Description

DNA replication in eukaryotes is initiated by the orchestrated assembly and association of initiator proteins on the replication origins. The hunt for initiator proteins in higher eukaryotes picked up pace after the discovery of the Origin Recognition Complex (ORC) comprising of six protein subunits of the ORC1-6 complex in budding yeast (Bell and Stillman, 1992). In Candida albicans, a human fungal pathogen, the ORC (CaORC1-6) and their associated proteins were identified by a BLAST analysis using the S. cerevisiae proteins as the query sequences in the Candida Genome Database (CGD). The phylogenetic analysis suggests that in spite of limited amino acid sequence similarity with their counterparts in other organisms, the CaORC proteins share most of the functional domains with them. Interestingly, the amino acid sequences of CaORC1, 2 and 4 share higher degree of similarities than CaORC3, 5 and 6 to those of S. cerevisiae. CaORC1, 4 and 5 tend to be homologous to the mammalian counterparts. Although, in general, the CaORC proteins show limited sequence similarities with their counterparts in various species, CaORC1, 2 and 6 show maximum similarities to their S. cerevisiae counterparts while CaORC3, 4 and 5 appear to be more similar to those of mammals which is evident from the phylogenetic map (Figure 1A, Extended Data Tables 1-3).

The Expasy PROSITE tool predicts the presence of an evolutionarily conserved Bromo-Adjacent Homology (BAH) domain spanning the region between 44^th and 179^th amino acids at the N-terminal of CaORC1 (Figure 1B). The BAH domain is involved in protein-protein interactions and has been found to be important in DNA methylation, replication and transcriptional regulation (Callebaut et al.,1999). The BAH domain present in CaORC1 along with the highly conserved basic residues (K-362 and R-367) (Kawakami et al., 2015) in its AAA domain is likely to play a key role in ORC-origin binding in C. albicans. The ATPase activity is indispensable for the origin-ORC association and henceforth for the establishment of the pre-initiation complex. Like metazoans, the CaORC subunits 1, 4, and 5 and CaCdc6 containing AAA+ domains are likely to be engaged in ORC assembly and consequent MCM recruitment. The presence of AAA+ domains, along with Walker A and B motifs provides a plausible explanation for the significance of these domains in CaORC4 in DNA replication (Extended Data Tables 4-6). From the conservative nature of the tyrosine residue between the Walker B motif and sensor I domain observed in humans and yeast, it is possible that the presence of this residue in CaORC4 (Tyr²⁷³) might play a regulatory role in the cell cycle (Extended data Figure 1). CaORC1 and CaORC4, each contains a consensus AAA+ domain (420-571 a.a. in CaORC1; 139-318 a.a. in CaORC4) (Figure 1B, 1C and 1D), which belongs to the AAA+ family that is pivotal to the initiation of eukaryotic DNA replication. There is an amino acid residue Tyr¹⁷⁴ in human ORC4 (Tyr²³² in S. cerevisiae) that is found between the Walker B motif and sensor I of the AAA+ domain which may be responsible for interacting with a conserved arginine residue on an adjacent helix structure of ORC4 (Bell and Dutta, 2002, Wigley 2009, Duncker et al., 2009, Kawakami and Katayama, 2010, Bell 2002, Guernsey et al., 2011). Identification of missense mutation in ORC4, Y174C in humans is reported in Meier-Gorlin syndrome (Bicknell et al., 2011, Guernsey et al., 2011) whereas in yeast, ORC4 mutation, Y232C resulted in slower growth rate with G1 to S phase transition defects (Ladha 2011) and locus specific chromosome breakage (Sanchez et al., 2017). This residue is present in CaORC4 (Tyr²⁷³) too probably doing a similar function. SNAP2 analysis suggests that this residue is crucial and whose mutation might cause functional defects in the protein (Extended data Figure1).

The Walker motifs are present in CaORC1, CaORC4 and CaORC5 (Walker B is absent in CaORC5) (Extended Data Table 4). Besides CaORC1, the perfect signature of the Walker motif is found in CaORC4 with a putative Walker A motif (147-153 a.a) and a putative Walker B motif (410-426 a.a), the amino acid sequences for which are shown in Extended Data Table 5. These motif signatures seem to be more closely related to the metazoan/higher eukaryotic sequences. A conserved Proliferating Cell Nuclear Antigen (PCNA) binding motif called the PCNA-interacting protein (PIP) box (QXXMXXFFFY) is found in the CaORC1 protein (524-536 a.a). Of the CaORC proteins, the PIP box is found to be unique to CaORC1. A conserved peptide motif named MIR (MOD1 interacting region – PXVHH) which is essential for their interaction with MOD1, a serotonin-gated chloride channel that modulates locomotory behavior in C. elegans (Ranganathan et al., 2000) is found in CaORC3 protein (435-448 a.a). In vivo studies demonstrate that the MIR domain of ORC3 is important in the HP1α interaction (Prasanth et al., 2010) suggesting a non-replicative role for CaORC3 too.

