Members of TALE and WUS subfamilies of homeodomain proteins with potentially important functions in development form dimers within each subfamily in rice

Transacting factors often form homo- and heterodimers and regulate various targets, the type of regulation depending on the dimeric combination. The WUS and TALE subfamilies are two atypical homeodomains in plants. A homeodomain mediates sequence-specific binding to its target DNA and usually consists of 60 amino acid residues, whereas atypical homeodomains have extra amino acid residues in the well-conserved region. The genes OsWUS and OsPRS, which encode atypical homeodomain proteins from the WUS subfamily, and OsBEL and OSH15, which encode those from the TALE subfamily, were isolated from rice and tested for their interactions by yeast two-hybrid analysis. OsWUS and OsPRS formed homodimers and formed heterodimers with each other but did not form dimers with the TALE family homeodomain proteins OSH15 or OsBEL. Likewise, OSH15 and OsBEL formed homodimers and heterodimers but did not form dimers with the WUS family homeodomain proteins OsWUS and OsPRS. These findings suggest that the combinations of dimers are well correlated with the classification of these proteins on the basis of sequence similarity. RT-PCR analysis revealed that expression of OsWUS and OsPRS was detected in the same organs, namely floral buds, roots, and suspension cells. Therefore, it is possible that the proteins encoded by both of these genes function as homo- and heterodimers in planta. These results suggest that, during the evolution of these subfamilies, various combinations of dimers within proteins encoded by paralogous genes were formed and generated independent regulatory networks that enabled complex patterns of plant development.     

Shoot apical meristem function in Arabidopsis requires the combined activities of three BEL1-like homeodomain proteins

In plants, most of the above-ground body is formed post-embryonically by the continuous organogenic potential of the shoot apical meristem (SAM). Proper function of the SAM requires maintenance of a delicate balance between the depletion of stem cell daughters into developing primordia and proliferation of the central stem cell population. Here we show that initiation and maintenance of the Arabidopsis SAM, including that of floral meristems, requires the combinatorial action of three members of the BELL-family of TALE homeodomain proteins, ARABIDOPSIS THALIANA HOMEOBOX 1 (ATH1), PENNYWISE (PNY) and POUND-FOOLISH (PNF). All three proteins interact with the KNOX TALE homeodomain protein STM, and combined lesions in ATH1 , PNY and PNF result in a phenocopy of stm mutations. Therefore, we propose that ath1 pny pnf meristem defects result from loss of combinatorial BELL-STM control. Further, we demonstrate that heterodimerization-controlled cellular localization of BELL and KNOX proteins involves a CRM1/exportin-1-mediated nuclear exclusion mechanism that is probably generic to control the activity of BELL and KNOX combinations. We conclude that in animals and plants corresponding mechanisms regulate the activity of TALE homeodomain proteins through controlled nuclear-cytosolic distribution of these proteins.     

Conformational changes in the PBX homeodomain and C-terminal extension upon binding DNA and HOX-derived YPWM peptides

PBX is a member of the three amino acid loop extension (TALE) class of homeodomains. PBX binds DNA cooperatively with HOX homeodomain proteins that contain a conserved YPWM motif. The amino acids immediately C-terminal to the PBX homeodomain increase the affinity of the homeodomain for its DNA site and HOX proteins. We have determined the structure of the free PBX homeodomain using NMR spectroscopy. Both the PBX homeodomain and the extended PBX homeodomain make identical contacts with a 5'-TGAT-3' DNA site and a YPWM peptide. A fourth alpha-helix, which forms upon binding to DNA, stabilizes the extended PBX structure. Variations in DNA sequence selectivity of heterodimeric PBX-HOX complexes depend on the HOX partner; however, a comparison of five different HOX-derived YPWM peptides showed that each bound to PBX in the same way, differing only in the strength of the association.     

Out of step: The function of TALE homeodomain transcription factors that regulate shoot meristem maintenance and meristem identity

The indeterminate growth pattern displayed by shoots is mediated by the proper maintenance of the shoot meristem.Meristem maintenance is dependent upon the balance of stem cell perpetuation in the cent...     

