The SPARC_Ca_bdg domain of Secreted Protein Acidic and Rich in Cysteine is responsible for the anti-spreading activity of human urothelial cells. It is rich in alpha-helices. This extracellular calcium-binding domain contains two EF-hands that each coordinates one Ca2+ ion, forming a helix-loop-helix structure that not only drives the conformation of the protein but is also necessary for biological activity. The anti-spreading activity was dependent on the coordination of Ca2+ by a Glu residue at the Z position of EF-hand 2 [1].

1: The C-terminal Ca2+-binding domain of SPARC confers anti-spreading activity to human urothelial cells. Delostrinos CF, Hudson AE, Feng WC, Kosman J, Bassuk JA; J Cell Physiol. 2006;206:211-220. PMID:16121393