The K homology (KH) domain was first identified in the human heterogeneous nuclear ribonucleoprotein (hnRNP) K. An evolutionarily conserved sequence of around 70 amino acids, the KH domain is present in a wide variety of nucleic acid-binding proteins. The KH domain binds RNA, and can function in RNA recognition [1]. It is found in multiple copies in several proteins, where they can function cooperatively or independently. For example, in the AU-rich element RNA-binding protein KSRP, which has 4 KH domains, KH domains 3 and 4 behave as independent binding modules to interact with different regions of the AU-rich RNA targets [2]. The solution structure of the first KH domain of FMR1 [2] and of the C-terminal KH domain of hnRNP K [3] determined by nuclear magnetic resonance(NMR) revealed a beta-alpha-alpha-beta-beta-alpha structure. Proteins containing KH domains include:

• Bacterial and organelle PNPases [4].
• Archaeal and eukaryotic exosome subunits [5].
• Eukaryotic and prokaryotic RS3 ribosomal proteins [6].
• Vertebrate fragile X mental retardation protein 1 (FMR1) [7].
• Vigilin, which has 14 KH domains [8].
• AU-rich element RNA-binding protein KSRP.
• hnRNP K, which contains 3 KH domains.
• Human onconeural ventral antigen-1 (NOVA-1) [9].

According to structural analyses [10, 10, 10], the KH domain can be separated in two groups - type 1 and type 2.

1. The structure of the C-terminal KH domains of KSRP reveals a noncanonical motif important for mRNA degradation. Structure 15, 485-98
2. The solution structure of the first KH domain of FMR1, the protein responsible for the fragile X syndrome. Nat. Struct. Biol. 4, 712-6
3. High precision solution structure of the C-terminal KH domain of heterogeneous nuclear ribonucleoprotein K, a c-myc transcription factor. J. Mol. Biol. 289, 949-62
4. The KH and S1 domains of Escherichia coli polynucleotide phosphorylase are necessary for autoregulation and growth at low temperature. Biochim. Biophys. Acta 1769, 194-203
5. Structural and biochemical characterization of the yeast exosome component Rrp40. EMBO Rep. 8, 63-9
6. [Transport of 131I-hippuric acid and 22Na from the space between retina and pigment epithelium after experimental amotio] Genes Dev. 15, 115-8
7. Kissing complex RNAs mediate interaction between the Fragile-X mental retardation protein KH2 domain and brain polyribosomes. Cell. Mol. Life Sci. 19, 903-18
8. The multi-KH protein vigilin associates with free and membrane-bound ribosomes. null 60, 2219-27
9. Crystal structures of Nova-1 and Nova-2 K-homology RNA-binding domains. Structure 7, 191-203
10. KH domain: one motif, two folds. Nucleic Acids Res. 29, 638-43

The K homology (KH) domain was first identified in the human heterogeneous nuclear ribonucleoprot
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