Molecular Determinants of the binding mechanism between the LTSV40 and the Retinoblastoma protein

The short linear motif (SLiM) LxCxE facilitates protein-protein interactions, particularly with the human Retinoblastoma protein (pRb). This motif is found in proteins involved in chromatin regulation, histone modification, and cell cycle control. Among these proteins is the large T antigen of simian virus 40 (LTSV40), which possesses an LxCxE motif at the beginning of an intrinsically disordered region (IDR). This results in its flanking regions being conformationally asymmetric (structured towards its N-terminal region and unstructured towards the C-terminal region).

In this study, we used computational techniques to understand the molecular determinants of the binding mechanism between LTSV40 and pRb, mediated by the LxCxE motif. Through Molecular Dynamics (MD) simulations and Umbrella Sampling, we evaluated the interaction energies and molecular characteristics of the binding processes to pRb of five different LTSV40 models.

Our results show that the regions immediately flanking the LxCxE motif significantly increase the binding energy to pRb. Additionally, we observed that when a complete LTSV40 model is considered, the interaction with pRb increases compared to models that possess the LxCxE motif and all residues that directly interact with pRb. This suggests that the global structure of LTSV40 modulates the interaction affinity of the segments directly involved in the interaction with pRb. Furthermore, when analyzing the molecular binding mechanism between LTSV40 and pRb, we determined that the N-terminal flanking residues of the LxCxE motif in LTSV40 are the first to recognize the pRb pocket. A detailed analysis of the conformational ensemble of free LTSV40 shows that the side chains of these N-terminal residues to the motif are always available for interaction as they are located in an ordered (folded) region of the system. This recognition strategy involving an ordered region combined with interaction through a contiguous disordered region could be a common feature in proteins containing this type of LxCxE motif, facilitating rapid identification and binding to target proteins.

These findings provide significant insights into the molecular mechanisms involved in the LTSV40-pRb interaction, highlighting the importance of the flanking sequences in enhancing binding affinity. Additionally, the proposed recognition/binding mechanism results could pave the way for the design of drugs and therapies targeting interactions with pRb in various diseases.