Role of the N-terminal forkhead-associated domain in the cell cycle checkpoint function of the Rad53 kinase

The DNA Replication Checkpoint Targets the Kinetochore for Relocation of Collapsed Forks to the Nuclear Periphery

Hairpin forming expanded CAG/CTG repeats pose significant challenges to DNA replication which can lead to replication fork collapse. Long CAG/CTG repeat tracts relocate to the nuclear pore complex to maintain their integrity. Forks impeded by DNA structures are known to activate the DNA damage checkpoint, thus we asked whether checkpoint proteins play a role in relocation of collapsed forks to the nuclear periphery in S. cerevisiae . We show that relocation of a (CAG/CTG) 130 tract is dependent on activation of the Mrc1/Rad53 replication checkpoint. Further, checkpoint-mediated phosphorylation of the kinetochore protein Cep3 is required for relocation, implicating detachment of the centromere from the spindle pole body. Activation of this pathway leads to DNA damage-induced microtubule recruitment to the repeat. These data suggest a role for the DNA replication checkpoint in facilitating movement of collapsed replication forks to the nuclear periphery by centromere release and microtubule-directed motion.

Highlights

The DNA replication checkpoint initiates relocation of a structure-forming CAG repeat tract to the nuclear pore complex (NPC)The importance of Mrc1 (hClaspin) implicates fork uncoupling as the initial checkpoint signalPhosphorylation of the Cep3 kinetochore protein by Dun1 kinase allows for centromere release, which is critical for collapsed fork repositioningDamage-inducible nuclear microtubules (DIMs) colocalize with the repeat locus and are required for relocation to the NPCEstablishes a new role for the DNA replication and DNA damage checkpoint response to trigger repositioning of collapsed forks within the nucleus.

A new transcription factor for mitosis: in Schizosaccharomyces pombe, the RFX transcription factor Sak1 works with forkhead factors to regulate mitotic expression

Mitotic genes are one of the most strongly oscillating groups of genes in the eukaryotic cell cycle. Understanding the regulation of mitotic gene expression is a key issue in cell cycle control but is poorly understood in most organisms. Here, we find a new mitotic transcription factor, Sak1, in the fission yeast Schizosaccharomyces pombe. Sak1 belongs to the RFX family of transcription factors, which have not previously been connected to cell cycle control. Sak1 binds upstream of mitotic genes in close proximity to Fkh2, a forkhead transcription factor previously implicated in regulation of mitotic genes. We show that Sak1 is the major activator of mitotic gene expression and also confirm the role of Fkh2 as the opposing repressor. Sep1, another forkhead transcription factor, is an activator for a small subset of mitotic genes involved in septation. From yeasts to humans, forkhead transcription factors are involved in mitotic gene expression and it will be interesting to see whether RFX transcription factors may also be involved in other organisms.

Molecular basis of the essential s phase function of the rad53 checkpoint kinase

The essential yeast kinases Mec1 and Rad53, or human ATR and Chk1, are crucial for checkpoint responses to exogenous genotoxic agents, but why they are also required for DNA replication in unperturbed cells remains poorly understood. Here we report that even in the absence of DNA-damaging agents, the rad53-4AQ mutant, lacking the N-terminal Mec1 phosphorylation site cluster, is synthetic lethal with a deletion of the RAD9 DNA damage checkpoint adaptor. This phenotype is caused by an inability of rad53-4AQ to activate the downstream kinase Dun1, which then leads to reduced basal deoxynucleoside triphosphate (dNTP) levels, spontaneous replication fork stalling, and constitutive activation of and dependence on S phase DNA damage checkpoints. Surprisingly, the kinase-deficient rad53-K227A mutant does not share these phenotypes but is rendered inviable by additional phosphosite mutations that prevent its binding to Dun1. The results demonstrate that ultralow Rad53 catalytic activity is sufficient for normal replication of undamaged chromosomes as long as it is targeted toward activation of the effector kinase Dun1. Our findings indicate that the essential S phase function of Rad53 is comprised by the combination of its role in regulating basal dNTP levels and its compensatory kinase function if dNTP levels are perturbed.

