Codanin-1 interacts with C15Orf41 protein.
To identify Codanin-1 interacting proteins, we immunoprecipitated either the endogenous Codanin-1 or overexpressed Flag-tagged Codanin-1 from human HeLa cells, and identified the interacting proteins by MS/MS analysis. One of the proteins precipitated using both approaches was C15Orf41. C15Orf41 was reported earlier as an additional etiological agent for CDA I [14]. However, physical interaction between Codanin-1 and C15Orf41 has not been demonstrated. Reciprocal co-immunoprecipitations using both tagged-Codanin-1and tagged C15Orf41 confirmed the association between C15Orf41 and Codanin-1, while the prominent protein, actin, and an unrelated protein (Nek7 kinase) did not co-precipitated (Fig.1A, B).In order to map the Codanin-1’s region responsible for the interaction with C15Orf41, sub-fragments of Codanin-1 were created (Fig. 2A). Subsequent co-immunoprecipitation assays confirmed previous observations that the histone H3/H4 chaperone Anti-Silencing Function 1 (ASF1) binds to the N-terminus of Codanin-1 [15](Fig. 2B). In contrast to ASF1, C15Orf41 interacted with the C-terminal 222 amino acids of Codanin-1 (a.a.1,005 to 1,227) (Fig. 2B). Indeed, Codanin-1 protein lacking the last 224 amino acids did not precipitate C15Orf4 (Fig. 2B). Even though the mutation in Codanin-1 protein found in the Israeli Bedouin families (R1042W) is located within this fragment, this mutation did not affect the binding to C15Orf4 (Fig. 2B)
Codanin-1 influences C15Orf41 levels and localization
Interestingly, we noticed that upon co-transfection of C15orf41 with Codanin-1 there was a sharp elevation in the levels of C15Orf41 protein (4.7 ± 2.0 times) (Fig. 1A, B; Fig. 2B). This rise in the levels of C15Orf41 protein was codanin-1 dose-dependent (Fig.2C). Thus in the following experiments in which transfection of C15Orf41 alone was compared to co-transfection of C15Orf41 and Codanin-1, the levels of C15Orf41 DNA were 2-3 times higher (than the levels in the co-transfection) in order to get comparable levels of C15Orf41 protein (Fig. 3, 4) Mapping of the Codanin-1 domain responsible for this elevation suggested that it is mainly dependent of the C-terminus of Codanin-1, and the construct lacking the last 224 a.a. has much lower ability to enhance the levels of C15Orf41 levels (Fig. 2B). To discount the possibility that the enhancement is the result of co-transfection of the two constructs, we established a HeLa Tet-On cell line in which codanin-1 is expressed under the tet-responsive element (TRE), and HeLa Tet-Off cell line in which C15Orf41 is expressed under the TRE. As expected, treatment with doxycycline or its abolishment, enhanced the levels of Codanin-1 or C15Orf41, respectively. In both cases, higher levels of Codanin-1 enhanced the levels of C15Orf41, revealing that co-transfection is not needed for C15Orf41 protein elevation (Fig. S1).
We next asked whether the higher levels of C15Orf41 in the presence of Codanin-1 are the result of an effect on the half-life of the C15Orf41 protein. To this end, we treated the transfected cells with the translation inhibitor, cycloheximide (CHX). HeLa cells were transfected with C15Orf41 alone or were co-transfected with C15Orf41 and Codanin-1, followed by incubation with CHX. As can be seen in figure 3, C15Orf41 is unstable protein and following 3 hours of incubation with cycloheximide it almost completely disappeared. However, co-transfection of Codanin-1 resulted in extension of the half-life of C15Orf41. As a control, the half-life of P53 protein was not influenced by co-expression of Codanin-1 (Fig. 3).
One option for Codanin-1’s influence on the levels of C15Orf41 protein is by inhibiting its degradation by the proteasome. MG132 is a potent, reversible, and cell-permeable proteasome inhibitor. Thus, if Codanin-1 influences C15Orf41 protein’s levels by inhibition of its degradation by the proteasome, we would expect that in the presence of MG132, Codanin-1 will not have a significant effect on C15Orf41 levels. HeLa cells were transfected with C15Orf41, with or without Codanin-1, and were incubated with MG132 or with its solvent, DMSO. Importantly, even in the presence of MG132, high levels of Codanin-1 correlated with higher levels of C15Orf41 (Supplementary Fig. S2). This suggests that at least part of the Codanin-1 effect on C15Orf41 levels is not due to escape from the proteasome. Quite surprisingly, a decrease in the levels of both Codanin-1 and C15Orf41 were seen following the treatment with MG132 (Supplementary Fig. S2). The lower levels of C15Orf41 in the MG132 treatment could be due to the lower levels of Codanin-1. However, the lower levels of Codanin-1 are harder to explain, and are in contrast with most proteins, whose levels rise following the inhibition of the proteasome. It is thus possible that Codanin-1 is normally degraded by a pathway that is downregulated by the proteasome, and thus MG132 relieves Codanin-1’s degradation.
