p60-S6K1 represents a novel kinase active isoform with the mode of regulation distinct from p70/p85-S6K1 isoforms

Co-expression of the RPS6KB1 and PDPK1 genes for production of activated p70S6K1 using bac-to-bac baculovirus expression system

BACKGROUND

Ribosomal protein S6 kinase 1 (p70S6K1) is a member of the AGC family of serine/threonine kinases which plays a role in various cellular processes, including protein synthesis, cell growth, and survival. Dysregulation of p70S6K1, characterized by its overexpression and/or hyperactivation, has been implicated in numerous human pathologies, particularly in several types of cancer. Therefore, generating active, recombinant p70S6K1 is critical for investigating its role in cancer biology and for developing novel diagnostic or therapeutic approaches.

METHODS

The baculovirus dual expression system was utilized, enabling the co-expression of two recombinant proteins in infected cells: (a) His-tagged S6K1 with a deletion of the C-terminal autoinhibitory motif and a phosphomimetic mutation at the mTORC1 phosphorylation site (T389D), and (b) untagged PDPK1 lacking the PH domain. The high activity of the purified kinase was confirmed by immunoblotting, as well as by Kinase-Glo and AlphaScreen kinase assays.

RESULTS

Efficient expression of both recombinant proteins was achieved, resulting in highly pure preparations of His-tagged p70S6K1. The high activity of the purified kinase was confirmed through multiple kinase assays, demonstrating significantly higher levels of substrate phosphorylation compared to the tested commercial product.

CONCLUSION

Here, we report a reliable and efficient methodology for the expression and purification of highly active p70S6K1 (His-actS6K1) in quantity and quality that is suitable for biochemical/biophysical studies and high-throughput enzymatic assays. Our developed methodology offers a rapid and cost-effective approach for producing constitutively active His-actS6K1, which can be utilized in academic research and biotechnology.

Investigating the Regulation of Ribosomal Protein S6 Kinase 1 by CoAlation

Ribosomal protein S6 kinases belong to a family of highly conserved enzymes in eukaryotes that regulate cell growth, proliferation, survival, and the stress response. It is well established that the activation and downstream signalling of p70S6Ks involve multiple phosphorylation events by key regulators of cell growth, survival, and energy metabolism. Here, we report for the first time the covalent modification of p70S6K1 by coenzyme A (CoA) in response to oxidative stress, which regulates its kinase activity. The site of CoA binding (CoAlation) was mapped by mass spectrometry to cysteine 217 (Cys217), located in the kinase activation loop and only one amino acid away from the tripeptide DFG motif, which facilitates ATP-binding. The CoAlation of recombinant p70S6K1 was demonstrated in vitro and was shown to inhibit its kinase activity. Our molecular docking and dynamics analysis revealed the most likely mode for CoA binding to p70S6K1. This mechanism involves the non-covalent binding of the CoA ADP moiety to the p70S6K1 nucleotide-binding pocket, positioning the CoA thiol group in close proximity to form a covalent bond with the surface-exposed Cys217 residue. These findings support a "dual anchor" mechanism for protein kinase inhibition by CoAlation in cellular response to oxidative stress. Furthermore, the inhibition of S6K1 by CoAlation may open new avenues for developing novel inhibitors.

A few notes on science in Ukraine

As a person who has had a long scientific career in Ukraine, both before and after its re-acquisition of independence thirty years ago, I would like to share my observations with the readership of this Special Issue. By no means are these observations meant to provide a systematic presentation, which requires a different format. Rather, they are highly personal notes, providing snippets of the past and present and a discussion of the future of Ukrainian science. They also allow me to acknowledge my wonderful colleagues and bright students. I am delighted to see that many of them have contributed excellent reviews and original manuscripts to this Special Issue. (I am also keenly aware of the fact that because of the brutal invasion and bombardments by our imperial neighbor, many of my colleagues have been unable to share their latest work). It will be up to this next generation of Ukrainian scientists to develop Biological Sciences in Ukraine in the future.

p70 S6 kinase as a therapeutic target in cancers: More than just an mTOR effector

p70 S6 kinase (p70^S6K) is best-known for its regulatory roles in protein synthesis and cell growth by phosphorylating its primary substrate, ribosomal protein S6, upon mitogen stimulation. The enhanced expression/activation of p70^S6K has been correlated with poor prognosis in some cancer types, suggesting that it may serve as a biomarker for disease monitoring. p70^S6K is a critical downstream effector of the oncogenic PI3K/Akt/mTOR pathway and its activation is tightly regulated by an ordered cascade of Ser/Thr phosphorylation events. Nonetheless, it should be noted that other upstream mechanisms regulating p70^S6K at both the post-translational and post-transcriptional levels also exist. Activated p70^S6K could promote various aspects of cancer progression such as epithelial-mesenchymal transition, cancer stemness and drug resistance. Importantly, novel evidence showing that p70^S6K may also regulate different cellular components in the tumor microenvironment will be discussed. Therapeutic targeting of p70^S6K alone or in combination with traditional chemotherapies or other microenvironmental-based drugs such as immunotherapy may represent promising approaches against cancers with aberrant p70^S6K signaling. Currently, the only clinically available p70^S6K inhibitors are rapamycin analogs (rapalogs) which target mTOR. However, there are emerging p70^S6K-selective drugs which are going through active preclinical or clinical trial phases. Moreover, various screening strategies have been used for the discovery of novel p70^S6K inhibitors, hence bringing new insights for p70^S6K-targeted therapy.