CaORC2 (130-172 a.a.) and CaORC3 (6-33 a.a.) contain PEST motif. Analysis of PEST signals in human and mouse ORC proteins suggests that only ORC1 is targeted for ubiquitination which is likely to hold good for all mammals (Li and DePamphilis, 2002). The domains of CaORC proteins are compared with other eukaryotes and are tabulated in Extended Data Table 6 and compared with S. cerevisiae in Figure 1D. The evolution of phospho regulation pattern in replication proteins of various yeast species including C. albicans is reported (Beltrao et al., 2009).

Experimental evidence would be required to find out if some or all of these subunits are involved in ATP binding and hydrolysis in C. albicans. The cryo-EM structural studies demonstrate that Cdt1 helps in the recruitment of Cdc6 and Mcm2-7 thus forming the ORC-Cdc6-Cdt1-Mcm2-7 (OCCM) intermediate (Yuan et al., 2017). The absence of a Cdt1 homolog in C. albicans suggests that this important task may be accomplished by a different mechanism/factor (Extended Data Tables 6 and 7). The unique presence of the PEST motif in CaORC2 and CaORC3 indicates that these components might be facilitating ORC turnover.

Moreover, the CaORC proteins are also compared across CTG clade and other yeast species to provide a robust roadmap for further comparative yeast subphylum analysis (Figure 1E and F and Extended data Figure 2). The ORC1, ORC2 and ORC5 proteins from yeast to humans are found to have common nodes. Subsequently, the ORC proteins from the related species of C. albicans in the CTG clade were also compared and a phylogenetic tree was constructed (Figure 1E). The time tree demonstrates the diversification rate of these ORC proteins across the species of which ORC1 and ORC4 seem to be older than their counterparts (Figure 1F). In order to understand the sequence identity of the ORC sequences across various yeast species, Sequenceserver (http://blast.wei.wisc.edu/) (Priyam et al., 2019, Shen et al.,2016) was used across 86 publicly available yeast genomes (Extended data Figure 2; Extended Dataset).

We used I-TASSER (Iterative Threading ASSEmbly Refinement) (Zhang 2008) for structure prediction of CaORC proteins. Of all the CaORC proteins, CaORC4 was found to be one of the putative candidates for further fine refinement studies of the protein structure due to its higher Cscore (combined measure, See Methods section) which indicates a better confidence in predicting the function using the template (Extended Data Table 8). Hence, we proceeded for predicting the structure of CaORC4 using Phyre. We were able to build the 3D protein structure of CaORC4 only whereas the other CaORC proteins did not have good homology with the known PDB (Protein Data Bank) structures (Figure 1G and 1H). From our in silico analysis of interactive studies, it is evident that CaORC3, CaORC5 and CaORC6 do not interact with the other ORC counterparts. It is possible that only CaORC1, CaORC2 and CaORC4 would be involved in DNA binding during the process of DNA replication and the other counterparts may aid in tethering or in conformational organization. It is not known as to whether the CaORC exists as a complex which requires experimental validation. CaCdc6 and CaMCM4, the apparent common binding partners of CaORC1, CaORC2 and CaORC4 and many MCMs are also predicted to play important role(s) in preRC assembly and functioning. We also find a potential ATP binding site in CaORC4 which might help in the regulation of origin binding. The mode of ORC assembly at origins in C. albicans might be different from that in other yeasts. The in silico detection of the presence of AAA+ ATPase and Walker motifs in CaORC4 and its likely interaction with MCM proteins suggest that CaORC4 might be involved in stable binding to origin DNA and loading MCM proteins to origins. While possibilities of a physical association between CaORC4 and other CaORC proteins were not obvious (Extended Data Table 9), the role of some unknown factors mediating ORC assembly in C. albicans is not ruled out. CDC6, MCM4 and MCM proteins interact with CaORC1, CaORC2 and CaORC4. In absence of a direct interaction of CaORC4 with other ORC counterparts, these proteins might be mediating interaction between them. Moreover, the absence of Cdt1 in C. albicans might provide an additional role for CaORC4. The recent cryo-EM data of the ORC in yeast (Yuan et al., 2017) and humans (Li et al., 2018) might open up more avenues in understanding the structural dynamics of this complex in other species too.