Description

We report the identification of C. elegans F42G8.19 as the ortholog of human CEP41. This conclusion was based on database searches, reciprocal BLAST analysis, and cilia-specific localization. To look for the orthologous gene of human CEP41 in C. elegans, we used three databases: the model organism Alliance of Genome Resources (https://www.alliancegenome.org/, Release 3.2.0), OrthoList 2 (http://ortholist.shaye-lab.org/, Release 2017), and ConVarT (https://convart.org/, Release 2020) (Kim et. al., 2018; Pir et. al., 2021). On the Alliance of Genome Resources and ConVarT, we discovered that C. elegans F42G8.19 is the ortholog of human CEP41. To validate this conclusion, we ran a manual reciprocal BLAST analysis (the Reciprocal Best Hits BLAST (RBHB)). The protein-protein BLAST (BLASTp) search of human CEP41 protein sequence (NP 061188.1) revealed F42G8.19 as the top C. elegans hit (Altschul et. al., 1990). In the following step, the protein sequence from the best hit C. elegans F42G8.19 (NP_001294206.1) was compared to human proteins, with human CEP41 emerging as the best match. C. elegans F42G8.19 encodes a 182-amino-acid protein that is shorter than the 373-amino-acid human CEP41 protein (Figure 1A). We subsequently performed the amino acid alignments of human CEP41 (169–373 amino acid) and C. elegans F42G8.19 (1–182 amino acid), which revealed over 24 % identity to each other (Figure 1A). We eventually determined whether C. elegans F42G8.19 has a rhodanese domain as human CEP41 (169-266 aa), and our query showed that C. elegans F42G8.19 has a rhodanese domain (30-121 aa) (Figure 1A) (Lee et. al., 2012). Taken together, our analysis revealed that C. elegans F42G8.19 is orthologous to human CEP41, and C. elegans F42G8.19 was therefore assigned CEPH-41 (CEntrosomal Protein Homolog 41).

We generated a transgenic strain bearing the ceph-41 promoter (1000 bp) together with a full-length of ceph-41 (1173 bp) tagged with wrmScarlet, and determined the subcellular localization of C. elegans CEPH-41 (El Mouridi et. al., 2017). Our super-resolution confocal laser scanning microscope analysis revealed that CEPH-41::wrmScarlet is exclusively expressed in the ciliated sensory neurons and is localized to cilia in the head (amphid) and tail (phasmid), suggesting cilia localization of CEPH-41 is evolutionary conserved in both humans and C. elegans (Figure 1B) (Lee et. al., 2012). Additionally, our co-localization analysis showed that CEPH-41 is present in the transition zone (TZ) and the proximal cilia region known as the middle segment (the microtubule doublet containing segment) in C. elegans, but not in the distal cilia region (only A-tubule containing segment) (Figure 1B and C). To independently validate the exclusion of CEPH-41 from the distal segment in the ciliated sensory neuron, we generated a new fluorescence marker for CEPH-41, tagging GFP to the C-terminus of C. elegans CEPH-41. A similar distal segment exclusion was detected when we examined the localization of CEPH-41::GFP (Figure 1D and E). It would be interesting to check whether CEP41 would display a similar localization pattern in mammals and other organisms. CEPH-41 joins the club of middle segment localizing ciliary proteins, as another JBTS ARL-13/ARL13B protein is primarily enriched in the proximal cilia zone in some cells in both C. elegans (in amphid and phasmid cilia but not in AWB cilia) and mammals (MDCKII cilia, mouse oviduct, and tracheal tissue) (Cevik et. al., 2010; Cevik et. al., 2013; Li et. al., 2010). Furthermore, CEP41 and ARL13B are both microtubule/tubulin-binding proteins that control ciliary tubulin glutamylation (Lee et. al., 2012; He et. al., 2018; Revenkova et. al., 2018; Gache et. al., 2010), but it is currently unknown how these two JBTS proteins are related. Finally, unlike ARL-13/ARL13B, which translocates in both directions along cilia, our time-lapse video analysis reveals that CEPH-41 does not appear to have IFT-like motility within cilia (Cevik et. al., 2013).

Our current work reveals that CEPH-41 displays the mutually exclusive expression pattern in the ciliated sensory neurons, and the previous work already established that DAF-19, a sole Regulatory Factor X (RFX) transcription factor, is responsible for the mechanisms underlying ciliated-cell-specific expression of ciliary genes in C. elegans. The binding site for DAF-19 is the X-box sequence (13-15 base pair sequence) that is a regulatory motif in the promoter of cilia-specific genes in C. elegans (Blacque et. al., 2005; Chen et. al., 2006; Efimenko et. al., 2005; Phirke et. al., 2011; Swoboda et. al., 2000). We next determined whether ceph-41 has an X-box motif in its promoter. Scanning of the promoter of ceph-41 (1000 bp upstream of the start codon) for the presence of an X-box motif revealed a putative X-box motif within the promoter of ceph-41 (Figure 1F). We predict that ciliated-cell-specific expression of ceph-41 is likely driven by DAF-19. We, therefore, crossed the transgenic strain expressing CEPH-41::GFP under its promoter into daf-19 mutants and found that the ciliated-cell-specific expression pattern of CEPH-41 is dramatically reduced in daf-19 mutants, suggesting the involvement of DAF-19 in the regulation of ceph-41 expression (Figure 1G, H and I)

In conclusion, we present CEPH-41 as a ciliary protein that is absent from the distal segment of the amphid and phasmid cilia in C. elegans, and our future efforts are directed toward investigating the function of this middle segment protein in cilia in C. elegans.