A strategy for interaction site prediction between phospho-binding modules and their partners identified from proteomic data

Small and large scale proteomic technologies are providing a wealth of potential interactions between proteins bearing phospho-recognition modules and their substrates. Resulting interaction maps reveal such a dense network of interactions that the functional dissection and understanding of these networks often require to break specific interactions while keeping the rest intact. Here, we developed a computational strategy, called STRIP, to predict the precise interaction site involved in an interaction with a phospho-recognition module. The method was validated by a two-hybrid screen carried out using the ForkHead Associated (FHA)1 domain of Rad53, a key protein of Saccharomyces cerevisiae DNA checkpoint, as a bait. In this screen we detected 11 partners, including Cdc7 and Cdc45, essential components of the DNA replication machinery. FHA domains are phospho-threonine binding modules and the threonines involved in both interactions could be predicted using the STRIP strategy. The threonines T484 and T189 in Cdc7 and Cdc45, respectively, were mutated and loss of binding could be monitored experimentally with the full-length proteins. The method was further tested for the analysis of 63 known Rad53 binding partners and provided several key insights regarding the threonines likely involved in these interactions. The STRIP method relies on a combination of conservation, phosphorylation likelihood, and binding specificity criteria and can be accessed via a web interface at http://biodev.extra.cea.fr/strip/.

Recruitment of Saccharomyces cerevisiae Dnl4-Lif1 complex to a double-strand break requires interactions with Yku80 and the Xrs2 FHA domain

Nonhomologous end joining (NHEJ) in yeast depends on eight different proteins in at least three different functional complexes: Yku70-Yku80 (Ku), Dnl4-Lif1-Nej1 (DNA ligase IV), and Mre11-Rad50-Xrs2 (MRX). Interactions between these complexes at DNA double-strand breaks (DSBs) are poorly understood but critical for the completion of repair. We previously identified two such contacts that are redundantly required for NHEJ, one between Dnl4 and the C terminus of Yku80 and one between the forkhead-associated (FHA) domain of Xrs2 and the C terminus of Lif1. Here, we first show that mutation of the Yku80 C terminus did not impair Ku binding to DSBs, supporting specificity of the mutant defect to the ligase interaction. We next show that the Xrs2-Lif1 interaction depends on Xrs2 FHA residues (R32, S47, R48, and K75) analogous to those known in other proteins to contact phosphorylated threonines. Two potential target threonines in Lif1 (T417 and T387) were inferred by identifying regions similar to a site in the human Lif1 homolog, XRCC4, known to be bound by the FHA domain of polynucleotide kinase. Mutating these threonines, especially T417, abolished the Xrs2-Lif1 interaction and impaired NHEJ epistatically with Xrs2 FHA mutation. Combining mutations that selectively disable the Yku80-Dnl4 and Xrs2-Lif1 interactions abrogated both NHEJ and DNA ligase IV recruitment to a DSB. The collected results indicate that the Xrs-Lif1 and Yku80-Dnl4 interactions are important for formation of a productive ligase-DSB intermediate.

Mdt1 facilitates efficient repair of blocked DNA double-strand breaks and recombinational maintenance of telomeres

DNA recombination plays critical roles in DNA repair and alternative telomere maintenance. Here we show that absence of the SQ/TQ cluster domain-containing protein Mdt1 (Ybl051c) renders Saccharomyces cerevisiae particularly hypersensitive to bleomycin, a drug that causes 3'-phospho-glycolate-blocked DNA double-strand breaks (DSBs). mdt1Delta also hypersensitizes partially recombination-defective cells to camptothecin-induced 3'-phospho-tyrosyl protein-blocked DSBs. Remarkably, whereas mdt1Delta cells are unable to restore broken chromosomes after bleomycin treatment, they efficiently repair "clean" endonuclease-generated DSBs. Epistasis analyses indicate that MDT1 acts in the repair of bleomycin-induced DSBs by regulating the efficiency of the homologous recombination pathway as well as telomere-related functions of the KU complex. Moreover, mdt1Delta leads to severe synthetic growth defects with a deletion of the recombination facilitator and telomere-positioning factor gene CTF18 already in the absence of exogenous DNA damage. Importantly, mdt1Delta causes a dramatic shift from the usually prevalent type II to the less-efficient type I pathway of recombinational telomere maintenance in the absence of telomerase in liquid senescence assays. As telomeres resemble protein-blocked DSBs, the results indicate that Mdt1 acts in a novel blocked-end-specific recombination pathway that is required for the efficiency of both drug-induced DSB repair and telomerase-independent telomere maintenance.