Characterization of PREP2, a paralog of PREP1, which defines a novel sub-family of the MEINOX TALE homeodomain transcription factors

TALE (three amino acid loop extension) homeodomain proteins include the PBC and the MEINOX sub-families. MEINOX proteins form heterodimer complexes with PBC proteins. Heterodimerization is crucial to DNA binding and for nuclear localization. PBC-MEINOX heterodimers bind DNA also in combination with HOX proteins, thereby modulating their DNA-binding specificity. TALE proteins therefore play crucial roles in multiple developmental and differentiation pathways in vivo. We report the identification and characterization of a novel human gene homologous to PREP1, called PREP2. Sequence comparisons indicate that PREP1 and PREP2 define a novel sub-family of MEINOX proteins, distinct from the MEIS sub-family. PREP2 is expressed in a variety of human adult tissues and displays a more restricted expression pattern than PREP1. PREP2 is capable of heterodimerizing with PBC proteins. Heterodimerization with PBX1 appears to be essential for nuclear localization of both PREP2 and PBX1. A comparison between the functional properties of PREP1 and PREP2 reveals that PREP2-PBX display a faster DNA-dissociation rate than PREP1-PBX heterodimers, suggesting different roles in controlling gene expression. Like PREP1, PREP2-PBX heterodimers are capable of forming ternary complexes with HOXB1. The analysis of some PREP2 in vitro properties suggests a functional diversification among PREP and between PREP and MEIS MEINOX proteins.     

Cell-fate switch of synergid to egg cell in Arabidopsis eostre mutant embryo sacs arises from misexpression of the BEL1-like homeodomain gene BLH1

In Arabidopsis thaliana, the female gametophyte is a highly polarized structure consisting of four cell types: one egg cell and two synergids, one central cell, and three antipodal cells. In this report, we describe the characterization of a novel female gametophyte mutant, eostre, which affects establishment of cell fates in the mature embryo sac. The eostre phenotype is caused by misexpression of the homeodomain gene BEL1-like homeodomain 1 (BLH1) in the embryo sac. It is known that BELL-KNAT proteins function as heterodimers whose activities are regulated by the Arabidopsis ovate family proteins (OFPs). We show that the phenotypic effect of BLH1 overexpression is dependent upon the class II knox gene KNAT3, suggesting that KNAT3 must be expressed and functional during megagametogenesis. Moreover, disruption of At OFP5, a known interactor of KNAT3 and BLH1, partially phenocopies the eostre mutation. Our study indicates that suppression of ectopic activity of BELL-KNOX TALE complexes, which might be mediated by At OFP5, is essential for normal development and cell specification in the Arabidopsis embryo sac. As eostre-1 embryo sacs also show nuclear migration abnormalities, this study suggests that a positional mechanism might be directing establishment of cell fates in early megagametophyte development.     

Genome-wide characterization of the TALE homeodomain family and the KNOX-BLH interaction network in tomato

Plant Molecular Biology - Comprehensive yeast and protoplast two-hybrid analyses illustrated the protein–protein interaction network of the TALE homeodomain protein family, KNOX and BLH...     

The Arabidopsis BEL1-LIKE HOMEODOMAIN proteins SAW1 and SAW2 act redundantly to regulate KNOX expression spatially in leaf margins

In Arabidopsis thaliana, the BEL1-like TALE homeodomain protein family consists of 13 members that form heterodimeric complexes with the Class 1 KNOX TALE homeodomain proteins, including SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP). The BEL1-like protein BELLRINGER (BLR) functions together with STM and BP in the shoot apex to regulate meristem identity and function and to promote correct shoot architecture. We have characterized two additional BEL1-LIKE HOMEODOMAIN (BLH) proteins, SAWTOOTH1 (BLH2/SAW1) and SAWTOOTH2 (BLH4/SAW2) that, in contrast with BLR, are expressed in lateral organs and negatively regulate BP expression. saw1 and saw2 single mutants have no obvious phenotype, but the saw1 saw2 double mutant has increased leaf serrations and revolute margins, indicating that SAW1 and SAW2 act redundantly to limit leaf margin growth. Consistent with this hypothesis, overexpression of SAW1 suppresses overall growth of the plant shoot. BP is ectopically expressed in the leaf serrations of saw1 saw2 double mutants. Ectopic expression of Class 1 KNOX genes in leaves has been observed previously in loss-of-function mutants of ASYMMETRIC LEAVES (AS1). Overexpression of SAW1 in an as1 mutant suppresses the as1 leaf phenotype and reduces ectopic BP leaf expression. Taken together, our data suggest that BLH2/SAW1 and BLH4/SAW2 establish leaf shape by repressing growth in specific subdomains of the leaf at least in part by repressing expression of one or more of the KNOX genes.