This domain is SSO6904 present in Sulfolobus solfataricus. SSO6904 is a calcium binding protein thought to have a weak affinity for other cations such as Mg2+ and Zn2+. The structure of SSO6904 is similar to that of saposin-fold proteins. Saposin proteins are membrane-interacting glycoproteins required for the hydrolysis of certain sphingolipids by specific lysosomal hydrolases [1].

1: Solution structure and calcium binding of protein SSO6904 from the hyperthermophilic archaeon Sulfolobus solfataricus. Feng Y, Yao H, Wang J; Proteins. 2010;78:474-479. PMID:19768683