ciliary tip, IFT25/27 promotes BBSome reassembly independent of IFT27 nucleotide state, making post-remodeled BBSomes available for PLD to interact with. Therefore, IFT25/27 facilitates BBSome-dependent PLD export from cilia via controlling availability of intact BBSomes at the ciliary tip, providing a regulatory mechanism for IFT27 to mediate phototaxis in C. reinhardtii.