Codanin-1 is primarily cytoplasmic (Fig. 4A). The sequence of C15Orf41 suggests that it might serve as an endonuclease, related to restriction endonucleases. As could be surmised from this assumption, overexpressed C15Orf41 tagged with HA was observed almost exclusively in the nucleus. (Fig. 4B). However, when C15Orf41 was co-transfected with codanin-1, C15Orf41 was shifted to the cytoplasm (Fig. 4C). The ratio of the levels of C15Orf41 in the cytoplasm compared to its levels in the nucleus was 0.75 ± 0.4, while following co-expression of Codanin-1 the ratio was 5.7 ± 2.1 (p-value < .00001). Fractionation of HeLa cells extracts transfected with either C15Orf41 alone or C15Orf41 and Codanin-1 confirmed the enrichment of C15Orf41 in the cytoplasm following co-transfection with Codanin-1 (Fig. 4E). However, presumably due to instability of the nuclear C15Orf41 protein, only low levels of the protein were detected in nucleus (Fig. 4E).
To corroborate whether C15Orf41 and Codanin-1 binding is essential for the change in C15Orf41 localization, fragments of Codanin-1 were co-expressed with HA-C15Orf41, and C15Orf41 localization was observed by immunofluorescence. The C15Orf41 localization shift was observed with Codanin-1’s fragments 3 and 6, similarly to those which bind C15Orf41 (supplementary Fig. S3). As a control, only the N-terminal fragment of Codanin-1 influences ASF1 localization (Supplementary Fig. S4). In line with the ability of Codanin-1 R1042W mutant protein to bind C15Orf41, it also caused a similar change in the localization in C15Orf41 (not shown). Thus, Codanin-1 may serve as a scaffold protein holding C15Orf41 (and ASF1) in the cytoplasm and stabilizing C15Orf41 levels.
Codanin-1 and C15Orf41 share very similar phylogenetic profiles.
The concept of “phylogenetic profiling” assumes that two proteins which participate in the same pathway, or are part of the same structure, will have an evolutionary tendency to be eliminated or to be preserved in the same taxa [16]. We therefore examined whether Codanin-1 and C15Orf41 share similar phylogenetic distributions. Codanin-1 was first identified in human [7] and in Drosophila [13]. To follow its ancestral origin and phyologenetic distribution we searched for homologues of human Codanin-1 in all eukaryotic major groups using the NCBI BlastP program.
Codanin-1 was found to be quite highly conserved during metazoan evolution. Codanin-1 orthologues are present in most animals (including the ancestral Placozoa and Cnidaria). However, even though it is an essential protein for mammalian development and for mammalian cell survival [12], it was apparently lost from several diverse taxa including Porifera (sponges), Nematoda, Tardigrada (“water bears”), Platyhelminthes (flat worms) and Mesozoa (worm-like parasites of marine invertebrates) (Fig. 5). In addition, there are Codanin-1 orthologues in several protists including the fungus-like oomycetes and the microalgae diatoms (both belonging to the Stramenopiles taxa), but not in choanoflagellates, the closest living protists of the animals. The presence of Codanin-1-like protein in only a few protists can suggest horizontal transfer from metazoans into these groups or (less plausibly) a massive loss in most of the protists’ groups.
In comparison, C15Orf41 roots are more ancient than those of Codanin-1, and its orthologues are already found in Euryarchaeotes, a phylum of Archaea (but not in other Archaeal phyla) (Fig.5). In addition, it is found in several protists’ groups including those in which Codanin-1 is present (diatoms and oomycetes). Quite strikingly, no homologues of C15Orf41 are present in the animal groups Porifera, Nematoda, Tardigrada, Platyhelminthes and Mesozoa, exactly the same groups from which Codanin-1 was lost (Fig. 5). Thus, C15Orf41 exists in all of the taxa in which Codanin-1 exists (but not vice versa), and they were simultaneously lost in several unrelated animal groups.
