Structural Basis of Enhanced Facilitated Diffusion of DNA Binding Proteins in Crowded Cellular Milieu

Abstract

DNA binding proteins (DBPs) rapidly recognize and specifically associate with their target DNA sites inside cell nucleus that contains up to 400 g/L macromolecules, most of which are proteins. While the fast association between DBPs and DNA is explained by a facilitated diffusion mechanism, where DBPs adopt a weighted combination of 3D diffusion and 1D sliding and hopping modes of transportation, the role of cellular environment that contains many nonspecifically interacting proteins and other biomolecules is mostly overlooked. By performing large scale computational simulations with an appropriately tuned model of protein and DNA in the presence of nonspecifically interacting bulk and DNA bound crowders (genomic crowders), we demonstrate the structural basis of the enhanced facilitated diffusion of DBPs inside a crowded cellular milieu through novel 1D scanning mechanisms. In the presence of bulk crowders, we identify the protein to float along the DNA under the influence of protein-crowder nonspecific interactions. The search mode is distinctly different compared to usual 1D sliding and hopping dynamics where protein diffusion is regulated by the DNA electrostatics. In contrast, the presence of genomic crowders expedite the target search process by transporting the protein over DNA segments through the formation of a transient protein-crowder bridged complex. By analyzing the ruggedness of the associated potential energy landscape, we underpin the molecular origin of the kinetic advantages of these search modes and show that they successfully explain the experimentally observed acceleration of facilitated diffusion of DBPs by molecular crowding agents and crowder concentration dependent enzymatic activity of transcription factors. Our findings provide crucial insights into gene regulation kinetics inside the crowded cellular milieu.SIGNIFICANCE 10-40% of the intracellular volume is occupied by proteins, and other biomolecules, collectively known as macromolecular crowders. Their presence has been found to promote faster translocation of DNA binding proteins (DBPs) during the search of their target DNA sites for crucial cellular processes. Using molecular simulations, we probe the underlying structural basis and underscore the existence of novel DNA scanning mechanisms actuated by protein-crowder nonspecific interactions. We show that the observed search modes are kinetically beneficial and can successfully explain the acceleration of facilitated diffusion of DBPs by molecular crowding agents and crowderconcentration dependent enzymatic activity of transcription factors.Our study sheds new light on the long-standing facilitated diffusion problem of DBPs in the crowded cellular environment for regulating gene expression.