Constitutively high dNTP concentration inhibits cell cycle progression and the DNA damage checkpoint in yeast Saccharomyces cerevisiae

In eukaryotic cells the concentration of dNTP is highest in S phase and lowest in G1 phase and is controlled by ribonucleotide reductase (RNR). RNR activity is eliminated in all eukaryotes in G1 phase by a variety of mechanisms: transcriptional regulation, small inhibitory proteins, and protein degradation. After activation of RNR upon commitment to S phase, dATP feedback inhibition ensures that the dNTP concentration does not exceed a certain maximal level. It is not apparent why limitation of dNTP concentration is necessary in G1 phase. In principle, dATP feedback inhibition should be sufficient to couple dNTP production to utilization. We demonstrate that in Saccharomyces cerevisiae constitutively high dNTP concentration transiently arrests cell cycle progression in late G1 phase, affects activation of origins of replication, and inhibits the DNA damage checkpoint. We propose that fluctuation of dNTP concentration controls cell cycle progression and the initiation of DNA replication.

Ccr4-not complex mRNA deadenylase activity contributes to DNA damage responses in Saccharomyces cerevisiae

DNA damage checkpoints regulate gene expression at the transcriptional and post-transcriptional level. Some components of the yeast Ccr4-Not complex, which regulates transcription as well as transcript turnover, have previously been linked to DNA damage responses, but it is unclear if this involves transcriptional or post-transcriptional functions. Here we show that CCR4 and CAF1, which together encode the major cytoplasmic mRNA deadenylase complex, have complex genetic interactions with the checkpoint genes DUN1, MRC1, RAD9, and RAD17 in response to DNA-damaging agents hydroxyurea (HU) and methylmethane sulfonate (MMS). The exonuclease-inactivating ccr4-1 point mutation mimics ccr4Delta phenotypes, including synthetic HU hypersensitivity with dun1Delta, demonstrating that Ccr4-Not mRNA deadenylase activity is required for DNA damage responses. However, ccr4Delta and caf1Delta DNA damage phenotypes and genetic interactions with checkpoint genes are not identical, and deletions of some Not components that are believed to predominantly function at the transcriptional level rather than mRNA turnover, e.g., not5Delta, also lead to increased DNA damage sensitivity and synthetic HU hypersensitivity with dun1Delta. Taken together, our data thus suggest that both transcriptional and post-transcriptional functions of the Ccr4-Not complex contribute to the DNA damage response affecting gene expression in a complex manner.

Rad53 Kinase Activation-independent Replication Checkpoint Function of the N-terminal Forkhead-associated (FHA1) Domain

Saccharomyces cerevisiae Rad53 has crucial functions in many aspects of the cellular response to DNA damage and replication blocks. To coordinate these diverse roles, Rad53 has two forkhead-associated (FHA) phosphothreonine-binding domains in addition to a kinase domain. Here, we show that the conserved N-terminal FHA1 domain is essential for the function of Rad53 to prevent the firing of late replication origins in response to replication blocks. However, the FHA1 domain is not required for Rad53 activation during S phase, and as a consequence of defective downstream signaling, Rad53 containing an inactive FHA1 domain is hyperphosphorylated in response to replication blocks. The FHA1 mutation dramatically hypersensitizes strains with defects in the cell cycle-wide checkpoint pathways (rad9Delta and rad17Delta) to DNA damage, but it is largely epistatic with defects in the replication checkpoint (mrc1Delta). Altogether, our data indicate that the FHA1 domain links activated Rad53 to downstream effectors in the replication checkpoint. The results reveal an important mechanistic difference to the homologous Schizosaccharomyces pombe FHA domain that is required for Mrc1-dependent activation of the corresponding Cds1 kinase. Surprisingly, despite the severely impaired replication checkpoint and also G(2)/M checkpoint functions, the FHA1 mutation by itself leads to only moderate viability defects in response to DNA damage, highlighting the importance of functionally redundant pathways.

