EF-hand, extracellular calcium-binding (EC) motif, found in testicans
Testicans are nervous system-expressed proteoglycans that play important roles in the regulation of protease activity, as well as in the determination of age at menarche. Testican-1 (TICN1, also termed protein SPOCK) is a secreted chimeric proteoglycan that is highly expressed in brain and carries both chondroitin and heparan sulfate glycosaminoglycan side chains. It has been implicated in autoimmune disease. It also acts as a regulator of bone morphogenetic protein (BMP) signaling and show critical functions in the nervous system. Testican-2 (TICN2, also termed protein SPOCK2) is an extracellular heparan sulphate proteoglycan highly expressed in brain. It may play regulatory roles in the development of the central nervous system. It also participates in diverse steps of neurogenesis. TICN1, but not TICN2, inhibits cathepsin L. TICN1 also inhibits attachment and neurite outgrowth in cultures of N2A neuroblastoma cells, While TICN2 is able to inhibit neurite outgrowth from primary cerebellar cells. Testicans contain an N-terminal signal peptide, a testican-specific domain followed by a follistatin-like (FS) domain, an extracellular calcium-binding (EC) domain including a pair of EF hands, a thyroglobulin-like domain (TY), and a C-terminal region with two putative glycosaminoglycan attachment sites. The substitution of a ligating Asp residue by Tyr orTyr in the +Y position of EF hand 2 in testican-2 could prevent Ca2+ binding to this site and also cause EF-hand 1 to bind one Ca2+ with low affinity. The substitution of a ligating Asp residue by Phe or Tyr in the +Y position of EF-hand 2 in testicans could prevent Ca2+ binding to this site and also cause EF-hand 1 to bind one Ca2+ ion with low affinity.
Feature 1:putative Ca binding site [ion binding site]
Evidence:
Comment:Based on the structure evidence how the first EF-hand motif in Homo sapiens SPARC (1SRA) binds Ca2+ ion.
Comment:Unlike other members in SPARC family, the substitution of a ligating Asp residue in the +Y position of EF hand 2 in testicans could prevent Ca2+ binding to this site and also cause EF-hand 1 to bind one Ca2+ with low affinity.
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