Background The THAP (Thanatos Associated Proteins) protein family in humans is implicated in various important cellular processes like epigenetic regulation, maintenance of pluripotency, transposition and disorders like cancers and hemophilia. The human THAP protein family which consists of twelve members of different lengths has a well characterized amino terminal, zinc-coordinating, DNA-binding domain called the THAP domain. However, the carboxy terminus of most THAP proteins is yet to be structurally characterized. A coiled coil region is known to help in protein oligomerization in THAP1 and THAP11. It is not known if other human THAP proteins oligomerize. We have used bioinformatic tools to explore the possibility of dimerization of THAP proteins via a coiled coil region. Results Classification of human THAP protein into three size based groups led to the identification of an evolutionarily conserved alpha helical region, downstream of the amino terminal THAP domain. Secondary structure predictions, alpha helical wheel plots and protein models demonstrated the strong possibility of coiled coil formation in this conserved, leucine rich region of all THAP proteins except THAP10. Conclusions The identification of a predicted oligomerization region in the human THAP protein family opens new directions to investigate the members of this protein family. Electronic supplementary material The online version of this article (10.1186/s12900-019-0102-2) contains supplementary material, which is available to authorized users.

Active DNA transposases like the Drosophila P element transposase (DmTNP) undergo oligomerization as a prerequisite for transposition. Human THAP9 (hTHAP9) is a catalytically active but functionally uncharacterized homologue of DmTNP. Here we report (using co-immunoprecipitation, pull down, colocalization, and proximity ligation assays) that both full length and truncated hTHAP9 (corresponding to amino-terminal DNA binding and predicted coiled coil domains) undergo homo-oligomerization, predominantly in the nuclei of HEK293T cells. Interestingly, the oligomerization is shown to be partially mediated by DNA. However, mutating the leucines (either individually or together) or deleting the predicted coiled coil region did not significantly affect oligomerization. Thus, we highlight the importance of DNA and the amino-terminal regions of hTHAP9 for their ability to form higher-order oligomeric states. We also report that Hcf-1, THAP1, THAP10, and THAP11 are possible protein interaction partners of hTHAP9. Elucidating the functional relevance of the different putative oligomeric state(s) of hTHAP9 would help answer questions about its interaction partners as well as its unknown physiological roles.

The THAP (Thanatos-associated protein) domain is a DNA-binding domain which binds DNA via a zinc coordinating C2CH motif. Although THAP domains share a conserved structural fold, they bind different DNA sequences in different THAP proteins, which in turn perform distinct cellular functions. In this study, we investigate (using multiple sequence alignment, in silico motif and secondary structure prediction) THAP domain conservation within the homologs of the human THAP (hTHAP) protein family. We report that there is significant variation in sequence and predicted secondary structure elements across hTHAP homologs. Interestingly, we report that the THAP domain can be either longer or shorter than the conventional 90 residues and the amino terminal C2CH motif within the THAP domain serves as a hotspot for insertion or deletion. Our results lay the foundation for future studies which will further our understanding of the evolution of THAP domain and regulation of its function.

BackgroundActive DNA transposases like the Drosophila P element transposase (DmTNP) undergo oligomerisation as a prerequisite for transposition. Human THAP9 (hTHAP9) is a catalytically active but functionally uncharacterised homologue of DmTNP. ResultsHere we report (using co-IP, pull down, co-localization, PLA) that both the full length as well as truncated hTHAP9 and DmTNP (corresponding to amino-terminal DNA binding and Leucine-rich coiled coil domains) undergo homo-oligomerisation, predominantly in the nuclei of HEK293T cells. Interestingly, the oligomerisation is shown to be partially mediated by DNA. However, mutating the leucines (either individually or together) or deleting the predicted coiled coil region did not significantly affect oligomerisation. We also report that Hcf-1, THAP1, THAP10 and THAP11 are possible protein interaction partners of hTHAP9. ConclusionsThus, we highlight the importance of DNA as well as the amino-terminal regions of both hTHAP9 and DmTNP, for their ability to form higher order oligomeric states. Elucidating the functional relevance of the different putative oligomeric state/s of hTHAP9 would help answer questions about its interaction partners as well as its unknown physiological roles.

Guanine nucleotide binding proteins are characterized by a structurally and mechanistically conserved GTP-binding domain, indispensable for binding GTP. The G domain comprises of five adjacent consensus motifs called G boxes, which are separated by amino acid spacers of different lengths. Several G proteins, discovered over time, are characterized by diverse function and sequence. This sequence diversity is also observed in the G box motifs (specifically the G5 box) as well as the inter-G box spacer length. The Spacers and Mismatch Algorithm (SMA) introduced in this study, can predict G-domains in a given G protein sequence, based on user-specified constraints for approximate G-box patterns and inter-box gaps in each G protein family. The SMA parameters can be customized as more G proteins are discovered and characterized structurally. Family-specific G box motifs including the less characterized G5 motif as well as G domain boundaries were predicted with higher precision. Overall, our analysis suggests the possible classification of G protein families based on family-specific G box sequences and lengths of inter-G box spacers. Significance StatementIt is difficult to define the boundaries of a G domain as well as predict G boxes and important GTP-binding residues of a G protein, if structural information is not available. Sequence alignment and phylogenetic methods are often unsuccessful, given the sequence diversity across G protein families. SMA is a unique method which uses approximate pattern matching as well as inter-motif separation constraints to predict the locations of G-boxes. It is able to predict all G boxes including the less characterized G5 motif which marks the carboxy-terminal boundary of a G domain. Thus, SMA can be used to predict G domain boundaries within a large multi-domain protein as long as the user-specified constraints are satisfied. Main Text