This is the C-terminal domain found in archeal proteins that carry a von Willebrand factor type A domain such as ArnB from Sulfolobus acidocaldarius. ArnB is involved in negative regulation of the archaellum (former: archaeal flagellum), and the C-terminal domain is phosphorylated by ArnC and ArnD on serine and threonine residues affecting docking of ArnA [1].

1: Expanding the archaellum regulatory network - the eukaryotic protein kinases ArnC and ArnD influence motility of Sulfolobus acidocaldarius. Hoffmann L, Schummer A, Reimann J, Haurat MF, Wilson AJ, Beeby M, Warscheid B, Albers SV; Microbiologyopen. 2017; [Epub ahead of print] PMID:27771939