SMC Proteins - Molecular DNA Condensation Motors?

Arsen Volkov, Judita Mascarenhas, L. Cornelia Andrei-Selmer, and Peter L. Graumann Laboratory of Biochemistry I, Department of Chemistry, Philipps-University, Hans-Meerwein Str., D-35043 Marburg, Germany e-mail: graumann@chemie.uni-marburg.de URL: http://www.chemie.uni-marburg.de/~graumann

SMC (structural maintenance of chromosomes) proteins are ubiquitous proteins that are essential for chromosome condensation and segregation in almost all organisms. They consist of antiparallel symmetrical dimers with two head domains connected by two long coiled coil regions and a central hinge domain. SMC proteins are ATPases associated with additional proteins, and can wrap up DNA by introduction of supercoils into DNA. ATP induces a conformational switch within the head domains that are thought to bind to DNA. The SMC dimer can be in an open conformation with head domains spread apart up to 100 nm or in a closed conformation with the head domains together. We have purified prokaryotic SMC and its two interacting proteins, ScpA and ScpB to investigate the condensation activity on a molecular level. We found that SMC loads onto DNA, and is released from DNA by addition of ATP. ScpA associates with the head domain(s) of SMC, which is stabilized by ScpB. Preliminary data suggest that ScpA changes the state of SMC to a closed conformation. Thus, a cycle between DNA binding of well spaced parts on the DNA, binding of ScpA and ScpB that brings bound DNA regions together and binding and hydrolysis of ATP could lead to DNA condensation in a motor-like fashion. We are attempting to use SMC different constructs as a DNA-based motor.