Nuclear receptors (NRs) are defined as ligand-inducible transcriptional regulators for thousands of their target genes that play critical roles in specific vital functions, including reproduction, embryonic development, metabolism, homeostasis, and other physiological processes within an organism (Dash and Tyagi, 2016; Mazaira et al., 2019) The Vitamin D Receptor (VDR) is a typical nuclear receptor that requires Retinoic Acid X Receptor (RXR) as its heterodimeric partner to execute VDR-associated physiological functions when sensitized with the active form of Vitamin D, i.e., 1α, 25 dihydroxy vitamin D3 (Jones et al., 2020; Mangelsdorf' and Evanst, 1995).

Several studies suggest that some transcriptional regulators or ancillary nuclear proteins remain associated with the mitotic chromatin and may include chromosome scaffold proteins, basal transcriptional machinery (TBP, TFIID), and DNA repair enzymes. This phenomenon of association with mitotic chromatin is now referred to as ‘mitotic genome bookmarking’ (Kumar et al., 2012; Michieletto et al., 2018; Zaidi et al., 2011). ‘Genome bookmarking,’ also known as mitotic genome bookmarking, refers to the phenomenon by which specific regions of the genome are ‘bookmarked’ by cell-specific transcription factors during cell division to maintain cellular transcriptional memory and ensure accurate post-mitotic reactivation of the transcriptome, proteome, specific cellular traits, and phenotypes in progeny cells (Rana et al., 2018; Rizvi et al., 2023; Zaidi et al., 2010). Furthermore, several NRs have also been reported to exhibit distinct modes of mitotic chromatin association throughout the mitotic phases of the cell cycle, either constitutively (e.g., PXR and VDR), ligand-mediated (e.g., AR and ERα) or partner-mediated (e.g., RXR, and SHP) (Kashyap and Tyagi, 2022; Kumar et al., 2008, 2021b; Rana et al., 2018; Saradhi et al., 2005). Findings from our laboratory on mitotic chromatin-receptor interactions have highlighted the importance of ‘mitotic chromatin binding-determining regions (MCBRs)' present in NRs. The nuclear localization signal (NLS) region stretches in the DNA binding domain (DBD) region of receptors like PXR and AR and has been reported to serve as MCBR (Kumar and Tyagi, 2012; Rana et al., 2018).

The NLS is a short stretch of amino acid sequences specifically rich in basic amino acid residues and required for the nuclear import of protein in eukaryotes (Lu et al., 2021). Typical NLS can be classified into two major categories: ‘monopartite’ and ‘bipartite’ sequences. Monopartite signals are characterized by the presence of a single stretch of 4–8 positively charged basic amino acid residues, mainly arginine (R) or lysine (K), usually found as a K(K/R) X(K/R) motif where X can be any other amino acid residue (Lange et al., 2007). Unlike monopartite signals, bipartite NLS consists of two short stretches of positively charged basic amino acid residues at both ends separated by a long stretch of 9–12 amino acid residues linker bridge identified primarily as consensus sequences of R/K(X)9-12 KRXK. The typical example of monopartite NLS is SV40 large T-antigen NLS (PKKKRKV), and of bipartite NLS is nucleoplasmin whose sequence is present at the C-terminal as KRPAATKKAGQAKKKK (Lange et al., 2007; Lu et al., 2021; Miyamoto et al., 1997). In addition to the primary functional involvement of NLS in the targeted transport of proteins, the probable role of NLS in the ‘genome bookmarking’ of transcription factors has recently come to light in investigations with NRs (Rizvi et al., 2023).

Our recent report revealed that VDR interacts with the mitotic chromatin constitutively, and its DBD region is essential for the enrichment of receptors onto the mitotic chromatin platform (Kashyap and Tyagi, 2022). However, detailed insights about the involvement of the NLS region of VDR in the ‘mitotic genome bookmarking’ are yet to be revealed. The NLS-1 region of VDR is studied in significant detail and is known to be present between amino acids Arg49-Lys55 (49RRSMKRK55) in the DBD region, also classified as mini-bipartite NLS (Hsieh et al., 1998). However, another study reported the presence of NLS-2 of VDR between 79-105 amino acids at the C-terminal of the second zinc finger of VDR (Luo et al., 1994; Michigami et al., 1999). However, our studies show that only NLS-1 of VDR is essential for its mitotic chromatin association property. It has been observed that positively charged basic amino acids in the NLS region of VDR are essential in the mitotic gene bookmarking of receptor. It has been shown that two consecutive arginine residues at the 49th and 50th positions in the NLS of VDR are critical for its interaction with the mitotic chromatin. Since VDR is majorly involved in the regulation of calcium and phosphate homeostasis as well as in other non-calcaemic functions, exploration of the underlying mechanisms executed by mitotic chromatin binding by VDR would allow us to understand the molecular basis of genetic disorder and also suggest novel therapeutic strategies for the treatment of several inherited disorders (Gil et al., 2018).