Abstract 877: Protein-protein interactions in cancer: An in-silico analysis of Cdc6 interactions
Authors: Eichner Andriele Silva , Philip Jasmin , Ikui Amy , Singh Shaneen 
Published: April 2023

Abstract
Aberrant activity of cell cycle proteins is a hallmark of cancer and therefore, cell cycle regulators are considered attractive targets in cancer therapy. One such regulator, Cell division cycle 6 (Cdc6), is a key conserved cell cycle protein responsible for the initiation of eukaryotic DNA replication. Cdc6 overexpression has been observed as an early event in malignancies. Additionally, Cdc6 has also been implicated in contributing to the aggressiveness of cervical and bladder cancers. In budding yeast, Cdc6 collaborates with proteins such as Cyclin B2 (Clb2) and Cell division control protein 4 (Cdc4) to ensure the fidelity of cell division by preventing re-replication. The perfectly orchestrated sequence of events requires Cdc6 to interact with other cell cycle proteins during the correct stage of the cell cycle. During S-phase, Cyclin-dependent kinase 1 (Cdk1) regulates Cdc6 levels and Cdc6 is targeted for degradation by polyubiquination via Cdc4 (SCF-complex). In mitosis, phosphorylation by Cdk1, induces the binding of Cdc6 to Clb2-Cdk1. Clb2 binds to Cdc6’s 44PEKLQF49 motif at the Cdc6 N-terminus thereby preventing Cdc6 from licensing origins, preventing DNA re-replication, and ensuring genomic integrity. Although, it has been well studied how each Cdc6 phosphorylation site regulates protein binding, in the absence of full-length structures for many of these proteins, the interaction modes and molecular details for Cdc6’s interaction with its protein partners remain unknown. In this study, we have characterized and analyzed Cdc6’s interactions with some of its key interactors. We present a robust computational approach to understand the molecular mechanisms of Cdc6 interactions with Cdc4 and Clb2, which includes homology/ab initio modeling of the full-length structures including their intrinsically disordered regions (IDRs), and predicted interaction scenarios. Our results show that the Cdc6-Cdc4 interaction displays a dynamic shift between S-phase and mitosis. In S-phase, Cdc4 binds to the N’ phosphodegrons on Cdc6, but in mitosis, Cdc4 binding shifts to the C’ phosphodegrons. In addition, we show Clb2’s K270, within its hydrophobic patch, binds E45 in Cdc6’s 44PEKLQF49 motif. We also provide a comparison of Clb2’s interaction with Cdc6 with that of Bud3 to assess the conservation of the binding mechanism. We show that the molecular mechanism is conserved with similar residues and motifs being involved in the interaction. By exploring the molecular details underlying these protein-protein interactions, we can gain insightful information and identify interface residues that are responsible for specific protein-protein interactions. Such molecular details are critical to pave the way to the discovery of druggable targets and ultimately lead to the development of new cancer therapeutics.
Citation Format: Andriele Silva Eichner, Jasmin Philip, Amy Ikui, Shaneen Singh. Protein-protein interactions in cancer: An in-silico analysis of Cdc6 interactions [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 877.