Modeling post-translational modifications and cancer-associated mutations that impact the heterochromatin protein 1α-importin α heterodimers.

Michael T Zimmermann,Gwen A Lomberk,Monique M Williams,Eric W Klee,Raul Urrutia

doi:10.1002/prot.25752

Michael T Zimmermann, Gwen A Lomberk + Show 3 more

Open Access

https://doi.org/10.1002/prot.25752

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Abstract

Heterochromatin protein 1α (HP1α) is a protein that mediates cancer‐associated processes in the cell nucleus. Proteomic experiments, reported here, demonstrate that HP1α complexes with importin α (IMPα), a protein necessary for its nuclear transport. This data is congruent with Simple Linear Motif (SLiM) analyses that identify an IMPα‐binding motif within the linker that joins the two globular domains of this protein. Using molecular modeling and dynamics simulations, we develop a model of the IMPα‐HP1α complex and investigate the impact of phosphorylation and genomic variants on their interaction. We demonstrate that phosphorylation of the HP1α linker likely regulates its association with IMPα, which has implications for HP1α access to the nucleus, where it functions. Cancer‐associated genomic variants do not abolish the interaction of HP1α but instead lead to rearrangements where the variant proteins maintain interaction with IMPα, but with less specificity. Combined, this new mechanistic insight bears biochemical, cell biological, and biomedical relevance.

Highlights

The current study uses the Heterochromatin protein 1α (HP1α) linker domain, as a model for extending our understanding of how these proteins can be regulated via the linker domain through heterodimerization events that lead to the translocation of this protein to the nucleus, the cellular region where it is functionally needed
Our findings indicate that post-translational modifications of the HP1α linker are a critical factor in altering interaction with importin α (IMPα), and that genomic variants alter the pattern of interactions making them less specific
We found that HP1α copurified with several IMPα subunits and with IMPβ (Figure 1E)

Summary

| INTRODUCTION

Three known HP1 isoforms, HP1α (CBX5), HP1β (CBX1), and HP1γ (CBX3), are critical for regulating gene expression networks that are crucial for normal embryonic development[1] and cancer associated processes, including differentiation,[2,3] cell proliferation,[4,5] cell cycle control,[4,6,7,8,9] apoptosis,[8,10] and DNA damage response.[11,12,13,14,15,16,17,18] a conservation in sequence, structure, and function among these proteins in organisms ranging from Drosophila melanogaster to human reflects the biological importance and biomedical implications of their. Studies performed by our group and others have demonstrated that signaling-mediated mechanisms, yet to be fully understood, lead to the extensive post-translational modifications of HP1 proteins Some of these modifications fall in the globular domains of these proteins; the globular domains read histone marks and are responsible for homodimerization and heterodimerization. We model how post-translational modifications, deposited in the linker region in response to upstream regulators such as cancer-associated mutations, alter linker structure and thereby bonding patterns during interphase, likely impacting import and the downstream pathway. We have gained insight for how HP1α alterations may disrupt signals relayed through mitogenic signaling pathways and heterochromatin regulation during each cell cycle These results must be taken into consideration as mechanisms that are likely to influence the function of HP1 proteins during development, homeostasis control, and disease. We compared our threaded model to the results of full-length HP1α docking to IMPα via ClusPro.[56,57,58,59,60]

| MATERIALS AND METHODS

| RESULTS

| DISCUSSION