Mdt1, a novel Rad53 FHA1 domain-interacting protein, modulates DNA damage tolerance and G(2)/M cell cycle progression in Saccharomyces cerevisiae

The Rad53 kinase plays a central role in yeast DNA damage checkpoints. Rad53 contains two FHA phosphothreonine-binding domains that are required for Rad53 activation and possibly downstream signaling. Here we show that the N-terminal Rad53 FHA1 domain interacts with the RNA recognition motif, coiled-coil, and SQ/TQ cluster domain-containing protein Mdt1 (YBl051C). The interaction of Rad53 and Mdt1 depends on the structural integrity of the FHA1 phosphothreonine-binding site as well as threonine-305 of Mdt1. Mdt1 is constitutively threonine phosphorylated and hyperphosphorylated in response to DNA damage in vivo. DNA damage-dependent Mdt1 hyperphosphorylation depends on the Mec1 and Tel1 checkpoint kinases, and Mec1 can directly phosphorylate a recombinant Mdt1 SQ/TQ domain fragment. MDT1 overexpression is synthetically lethal with a rad53 deletion, whereas mdt1 deletion partially suppresses the DNA damage hypersensitivity of checkpoint-compromised strains and generally improves DNA damage tolerance. In the absence of DNA damage, mdt1 deletion leads to delayed anaphase completion, with an elongated cell morphology reminiscent of that of G(2)/M cell cycle mutants. mdt1-dependent and DNA damage-dependent cell cycle delays are not additive, suggesting that they act in the same pathway. The data indicate that Mdt1 is involved in normal G(2)/M cell cycle progression and is a novel target of checkpoint-dependent cell cycle arrest pathways.

Diverse but overlapping functions of the two forkhead-associated (FHA) domains in Rad53 checkpoint kinase activation

Forkhead-associated (FHA) domains are phosphothreonine-binding modules prevalent in proteins with important cell cycle and DNA damage response functions. The yeast checkpoint kinase Rad53 is unique in containing two FHA domains. We have generated novel recessive rad53 alleles with abolished FHA domain functions resulting from Ala substitution of the critical phosphothreonine-binding residues Arg70 and Arg605. In asynchronous cells, inactivation of the N-terminal FHA1 domain did not impair Rad53 activation and downstream functions, whereas inactivation of the C-terminal FHA2 domain led to reduced Rad53 activation and significantly increased DNA damage sensitivity. Simultaneous inactivation of both FHA domains abolished Rad53 activation and all downstream functions and dramatically increased the sensitivity to DNA damage and replication blocks similar to kinase-defective and rad53 null alleles, but did not compromise the essential viability function of Rad53. Interestingly, in G2/M synchronized cells, mutation of either FHA domain prevented Rad53 activation and impaired the cell cycle arrest checkpoint. Our data demonstrate that both FHA domains are required for normal Rad53 functions and indicate that the two FHA domains have differential but partially overlapping roles in Rad53 activation and downstream signaling.

Recruitment of Thr 319-phosphorylated Ndd1p to the FHA domain of Fkh2p requires Clb kinase activity: a mechanism for CLB cluster gene activation

Activation of the CLB gene cluster through the assembly of Mcm1p-Fkh2p complexes at target promoters is essential for mitotic entry and transition through M phase. We show that the activation of this mitotic transcriptional program is dependent on the recruitment of Ndd1p, a coactivator that performs its essential function by acting through Fkh2p. Although an essential gene, NDD1 is dispensable in cells expressing a truncated form of Fkh2p lacking its C terminus. When phosphorylated on T319, Ndd1p is recruited to CLB cluster promoters by association with the forkhead-associated (FHA) domain of Fkh2p. Substitution of T319 for alanine significantly reduces recruitment of Ndd1p, resulting in loss of normal transcriptional regulation, severe impairment of cell growth, and a budding defect reminiscent of cells with a Cdk-Clb kinase deficiency. Finally, we show that phosphorylation of T319 and recruitment of Ndd1p to CLB2 and SWI5 promoters is dependent on Cdc28-Clb kinase activity. These data provide a model describing the activation of G2/M transcription through the phosphorylation of Ndd1p by Cdc28-Clb kinase activity.

