<p>The K homology (KH) domain was first identified in the human heterogeneousnuclear ribonucleoprotein (hnRNP) K. It is a domain of around 70 amino acidsthat is present in a wide variety of quite diverse nucleic acid-bindingproteins [<cite idref="PUB00005188"/>]. It has been shown to bind RNA [<cite idref="PUB00005961"/>, <cite idref="PUB00005962"/>]. Like many other RNA-binding motifs, KH motifs are found in one or multiple copies (14 copies in chicken vigilin) and, at least for hnRNP K (three copies) and FMR-1 (two copies), each motif is necessary for <i>in vitro</i> RNA binding activity, suggesting that they may function cooperatively or, in the case of single KH motif proteins (for example, Mer1p), independently [<cite idref="PUB00005188"/>].</p><p>According to structural [<cite idref="PUB00005961"/>, <cite idref="PUB00005962"/>, <cite idref="PUB00007305"/>] analysis the KH domain can be separated in two groups. The first group or type-1 contain a beta-alpha-alpha-beta-beta-alpha structure, whereas in the type-2 the two last beta-sheet are located in the N-terminal part of the domain (alpha-beta-beta-alpha-alpha-beta). Sequence similarity between these two folds are limited to a short region (VIGXXGXXI) in the RNA binding motif. This motif is located between helice 1 and 2 in type-1 and between helice 2 and 3 in type-2. Proteins known to contain a type-1 KH domain include bacterial polyribonucleotide nucleotidyltransferases (<db_xref db="EC" dbkey="2.7.7.8"/>); vertebrate fragile X mental retardation protein 1 (FMR1); eukaryotic heterogeneous nuclear ribonucleoprotein K (hnRNP K), one of at least 20 major proteins that are part of hnRNP particles in mammalian cells; mammalian poly(rC) binding proteins; <taxon tax_id="85549">Artemia salina</taxon> glycine-rich protein GRP33; yeast PAB1-binding protein 2 (PBP2); vertebrate vigilin; and human high-density lipoprotein binding protein (HDL-binding protein).</p><p>More information about these proteins can be found at Protein of the Month: RNA Exosomes [<cite idref="PUB00035573"/>].</p> K Homology, type 1