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Figure 2. Schematic representation of the modular organization of SPARC and the EC-collagen-binding domain. The N-terminal domain of SPARC in vertebrates can bind up to 8 Ca2+-ions with low-affinity (Kd 10–3 to 10–5 M), whereas 2 EF-hands bind a single Ca2+-ion, each with high affinity (Kd 10–7 M). X-ray crystallographic studies have indicated that the EC-domain of SPARC binds to the triple-helical domains of several types of fibrillar collagen and the basal laminae-associated network-forming collagen type IV. The illustrations are based on schematics by Sasaki et al.(1998) and an NCBI Cn3D v4.1 3D structure viewer (NCBI, http://www.ncbi.nlm.nih.gov/Structure/mmdb/mmdbsrv.cgi?form=6&db=t&Dopt=s&uid=6694). Human SPARC (with signal peptide attached, lower-case letters) is shown as an example and color-coded to highlight the 3 modules.
