Chlamydomonas LZTFL1 mediates phototaxis via controlling BBSome recruitment to the basal body and its reassembly at the ciliary tip

Abstract

Many G protein–coupled receptors and other signaling proteins localize to the ciliary membrane for regulating diverse cellular processes. The BBSome composed of multiple Bardet–Biedl syndrome (BBS) proteins is an intraflagellar transport (IFT) cargo adaptor essential for sorting signaling proteins in and/or out of cilia via IFT. Leucine zipper transcription factor-like 1 (LZTFL1) protein mediates ciliary signaling by controlling BBSome ciliary content, reflecting how LZTFL1 mutations could cause BBS. However, the mechanistic mechanism underlying this process remains elusive thus far. Here, we show that LZTFL1 maintains BBSome ciliary dynamics by finely controlling BBSome recruitment to the basal body and its reassembly at the ciliary tip simultaneously in Chlamydomonas reinhardtii. LZTFL1 directs BBSome recruitment to the basal body via promoting basal body targeting of Arf-like 6 GTPase BBS3, thus deciding the BBSome amount available for loading onto anterograde IFT trains for entering cilia. Meanwhile, LZTFL1 stabilizes the IFT25/27 component of the IFT-B1 subcomplex in the cell body so as to control its presence and amount at the basal body for entering cilia. Since IFT25/27 promotes BBSome reassembly at the ciliary tip for loading onto retrograde IFT trains, LZTFL1 thus also directs BBSome removal out of cilia. Therefore, LZTFL1 dysfunction deprives the BBSome of ciliary presence and generates Chlamydomonas cells defective in phototaxis. In summary, our data propose that LZTFL1 maintains BBSome dynamics in cilia by such a dual-mode system, providing insights into how LZTFL1 mediates ciliary signaling through maintaining BBSome ciliary dynamics and the pathogenetic mechanism of the BBS disorder as well.

RABL2 Regulates Ciliation via Controlling IFT-B1 Basal Body Recruitment and ARL3-mediated BBSome Ciliary Retrieval

“…Rabbit-raised polyclonal antibodies against IFT38, IFT43, IFT46, IFT57, IFT70, IFT139, BBS1, BBS4, BBS5, BBS7, BBS8, CEP290, ARL3, and PLD have been reported previously (39–41, 53). Antibodies against YFP (mAbs 7.1 and 13.1, Roche), α-tubulin (mAb B512, Sigma-Aldrich) and acetylated-α-tubulin (mAb 6-11B-1, Sigma-Aldrich) were commercially bought.…”

“…It has been shown that IFT-A, IFT-B1, and IFT-B2 subcomplexes components of IFT trains become separated from one another in HMEKN buffer (Fig. 2 A ) (39). In the same buffer, RABL2 Q83L -HA-YFP immunoprecipitated IFT-B1 (represented by IFT46 and IFT70) but not IFT-A (represented by IFT43 and IFT139) and IFT-B2 (represented by IFT38 and IFT57) (Fig.…”

Chlamydomonas RABL4/IFT27 Mediates Phototaxis via Promoting BBSome-dependent Ciliary Export of Phospholipase D

“…As an IFT cargo adaptor, the BBSome delivers signaling proteins for either entering or exiting cilia via IFT (Wingfield et al ., 2018). BBSome malfunction thus causes loss or abnormal retention of signaling proteins in the ciliary membrane (Chiang et al , 2004; Lechtreck et al ., 2009a; Loktev et al , 2008; Nachury et al ., 2007; Scheidecker et al , 2014; Sun et al , 2021; Xue et al , 2020; Zhang et al , 2011). These defects impair signaling protein dynamics in cilia and eventually leads to Bardet-Biedl syndrome (BBS) in humans (Fliegauf et al , 2007) and phototactic defects in C. reinhardtii (Lechtreck et al ., 2009a; Liu & Lechtreck, 2018).…”

“…Thus far, small GTPases of various subfamilies have been found to play roles in regulating BBSome remodeling and BBSome-cargo interaction in cilia. For instance, Chlamydomonas Rab-like 4 (RABL4) GTPase IFT27 enables BBSome reassembly at the ciliary tip and Arf-like 6 (ARL6) GTPase BBS3 instead promotes BBSome interaction with phospholipase D (PLD) for its export form cilia (Liu et al , 2021b; Sun et al ., 2021). As with different mechanisms for controlling BBSome availability for signaling protein loading for their export form cilia via IFT, IFT27 and BBS3 dysfunction both can disrupt BBSome ciliary signaling, eventually causing phototactic defects in C. reinhardtii (Liu et al ., 2021b; Sun et al ., 2021).…”

“…Unlike other conventional IFT-B1 subunits essential for IFT-B assembly, IFT initiation, and ciliation (Fan et al , 2010; Hou et al , 2007), IFT25 and IFT27, by binding to form a heterodimer IFT25/27 (Wang et al , 2009), are dispensable for IFT-B assembly nor for ciliation in rodent somatic cells and green algae (Dong et al , 2017b; Eguether et al , 2014; Keady et al , 2012; Liew et al , 2014; Sun et al ., 2021). In rodent cells, knockout of IFT27 or IFT25 causes abnormal ciliary retention of BBSomes as well as signaling proteins like patched-1 (PTCH1), smoothened (Smo), Gli2, and GPR161 due to the defect in BBSome ciliary removal (Eguether et al ., 2014; Keady et al ., 2012; Liew et al ., 2014).…”