Figure 1Sequence similarity of the PRH proteins from different vertebrate species
Upper part of the Figure: phylogenetic analysis of PRH. Lower part of the Figure: the amino acid sequence of mouse PRH (AAH57986), rat PRH (NP_077361), zebrafish PRH (NP_571009), Xenopus PRH (NP_989420), avian PRH (Q05502) and human PRH (Q03014) were aligned according to the ClustalW method. Identical residues are highlighted in yellow and similar residues in blue and green. The green line represents the N-terminal domain of PRH, the blue line the PRH homeodomain and the red line the C-terminal domain of PRH.
A schematic showing the PRH protein divided into three main domains: an N-terminal proline-rich domain (green; NT), a homeodomain (blue; HD) and an acidic C-terminal domain (red; CT). The correlated function of each region of the protein is also illustrated in the Figure. huPRH, human PRH.
Figure 3A molecular model of the PRH homeodomain bound to DNA
Upper part of the Figure: the amino acid sequences of the homeodomain proteins Engrailed, Oct-2, Msx-1 and PRH were aligned according to their three-dimensional structures. The alignment was carried out using the three-dimensional superimpose program CE. Lower part of the Figure: the structure of the three aligned homeodomain proteins Engrailed (blue), Oct-2 (yellow), Msx-1 (green) and PRH (magenta) bound to DNA (red). This molecular model shows that the PRH homeodomain contacts the major groove of DNA via helix III (the recognition helix) and the minor groove via the N-terminal arm. The loop between helix I and helix II makes extra contacts with the opposing phosphate backbone of DNA. This model shows that the PRH homeodomain binds to DNA following the general mode of specific DNA recognition employed by other homeodomain proteins. An interactive three-dimensional version of this structure can be found at http://www.BiochemJ.org/bj/412/0399/bj4120399add.htm.
The representation on the left-hand side shows the two-dimensional (2D) open octamer. The representation on the right-hand side shows the three-dimensional (3D) model of the closed octamer. The P region represents amino acids 1–50, the R region amino acids 50–137 and the H region amino acids 137–270. In the octameric form of PRH, the R region interacts with the H region in a head-to-tail orientation via domain-swapping of two PRH molecules, whereas the P region interacts with another P region in a head-to-head orientation (dimerization).
Figure 5A representative model of transcription repression facilitated by PRH oligomerization
In the open state of chromatin, PRH octamers (green) bind specifically to multiple sites within a target promoter. Further self-association of PRH along the chromatin fibre will exclude the transcription machinery and bring about transcription repression (the repressed state). Recruitment of co-repressor proteins such as Groucho/TLE results in a condensed state of chromatin.
Figure 6Diagram showing the different steps of haematopoiesis and vasculogenesis
Haematopoiesis and vasculogenesis begin in the blood islands. The inner part of these blood islands becomes HSCs, whereas the outer part gives rise to the vascular endothelial progenitors (angioblasts). The endothelial and haematopoietic precursors are thought to originate from the common mesodermal progenitor, the haemangioblast. HSCs differentiate to all cells found in the circulating blood, and the angioblasts develop to form the vascular endothelial system. PRH is expressed in the haemangioblast/haematopoietic stem cell but not in angioblasts or their derivatives. PRH is expressed in differentiating haematopoietic cells of B-cell lineages and myelomonocytic and erythroid lineages, but not in T-cell lineages. dpc, days post coitum.
Figure 7The expression of PRH during the early development of mouse embryos [from 5–7 dpc (days post coitum)]
At 5 dpc PRH is expressed in the AVE precursors, located at the distal tip of the embryo. The AVE moves towards the anterior–proximal region signalling to the epiblast in the posterior region where the primitive streak is formed at 6.5 dpc. During gastrulation the primitive streak elongates and cells at the tip form the node. The mesoderm and the definitive endoderm are also formed at this stage (7 dpc). The ADE where PRH is expressed moves anteriorly, displacing the AVE towards the extra-embryonic region.
Figure 8Schematic representation of PRH expression in the endoderm after gastrulation
At 7 dpc (days post coitum) PRH is expressed in the ADE (shaded). The ADE develops to give rise to the foregut (9 dpc). This region is fated to form the liver, pancreas, lungs, thyroid and thymus. PRH expression continues primarily in tissues that are derived from the foregut endoderm including liver, lung, thyroid, thymus and pancreas. The expression of PRH in the ADE is required for the normal development of the forebrain, which is derived from the overlying ectoderm. PRH expression in the ADE underlying the presumptive cardiac mesoderm is also required for cardiogenesis in vertebrate embryos.
Figure 9Model of PRH regulating gene expression in response to multiple developmental and cell-growth-related cues
A single octamer of PRH is shown bound to DNA and also to multiple PRH partners allowing PRH to respond to multiple signalling pathways simultaneously or temporally. For clarity, each subunit of PRH is not shown bound to DNA or bound to every interacting partner.