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The Ig-like domain is probably the most widespread domain, at least in animals. This domain can be considered as an heterogeneous group built on a common fold. Proteins containing an Ig-like domain differ in their tissue distribution, amino acid composition, and biological role [1,2,3].

All Ig-like domains appear to be involved in binding functions. The ligands range from small molecules (antigens, chromophores), to hormones (growth hormone, interferons, prolactin), up to giant molecules (muscle proteins) [3]. Binding sites are localized either in the loop regions (the most variable parts of the immunoglobulins) or in strands. For instance, distinct areas of the sheets are used to bind the ligands of the MHC, CD8, CD4, and PapD molecules or of the growth hormone receptor (GHR). These binding sites may be formed by a single chain (CD2, CD4), by homodimers (GHR, CD8), or by heterodimers [3].

Classical Ig-like domains are composed of 7 to 10 β strands, distributed between two sheets with typical topology and connectivity described as a Greek key β-barrel (see <PDB:3HLA>). The general shape of Ig-like domains is well conserved, but they can differ significantly in their size, owing to high variability of the loops. While a classical domain contains about 100 residues, smaller ones (74-90 residues) have been observed in several Ig-related molecules (CD2, CD4). Large decorations within loops, sometimes including extra domains, are found in hemocyanin (238 amino acids) [4] and transcription factor NFkappaB (201 amino acids) [5]. The schematic representation of the structure of a typical Ig-like domain is shown below:

               ----------======D======>--
              |                          |
              |    ------<=====E=======--
              |   |
              |   |    --======B======>------
              |   |   |                      |
              |   |    --<=====A=======      |
              |   |                          |
              |   |                          |
              |   |      <=====G=======--    |
              |   |                      |   |
              |    ------======F======>--    |
              |                              |
               ----------<=====C=======------

     '=>': indicates the direction of the beta-strands 'A' to 'G'.

Ig-like domains can be classified according to the numbers of β strands [1,3]:

C1-type: classical Ig-like domain, described in the schematic representation. Sheet I: ABED, sheet II: CFG. Domain C1 is found exclusively in the molecules involved in the immune system:

• Immunoglobulins (Ig).
• Major Histocompatibility Complex (MHC) molecules.
• T-cell receptors (TcR).

C2-type: strand D is deleted and replaced by strand C' directly connected to strand E. Sheet I: ABE, sheet II: C'CFG:

• Second domain of the vascular cell adhesion molecule-1.
• Neural cell adhesion molecule 2.
• Vascular endothelial growth factor receptor 3.
• Fibroblast growth factor receptor 4.
• Interleukin-6 receptor α chain.

V-type:extra strands C' and C" between strand C and D. Sheet I: ABED, sheet II: C"C'CFG:

• Variable domain of the immunoglobulin heavy chain.
• T-cell surface glycoprotein CD8 α chain.
• Viral Hemagglutinin.
• Programed cell death protein 1.
• Neurocan core protein.
• Myelin protein zero.

H-type:extra strand C' between strand C and D. Sheet I: ABE, sheet II: CFG. Strand C'/D links sheet I and II:

• N-terminal domain of cellulase c.
• C-terminal domain of galactose oxidase.

We developed a profile based on structural alignments that covers the whole domain and recognizes Ig-like domains closely related to immunoglobulin. Our profiles does not recognise H-type or DNA binding Ig-like domains, because of their high divergence.

May 2003 / First entry.

PROSITE method (with tools and information) covered by this documentation:

IG_LIKE, PS50835; Ig-like domain profile  (MATRIX)

Sequences known to belong to this class detected by the profile: ALL Other sequence(s) detected in Swiss-Prot: 1

• Domain architecture view of Swiss-Prot proteins matching PS50835 • Retrieve an alignment of Swiss-Prot true positive hits:   Clustal format, color, condensed view  / Clustal format, color  / Clustal format, plain text  / Fasta format • Retrieve the sequence logo from the alignment • Taxonomic tree view of all Swiss-Prot/TrEMBL entries matching PS50835 • Retrieve a list of all Swiss-Prot/TrEMBL entries matching PS50835 • Scan Swiss-Prot/TrEMBL entries against PS50835 • view ligand binding statistics

Matching PDB structures: 15C8 1A0Q 1A14 1A1M ... [ALL]

1	Authors	Bork P., Holm L., Sander C.
	Title	The immunoglobulin fold. Structural classification, sequence patterns and common core.
	Source	J. Mol. Biol. 242:309-320(1994).
	PubMed ID	7932691

2	Authors	Halaby D.M., Poupon A., Mornon J.-P.
	Title	The immunoglobulin fold family: sequence analysis and 3D structure comparisons.
	Source	Protein Eng. 12:563-571(1999).
	PubMed ID	10436082

3	Authors	Halaby D.M., Mornon J.-P.
	Title	The immunoglobulin superfamily: an insight on its tissular, species, and functional diversity.
	Source	J. Mol. Evol. 46:389-400(1998).
	PubMed ID	9541533

4	Authors	Volbeda A., Hol W.G.
	Title	Crystal structure of hexameric haemocyanin from Panulirus interruptus refined at 3.2 A resolution.
	Source	J. Mol. Biol. 209:249-279(1989).
	PubMed ID	2585484

5	Authors	Rudolph M.J., Gergen J.P.
	Title	DNA-binding by Ig-fold proteins.
	Source	Nat. Struct. Biol. 8:384-386(2001).
	PubMed ID	11323707
	DOI	10.1038/87531

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