A homeobox is a DNA sequence, around 180 base pairs long, found within genes that are involved in the regulation of patterns of anatomical development (morphogenesis) in animals, fungi and plants. These genes encode homeodomain protein products that are transcription factors sharing a characteristic protein fold structure that binds DNA.
Homeoboxes were discovered independently in 1983 by Ernst Hafen, Michael Levine, and William McGinnis working in the lab of Walter Jakob Gehring at the University of Basel, Switzerland; and by Matthew P. Scott and Amy Weiner, who were then working with Thomas Kaufman at Indiana University in Bloomington. The existence of homeoboxes was first discovered in Drosophila, where the radical alterations that resulted from mutations in homeobox genes were termed homeotic mutations. One of the most famous such mutation is Antennapedia, in which legs grow from the head of a fly instead of the expected antennae.
A homeobox is about 180 base pairs long and encodes a protein domain that binds DNA. The following shows the consensus homeodomain (~60 amino acid residue chain), with typical insertion sites noted with dashes:
The characteristic homeodomain protein fold consists of a 60-amino acid helix-turn-helix (HTH) structure in which three alpha helices are connected by short loop regions. The N-terminal two helices are antiparallel and the longer C-terminal helix is roughly perpendicular to the axes established by the first two. It is this third helix that interacts directly with DNA via a number of hydrogen bonds and hydrophobic interactions, which occur between specific side chains and the exposed bases and thymine methyl groups within the major groove of the DNA.
Homeodomain proteins are found exclusively in eukaryotes but have high homology to lambda phage proteins that alter the expression of genes in prokaryotes. The motif is very similar in sequence and structure in a wide range of DNA-binding proteins (e.g., cro and repressor proteins, homeotic proteins, etc.). One of the principal differences between HTH motifs in these different proteins arises from the stereo-chemical requirement for glycine in the turn which is needed to avoid steric interference of the beta-carbon with the main chain: for cro and repressor proteins the glycine appears to be mandatory, whereas for many of the homeotic and other DNA-binding proteins the requirement is relaxed.
Homeodomains can bind both specifically and nonspecifically to B-DNA with the C-terminal recognition helix aligning in the DNA's major groove and the unstructured peptide "tail" at the N-terminus aligning in the minor groove. The recognition helix and the inter-helix loops are rich in arginine and lysine residues, which form hydrogen bonds to the DNA backbone; conserved hydrophobic residues in the center of the recognition helix aid in stabilizing the helix packing. Homeodomain proteins show a preference for the DNA sequence 5'-ATTA-3'; sequence-independent binding occurs with significantly lower affinity.
Through the DNA-recognition properties of the homeodomain, homeoproteins are believed to regulate the expression of targeted genes and direct the formation of many body structures during early embryonic development. Many homeodomain proteins induce cellular differentiation by initiating the cascades of coregulated genes required to produce individual tissues and organs. Other proteins in the family, such as NANOG are involved in maintaining pluripotency. Homeobox genes are critical in the establishment of body axes during embryogenesis.
Homeoprotein transcription factors typically switch on cascades of other genes. The homeodomain binds DNA in a sequence-specific manner. However, the specificity of a single homeodomain protein is usually not enough to recognize only its desired target genes. Most of the time, homeodomain proteins act in the promoter region of their target genes as complexes with other transcription factors. Such complexes have a much higher target specificity than a single homeodomain protein. Homeodomains are encoded both by genes of the Hox gene clusters and by other genes throughout the genome.
Hox genes are essential metazoan genes as they determine the identity of embryonic regions along the anterio-posterior axis. The first vertebrate Hox gene was isolated in Xenopus by Eddy De Robertis and colleagues in 1984, marking the beginning of the young science of evolutionary developmental biology ("evo-devo").
In vertebrates, the four paralog clusters are partially redundant in function, but have also acquired several derived functions. In particular, HoxA and HoxD specify segment identity along the limb axis.
The main interest in this set of genes stems from their unique behaviour. They are typically found in an organized cluster. The linear order of the genes within a cluster is directly correlated to the order of the regions they affect as well as the timing in which they are affected. This phenomenon is called colinearity. Due to this linear relationship, changes in the gene cluster due to mutations generally result in similar changes in the affected regions.
For example, when one gene is lost the segment develops into a more anterior one, while a mutation that leads to a gain of function causes a segment to develop into a more posterior one. This is called ectopia. Famous examples are Antennapedia and bithorax in Drosophila, which can cause the development of legs instead of antennae and the development of a duplicated thorax, respectively.
Molecular evidence shows that some limited number of Hox genes have existed in the Cnidaria since before the earliest true Bilatera, making these genes pre-Paleozoic. Homeobox genes have even been found in fungi, for example the unicellular yeasts, and in plants.
The plant homeotic genes code for the typical 60 amino acid long DNA-binding homeodomain or in case of the TALE (three amino acid loop extension) homeobox genes for an "atypical" homeodomain consisting of 63 amino acids. According to their conserved intron–exon structure and to unique codomain architectures they have been grouped into 14 distinct classes: HD-ZIP I to IV, BEL, KNOX, PLINC, WOX, PHD, DDT, NDX, LD, SAWADEE and PINTOX. Conservation of codomains suggests a common eukaryotic ancestry for TALE and non-TALE homeodomain proteins.