Nibrin forkhead-associated domain and breast cancer C-terminal domain are both required for nuclear focus formation and phosphorylation

The Mre11.Rad50.nibrin protein complex plays an essential role in the mammalian cellular response to DNA double-strand breaks. The disorder Nijmegen breakage syndrome (NBS) results from mutations in the NBS1 gene that encodes nibrin, and NBS cells are radiosensitive and defective in S-phase checkpoint activation following irradiation. In response to radiation, nibrin is phosphorylated by Atm, and the Mre11.Rad50.nibrin complex relocalizes to form punctate nuclear foci. The N terminus of nibrin contains a forkhead-associated (FHA) domain and a breast cancer C-terminal (BRCT) domain, the functions of which are unclear. To determine the role of the FHA and BRCT domains in nibrin function, we have performed site-directed mutagenesis of conserved residues in these motifs. Mutations in the nibrin FHA and BRCT domains did not affect interaction with Mre11.Rad50 or nuclear localization of the complex. However, mutation of conserved residues in either domain disrupted nuclear focus formation and blocked nibrin phosphorylation after irradiation, suggesting that these events may be functionally interdependent. Despite an effect on nibrin phosphorylation, expression of the FHA or BRCT mutants in NBS cells restored the downstream phosphorylation of Chk2 and Smc1, necessary for S-phase checkpoint activation. None of the mutations revealed separate functions for the FHA or BRCT domains, suggesting they do not function independently.

Cell-cycle responses to DNA damage in G2

Cellular reproduction, at its basic level, is simply the passing of genetic information from a single parent cell into two daughter cells. As the cellular genome encodes all the information that defines a cell, it is crucial that the genome be accurately replicated. Furthermore, the duplicated genome must be properly segregated so that each daughter cell contains the exact same information as the parent cell. The processes by which this occurs is known as the cell cycle. The failure of either duplication or segregation of the genome can have disastrous consequences for an organism, including cancer and death. This article discusses what is known about checkpoints, the surveillance mechanisms that monitor both the fidelity and accuracy of DNA replication and segregation. Specifically, we will focus on the G2 checkpoint that is responsible for ensuring proper segregation of the duplicated genome into the daughter cells and how this checkpoint functions to arrest entry into mitosis in response to DNA damage.

Direct kinase-to-kinase signaling mediated by the FHA phosphoprotein recognition domain of the Dun1 DNA damage checkpoint kinase

The serine-threonine kinase Dun1 contains a forkhead-associated (FHA) domain and functions in the DNA damage checkpoint pathway of Saccharomyces cerevisiae. It belongs to the Chk2 family of checkpoint kinases, which includes S. cerevisiae Rad53 and Mek1, Schizosaccharomyces pombe Cds1, and human Chk2. Dun1 is required for DNA damage-induced transcription of certain target genes, transient G(2)/M arrest after DNA damage, and DNA damage-induced phosphorylation of the DNA repair protein Rad55. Here we report that the FHA phosphoprotein recognition domain of Dun1 is required for direct phosphorylation of Dun1 by Rad53 kinase in vitro and in vivo. trans phosphorylation by Rad53 does not require the Dun1 kinase activity and is likely to involve only a transient interaction between the two kinases. The checkpoint functions of Dun1 kinase in DNA damage-induced transcription, G(2)/M cell cycle arrest, and Rad55 phosphorylation are severely compromised in an FHA domain mutant of Dun1. As a consequence, the Dun1 FHA domain mutant displays enhanced sensitivity to genotoxic stress induced by UV, methyl methanesulfonate, and the replication inhibitor hydroxyurea. We show that the Dun1 FHA domain is critical for direct kinase-to-kinase signaling from Rad53 to Dun1 in the DNA damage checkpoint pathway.

Forkhead-associated domains of the tobacco NtFHA1 transcription activator and the yeast Fhl1 forkhead transcription factor are functionally conserved

NtFHA1 encodes a novel protein containing the forkhead-associated (FHA) domain and the acidic domain in Nicotiana tabacum. NtFHA1 functions as a transactivator and is targeted to the nucleus. The sequence of the FHA domain of NtFHA1 is significantly homologous to that of the Fhl1 forkhead transcription factor of yeast. FHL1 was previously identified as a suppressor of RNA polymerase III mutations, and the fhl1 deletion mutant exhibited severe growth defects and impaired rRNA processing. Ectopic expression of the FHA domain of NtFHA1 (but not its mutant form) resulted in severe growth retardation in yeast. Similarly, expression of Fhl1, its FHA domain, or chimeric Fhl1 containing the NtFHA1 FHA domain also inhibited yeast growth. Yeast cells overexpressing the FHA domains of NtFHA1 and Fhl1 contained lower levels of mature rRNAs and exhibited rRNA-processing defects, similar to the fhl1 null mutant. Chimeric Fhl1 (but not the mutant form with a small deletion in its FHA domain) fully complemented the growth and rRNA-processing defects of the fhl1 null mutant, demonstrating that the FHA domain of NtFHA1 can functionally substitute for the FHA domain of Fhl1. These results demonstrate that the FHA domains of NtFHA1 and Fhl1 are conserved in their structure and function and that the FHA domain of Fhl1 is critically involved in regulation of rRNA processing in yeast. NtFHA1 function in plants may be analogous to Fhl1 function in yeast.

