SMC (structural maintenance of chromosomes) protein complexes act in chromosome processing in all domains of life. In this issue, a study of the prokaryotic SMC complex Smc–ScpAB reveals an unanticipated asymmetry despite Smc forming a symmetric homodimer. This asymmetry—contributed by two distinct binding sites in Smc for the kleisin ScpA—is crucial for function in vivo and bears similarities to the eukaryotic complexes formed by Smc heterodimers.
- Amy L Upton
- David J Sherratt
