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35  structures 324  species 0  interactions 3503  sequences 46  architectures

Family: Cys_knot (PF00007)

Summary: Cystine-knot domain

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Cystine knot Edit Wikipedia article

Cystine-knot domain
PDB 1hcn EBI.jpg
Structure of human chorionic gonadotropin.[1]
Pfam clanCL0079

A cystine knot is a protein structural motif containing three disulfide bridges (formed from pairs of cysteine residues). The sections of polypeptide that occur between two of them form a loop through which a third disulfide bond passes, forming a rotaxane substructure. The cystine knot motif stabilizes protein structure and is conserved in proteins across various species.[2][3][4] There are three types of cystine knot, which differ in the topology of the disulfide bonds:[5]

The growth factor cystine knot (GFCK) was first observed in the structure of Nerve Growth Factor, solved by X-ray crystallography and published in 1991 by Tom Blundell in Nature.[6] The GFCK comprises four superfamilies. These include nerve growth factor, transforming growth factor beta, platelet-derived growth factor, and glycoprotein hormones, including human chorionic gonadotropin. These are structurally related due to the presence of the cystine knot motif but differ in sequence.[7] All GFCK structures that have been determined are dimeric, but their dimerization modes in different classes are different.[8]

The presence of the cyclic cystine knot (CCK) motif was discovered when cyclotides were isolated from various plant families. The CCK motif has a cyclic backbone, triple stranded beta sheet, and cystine knot conformation.[10]

There are currently novel proteins being added to the cystine knot motif family, which are called the C-terminal cystine knot (CTCK) proteins. They share approximately 90 amino acid residues in their cysteine-rich C terminal regions.[9]

Inhibitor cystine knot (ICK) is a structural motif with a triple stranded antiparallel beta sheet linked by three disulfide bonds, forming a knotted core. ICK motif can be found under the category of phylum, such as animals and plants. It is usually found in many venom peptides, which is in the venoms of snails, spiders, and scorpions. Peptide K-PVIIA that contains an ICK can undergo a successful enzymatic backbone cyclization. The disulfide connectivity and the common sequence pattern of ICK motif provide the stability of the peptides that supports cyclization. [11]


  1. ^ Wu H, Lustbader JW, Liu Y, Canfield RE, Hendrickson WA (June 1994). "Structure of human chorionic gonadotropin at 2.6 A resolution from MAD analysis of the selenomethionyl protein". Structure. 2 (6): 545–58. doi:10.1016/s0969-2126(00)00054-x. PMID 7922031.
  2. ^ "Cystine Knots". The Cyclotide Webpage.
  3. ^ Sherbet, G.V. (2011), "Growth Factor Families", Growth Factors and Their Receptors in Cell Differentiation, Cancer and Cancer Therapy, Elsevier, pp. 3–5, doi:10.1016/b978-0-12-387819-9.00002-5, ISBN 9780123878199, retrieved 2019-05-01
  4. ^ Vitt, Ursula A.; Hsu, Sheau Y.; Hsueh, Aaron J. W. (2001-05-01). "Evolution and Classification of Cystine Knot-Containing Hormones and Related Extracellular Signaling Molecules". Molecular Endocrinology. 15 (5): 681–694. doi:10.1210/mend.15.5.0639. ISSN 0888-8809. PMID 11328851.
  5. ^ Daly NL, Craik DJ (June 2011). "Bioactive cystine knot proteins". Current Opinion in Chemical Biology. 15 (3): 362–8. doi:10.1016/j.cbpa.2011.02.008. PMID 21362584.
  6. ^ PDB: 1bet​; McDonald NQ, Lapatto R, Murray-Rust J, Gunning J, Wlodawer A, Blundell TL (December 1991). "New protein fold revealed by a 2.3-A resolution crystal structure of nerve growth factor". Nature. 354 (6352): 411–4. Bibcode:1991Natur.354..411M. doi:10.1038/354411a0. PMID 1956407.
  7. ^ Sun PD, Davies DR (1995). "The cystine-knot growth-factor superfamily". Annual Review of Biophysics and Biomolecular Structure. 24 (1): 269–91. doi:10.1146/ PMID 7663117.
  8. ^ Jiang X, Dias JA, He X (January 2014). "Structural biology of glycoprotein hormones and their receptors: insights to signaling". Molecular and Cellular Endocrinology. 382 (1): 424–451. doi:10.1016/j.mce.2013.08.021. PMID 24001578.
  9. ^ a b Iyer S, Acharya KR (November 2011). "Tying the knot: the cystine signature and molecular-recognition processes of the vascular endothelial growth factor family of angiogenic cytokines". The FEBS Journal. 278 (22): 4304–22. doi:10.1111/j.1742-4658.2011.08350.x. PMC 3328748. PMID 21917115.
  10. ^ Craik DJ, Daly NL, Bond T, Waine C (December 1999). "Plant cyclotides: A unique family of cyclic and knotted proteins that defines the cyclic cystine knot structural motif". Journal of Molecular Biology. 294 (5): 1327–36. doi:10.1006/jmbi.1999.3383. PMID 10600388.
  11. ^ Kwon, Soohyun; Bosmans, Frank; Kaas, Quentin; Cheneval, Oliver; Cinibear, Anne C; Rosengren, K Johan; Wang, Conan K; Schroeder, Christina I; Craik, David J (19 April 2016). "Efficient enzymatic cyclization of an inhibitory cystine knot‐containing peptide". Biotechnology and Bioengineering. 113 (10): 2202–2212. doi:10.1002/bit.25993. PMC 5526200. PMID 27093300.

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.

Cystine-knot domain Provide feedback

The family comprises glycoprotein hormones and the C-terminal domain of various extracellular proteins. It is believed to be involved in disulfide-linked dimerisation.

Literature references

  1. Bork P; , FEBS Lett 1993;327:125-130.: The modular architecture of a new family of growth regulators related to connective tissue growth factor. PUBMED:7687569 EPMC:7687569

Internal database links

External database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR006208

This domain is found at the C-terminal of glycoprotein hormones and various extracellular proteins. It is believed to be involved in disulphide-linked dimerisation [ PUBMED:7687569 ].

Domain organisation

Below is a listing of the unique domain organisations or architectures in which this domain is found. More...

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Pfam Clan

This family is a member of clan Cystine-knot (CL0079), which has the following description:

The cytokine families in this clan have the cystine-knot fold. In this 6 cysteines form three disulphide bridges that are interlinked.

The clan contains the following 14 members:

Coagulin Cys_knot Cys_Knot_tox D_CNTX DAN Hormone_6 IL17 m_DGTX_Dc1a_b_c NGF Noggin PDGF Sclerostin Spaetzle TGF_beta


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Curation and family details

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Curation View help on the curation process

Seed source: Published_alignment enriched with PDOC00234 members.
Previous IDs: none
Type: Domain
Sequence Ontology: SO:0000417
Author: Sonnhammer ELL
Number in seed: 24
Number in full: 3503
Average length of the domain: 95.70 aa
Average identity of full alignment: 26 %
Average coverage of the sequence by the domain: 33.75 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 57096847 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 21.3 21.3
Trusted cut-off 21.3 21.3
Noise cut-off 21.2 21.2
Model length: 105
Family (HMM) version: 24
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Species distribution

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Archea Archea Eukaryota Eukaryota
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Viruses Viruses Unclassified Unclassified
Viroids Viroids Unclassified sequence Unclassified sequence


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For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Cys_knot domain has been found. There are 35 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein sequence.

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