Structural study of SMC-based condensin system in archaea

Abstract

Structural maintenance and condensation (SMC) family is a member of condensin system which is involved in chromosome condensation and segregation during cell replication. Generally, prokarykotic SMC protein family is composed of two SMC homodimer, one ScpA as a kleisin subunit and two ScpB homodimer as a kite subunit. N-terminal domain and Ctermianl domain of ScpA bind to the neck of one Smc and the head of the other Smc, respectively, to make a closed ring structure, and ScpB homodimer binds to ScpA to stabilize the protein. While almost all bacterial species have all the three components of SMC family, ScpB does not exist in more than half of archaeal species. In this study we characterized archaeal SMC family in which ScpB is not included in sequential and structural manner. We found that ScpB homolog in some archaeal species does not interact with ScpA using in vitro pull-down assay. Determined structure of the two proteins and sequence comparison of archaeal ScpAs with bacterial proteins showed that the impotence of interaction between the two proteins is caused by the lack of amino acid sequence in archaeal ScpA, which is important for the interaction with ScpB in bacteria. Pull-down assay and cysteine crosslinking experiments of archaeal ScpA and Smc showed that the two proteins are able to make a closed ring structure in similar with bacterial complex. The study suggests a new type of SMC protein family which belongs to archaeal species and is a binary complex of Smc and ScpA only.