High-Speed Atomic Force Microscopy Of Smc Proteins

Condensin and cohesin belong to Structural Maintenance Complex (SMC) protein family that are involved in chromosome organization and chromosome segregation. The key feature of these SMC protein complexes is the ring-shaped structure consisting of 3 major components - two structural maintenance complex (SMC) subunits and one kleisin that together form a ring, as well as 1 or 2 vital co-factors. Through a mechanism where the SMC protein embraces the DNA, cohesin holds sister chromatids together after DNA replication while condensin condenses DNA using ATP as an energy source, possibly related to the linear motor function that was recently discovered [1]. Resolving dynamic conformational changes of the SMC complexes is key to resolve the mechanism underlying these important functionalities [2]. High-speed atomic force microscopy (HS-AFM) is uniquely fit to address this question since it allows video-imaging of entire protein structure with high spatial resolution (∼1 nm) as well as high temporal resolution (50 ms). Here, we will show HS-AFM video imaging of condensin [3] and cohesin proteins in physiological buffers. Conformational changes of SMC dimers of condensin and cohesin are directly imaged for various conditions (+/- ATP, +/- DNA). The real-time imaging reveals how structures of these protein complexes evolve over time, from which the structure-function relationship of condensin and cohesin can be obtained. These results provide insights into the molecular mechanism of chromosome organization and segregation. References [1] T. Terakawa et al. (2017), “The condensin complex is a mechanochemical motor that translocates along DNA” Science. [2] J. Eeftens, (2017) “Real-time detection of condensin-driven DNA compaction reveals a multistep binding mechanism” BioRxiv, https://doi.org/10.1101/149138s [3] J. Eeftens, (2016), “Condensin Smc2-Smc4 dimers are flexible and dynamic” Cell Rep.