Codanin-1 is structurally similar to the multifunctional protein, CNOT1
The identity of the ancestral protein(s) which gave rise to Codanin- 1 is not clear, as BlastP searches do not reveal any paralogous proteins or an obvious domain [11, 17], giving no clue about Codanin-1 functions. As protein structure is more conserved in evolution than protein sequence, we have used the protein structure homology-modeling server Phyre2 to predict the 3D structure of codanin-1, and to identify model proteins which carry similar structures (Kelley et al., 2015). Interestingly, the three highest scoring templates used to model human Codanin-1 structure were all derived from a single protein, CNOT1 (Fig. 6A,B). The confidence scores were 97.8%, 89.4% and 60.9% for a.a. 478-758, 855-988 and 299-418 of Codanin-1 using a.a. 1109-1292, 1381-1524 and 879-991 of human CNOT1 as a model (respectively).
The three domains are arranged in the same order and with similar spacing in Codanin-1 and CNOT1 (Fig. 6B). Notably, the region giving the highest confidence score (a.a. 478-758 in human Codanin-1) is included in the region which is highly conserved through the evolution of the codanin-1 proteins (~a.a. 400-850). The 3D structural similarity of Codanin-1 to CNOT1 is conserved and was found in all Codanin-1-like proteins examined. For example, the two highest scores for modeling Drosophila melanogaster Codanin-1 homolog, discs lost, were found in CNOT1 (similarity of 96.7% confidence to a.a. 1111-1265 and 56.1% confidence to a.a. 895-988 of human CNOT1). Similarly, the oomycetes fungus-like Phytophthora parasitica Codanin-1 homolog (protein F441_15879; 1340 a.a.) has similarity of 98.9 % confidence to a.a. 1352-1464 and of 98.8% confidence to a.a. 1143-1309 of human CNOT1. The predicted model of the most conserved codanin-1 domain in human, Drosophila and Phytophtora as well as the corresponding domain in the human CNOT1 template are seen in Fig. 7.
CNOT1 is the large, scaffolding subunit of the conserved CCR4–NOT complex, which is mainly involved in RNA-related processes including mRNA deadenylation, translational repression and transcriptional control [17, 18]. The deadenylation is performed by the CAF1 and CCR4A exoribonuclease, which are components of the CCR4–NOT complex. The interactions between CNOT1 and its binding proteins were mapped in yeast, human and Drosophila [19–22]. Interestingly, the two most conserved regions between Codanin-1 and CNOT1 are mapped to well documented CNOT1 domains. A central region in CNOT1 (spanning a.a. 1152-1376 in Drosophila CNOT1), designated NOT1 MIF4G domain, has been shown to bind the deadenylase CAF1 [22]. The CAF1 deadenylase binds the CCR4A deadenylase and thus bridges between CNOT1 and CCR4A [19]. NOT1 MIF4G domain has also been shown to bind to the DEAD-box protein DDX6, which functions as a translational repressor and decapping activator [20, 23]. The MIF4G domain is the region which served to model Codanin-1 with the highest confidence (97.8%)(Fig.6B). The second highest similar region (89.4% confidence), spanning a.a. 1381-1535 in human CNOT1, is designated CAF40/NOT9-binding domain (CN9BD). The CN9BD domain is also implicated in mRNA metabolism and function: it binds CAF40, a scaffold protein for several proteins which are involved in miRNA degradation [20], degradation of mRNAs containing AU-rich elements [24], and mRNA decay and translational repression [25].
CNOT1 has been demonstrated to bind and repress ligand-dependent transcriptional activation by estrogen receptor α. The binding and repression are dependent on nine LXXLL motifs (where L is leucine and X any amino acid; termed nuclear receptor boxes) present in CNOT1 [26]. Intriguingly, eight LXXLL motifs exist in human Codanin-1. To examine the statistical probability for a random existence of this motif in Codanin-1, we performed a protein sequence simulation in which the Codanin-1 sequence was shuffled 10000 times (i.e. keeping the single amino acids frequencies). In only 194 cases we got the same number (8) or higher number of the LXXLL pattern. This will amount to statistical significance of P-Value < 0.02, strengthening the structural similarity between the two proteins, and suggesting involvement of Codanin-1 in nuclear receptors signaling and transcription.
Taken together, the structural similarity between Codanin-1 and CNOT1 suggests that Codanin-1 is involved in similar pathways, namely in transcriptional and post-transcriptional control of RNA levels and functions.