Humans generally contain Hox genes in four clusters:
|HOXA (or sometimes HOX1) - HOXA@||chromosome 7||HOXA1, HOXA2, HOXA3, HOXA4, HOXA5, HOXA6, HOXA7, HOXA9, HOXA10, HOXA11, HOXA13|
|HOXB - HOXB@||chromosome 17||HOXB1, HOXB2, HOXB3, HOXB4, HOXB5, HOXB6, HOXB7, HOXB8, HOXB9, HOXB13|
|HOXC - HOXC@||chromosome 12||HOXC4, HOXC5, HOXC6, HOXC8, HOXC9, HOXC10, HOXC11, HOXC12, HOXC13|
|HOXD - HOXD@||chromosome 2||HOXD1, HOXD3, HOXD4, HOXD8, HOXD9, HOXD10, HOXD11, HOXD12, HOXD13|
There is also a "distal-less homeobox" family: DLX1, DLX2, DLX3, DLX4, DLX5, and DLX6. Dlx genes are involved in the development of the nervous system and of limbs. Other examples include HESX homeobox 1 (HESX1) and short stature homeobox gene (SHOX).
Additional human proteins containing this domain per UniProt annotation:
- ADNP; ALX3; ALX4; ARGFX; ARX;
- NKX3-2 (BAPX1); BARHL1; BARHL2; BARX1; BARX2;
- ALX1 (CART1); CDX1; CDX2; CDX4; CRX; CUTL1; CUTL2;
- DBX1; DBX2; DLX1; DLX2; DLX3; DLX4; DLX5; DLX6; DMBX1; DPRX; DRGX; DUX1; DUX2; DUX3; DUX4; DUX4c; DUX5; DUXA;
- EMX1; EMX2; EN1; EN2; ESX1L; EVX1; EVX2;
- GBX1; GBX2; GSC; GSC2; GSX1; GSX2;
- HDX; HESX1; HHEX; HLX1; HMBOX1; HMX1; HMX2; HMX3; HNF1A; HNF1B; HOMEZ; HOPX
- IRX1; IRX2; IRX3; IRX4; IRX5; IRX6; ISL1; ISL2; ISX
- LASS2; LASS3; LASS4; LASS5; LASS6; LBX1; LBX2; LHX1; LHX2; LHX3; LHX4; LHX5; LHX6; LHX8; LHX9; LMX1A; LMX1B
- MEIS1; MEIS2; MEIS3; MEOX1; MEOX2; MIXL1; MKX; MNX1; MSX1; MSX2
- NANOG; NANOGP1; NANOGP8; NKX2-1; NKX2-2; NKX2-3; NKX2-4; NKX2-5; NKX2-8; NKX3-1; NKX3-2; NKX6-1; NKX6-2; NKX6-3; NOBOX; NOTO
- ONECUT1; ONECUT2; ONECUT3; OTP; OTX1; OTX2
- PAX3; PAX4; PAX6; PAX7; PBX1; PBX2; PBX3; PBX4; PDX1; PHOX2A; PHOX2B; PITX1; PITX2; PITX3; PKNOX1; PKNOX2; POU1F1; POU2F1; POU2F2; POU2F3; POU3F1; POU3F2; POU3F3; POU3F4; POU4F1; POU4F2; POU4F3; POU5F1; POU5F1P1; POU5F1P4; POU5F2; POU6F1; POU6F2; PROP1; PRRX1; PRRX2
- RAX; RAX2; RHOXF1; RHOXF2
- SATB1; SATB2; SEBOX; SHOX; SHOX2; SIX1; SIX2; SIX3; SIX4; SIX5; SIX6
- TGIF1; TGIF2; TGIF2LX; TGIF2LY; TLX1; TLX2; TLX3; TSHZ1; TSHZ2; TSHZ3
- UNCX; VAX1; VAX2; VENTX; VSX1; VSX2
- ZEB1; ZEB2; ZFHX2; ZFHX3; ZFHX4; ZHX1
Mutations to homeobox genes can produce easily visible phenotypic changes.
Two examples of homeobox mutations in the above-mentioned fruit fly are legs where the antennae should be (antennapedia), and a second pair of wings.
Duplication of homeobox genes can produce new body segments, and such duplications are likely to have been important in the evolution of segmented animals. However, Hox genes typically determine the identity of body segments.
Interestingly, there is one insect family, the xyelid sawflies, in which both the antennae and mouthparts are remarkably leg-like in structure. This is not uncommon in arthropods as all arthropod appendages are homologous.
The regulation of Hox genes is highly complex and involves reciprocal interactions, mostly inhibitory. Drosophila is known to use the Polycomb and Trithorax Complexes to maintain the expression of Hox genes after the down-regulation of the pair-rule and gap genes that occurs during larval development. Polycomb-group proteins can silence the HOX genes by modulation of chromatin structure.
Proteins containing a POU region consist of a homeodomain and a separate, structurally homologous POU domain that contains two helix-turn-helix motifs and also binds DNA. The two domains are linked by a flexible loop that is long enough to stretch around the DNA helix, allowing the two domains to bind on opposite sides of the target DNA, collectively covering an eight-base segment with consensus sequence 5'-ATGCAAAT-3'. The individual domains of POU proteins bind DNA only weakly, but have strong sequence-specific affinity when linked. The POU domain itself has significant structural similarity with repressors expressed in bacteriophages, particularly lambda phage.
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- The Homeodomain Resource (National Human Genome Research Institute, National Institutes of Health)
- HomeoDB: a database of homeobox genes diversity. Zhong YF, Butts T, Holland PWH, since 2008.
- Eukaryotic Linear Motif resource motif class LIG_HOMEOBOX
- Homeobox at the US National Library of Medicine Medical Subject Headings (MeSH)