Posttranscriptional regulation of the RAD5 DNA repair gene by the Dun1 kinase and the Pan2-Pan3 poly(A)-nuclease complex contributes to survival of replication blocks

The yeast Dun1 kinase has complex checkpoint functions including DNA damage-dependent cell cycle arrest in G(2)/M, transcriptional induction of repair genes, and regulation of postreplicative DNA repair pathways. Here we report that the Dun1 forkhead-associated domain interacts with the Pan3 subunit of the poly(A)-nuclease complex and that dun1pan2 and dun1pan3 double mutants are dramatically hypersensitive to replicational stress. This phenotype was independent of the function of Dun1 in regulating deoxyribonucleotide levels as it was also observed in strains lacking the ribonucleotide reductase inhibitor Sml1. dun1pan2 mutants initially arrested normally in response to replication blocks but died in the presence of persistent replication blocks with considerably delayed kinetics compared with mutants lacking the Rad53 kinase, indicating that the double mutation does not compromise the intra-S phase checkpoint. Interestingly, the RAD5 gene involved in error-free postreplication repair pathways was specifically up-regulated in dun1pan2 double mutants. Moreover, inducible overexpression of RAD5 mimicked the double mutant phenotype by hypersensitizing dun1 mutants to replication blocks. The data indicate that Dun1 and Pan2-Pan3 cooperate to regulate the stoichiometry and thereby the activity of postreplication repair complexes, suggesting that posttranscriptional mechanisms complement the transcriptional response in the regulation of gene expression by checkpoint signaling pathways in Saccharomyces cerevisiae.

The FHA domain

The forkhead-associated (FHA) domain is a small protein module recently shown to recognize phosphothreonine epitopes on proteins. It is present in a diverse range of proteins in eukaryotic cells, such as kinases, phosphatases, kinesins, transcription factors, RNA-binding proteins, and metabolic enzymes. It is also found in a number of bacterial proteins. This suggests that FHA domain-mediated phospho-dependent assembly of protein complexes is an ancient and widespread regulatory mechanism.

Solution structures of two FHA1-phosphothreonine peptide complexes provide insight into the structural basis of the ligand specificity of FHA1 from yeast Rad53

Rad53, a yeast checkpoint protein involved in regulating the repair of DNA damage, contains two forkhead-associated domains, FHA1 and FHA2. Previous combinatorial library screening has shown that FHA1 strongly selects peptides containing a pTXXD motif. Subsequent location of this motif within the sequence of Rad9, the target protein, coupled with spectroscopic analysis has led to identification of a tight binding sequence that is likely the binding site of FHA1: (188)SLEV(pT)EADATFVQ(200). We present solution structures of FHA1 in complex with this pT-peptide and with another Rad9-derived pT-peptide that has ca 30-fold lower affinity, (148)KKMTFQ(pT)PTDPLE(160). Both complexes showed intermolecular NOEs predominantly between three peptide residues (pT, +1, and +2 residues) and five FHA1 residues (S82, R83, S85, T106, and N107). Furthermore, the following interactions were implicated on the basis of chemical shift perturbations and structural analysis: the phosphate group of the pT residue with the side-chain amide group of N86 and the guanidino group of R70, and the carboxylate group of Asp (at the +3 position) with the guanidino group of R83. The generated structures revealed a similar binding mode adopted by these two peptides, suggesting that pT and the +3 residue Asp are the major contributors to binding affinity and specificity, while +1 and +2 residues could provide additional fine-tuning. It was also shown that FHA1 does not bind to the corresponding pS-peptides or a related pY-peptide. We suggest that differentiation between pT and pS-peptides by FHA1 can be attributed to hydrophobic interactions between the methyl group of the pT residue and the aliphatic protons of R83, S85, and T106 from FHA1.