Investigating the structure and function of SAF-A

SAF-A is a 90 kDa protein involved in the organisation of large-scale chromatin structure. SAF-A has four domains: a SAP domain with proposed DNA binding function, a SPRY domain of unknown function, an ATP binding domain and an unstructured RGG RNA binding domain. Previously, it was hypothesised that the ATP binding domain of SAF-A belonged to a class of proteins called AAA+ proteins. This study presents homology modelling that suggests that the ATP binding domain of SAF-A is more structurally similar to P-loop kinases. A previous study suggested that SAF-A oligomerises in vivo upon both ATP binding to the ATP binding domain and RNA binding to the RGG domain, and that oligomerisation of SAF-A is important for proper chromatin organisation. This study confirms that SAF-A binds ATP in vitro. Surprisingly, this study identified a novel RNA binding region within the SPRY domain of SAF-A that appears to preferentially bind longer RNA in vitro. Furthermore, binding of ATPS at the ATP binding domain appears to facilitate tighter RNA binding by the SPRY domain. Small angle X-ray scattering suggests that this effect is independent of a large conformational change. This study describes a novel RNA binding domain in SAF-A and presents an interplay between ATPS binding and RNA binding within the SPRY-AAA+ fragment of SAF-A