Plk1 self-organization and priming phosphorylation of HsCYK-4 at the spindle midzone regulate the onset of division in human cells.

Animal cells initiate cytokinesis in parallel with anaphase onset, when an actomyosin ring assembles and constricts through localized activation of the small GTPase RhoA, giving rise to a cleavage furrow. Furrow formation relies on positional cues provided by anaphase spindle microtubules (MTs), but how such cues are generated remains unclear. Using chemical genetics to achieve both temporal and spatial control, we show that the self-organized delivery of Polo-like kinase 1 (Plk1) to the midzone and its local phosphorylation of a MT-bound substrate are critical for generating this furrow-inducing signal. When Plk1 was active but unable to target itself to this equatorial landmark, both cortical RhoA recruitment and furrow induction failed to occur, thus recapitulating the effects of anaphase-specific Plk1 inhibition. Using tandem mass spectrometry and phosphospecific antibodies, we found that Plk1 binds and directly phosphorylates the HsCYK-4 subunit of centralspindlin (also known as MgcRacGAP) at the midzone. At serine 157, this modification creates a major docking site for the tandem BRCT repeats of the Rho GTP exchange factor Ect2. Cells expressing only a nonphosphorylatable form of HsCYK-4 failed to localize Ect2 at the midzone and were severely impaired in cleavage furrow formation, implying that HsCYK-4 is Plk1's rate-limiting target upstream of RhoA. Conversely, tethering an inhibitor-resistant allele of Plk1 to HsCYK-4 allowed furrows to form despite global inhibition of all other Plk1 molecules in the cell. Our findings illuminate two key mechanisms governing the initiation of cytokinesis in human cells and illustrate the power of chemical genetics to probe such regulation both in time and space.

The NDR/LATS family kinase Cbk1 directly controls transcriptional asymmetry.

Cell fate can be determined by asymmetric segregation of gene expression regulators. In the budding yeast Saccharomyces cerevisiae, the transcription factor Ace2 accumulates specifically in the daughter cell nucleus, where it drives transcription of genes that are not expressed in the mother cell. The NDR/LATS family protein kinase Cbk1 is required for Ace2 segregation and function. Using peptide scanning arrays, we determined Cbk1's phosphorylation consensus motif, the first such unbiased approach for an enzyme of this family, showing that it is a basophilic kinase with an unusual preference for histidine -5 to the phosphorylation site. We found that Cbk1 phosphorylates such sites in Ace2, and that these modifications are critical for Ace2's partitioning and function. Using proteins marked with GFP variants, we found that Ace2 moves from isotropic distribution to the daughter cell nuclear localization, well before cytokinesis, and that the nucleus must enter the daughter cell for Ace2 accumulation to occur. We found that Cbk1, unlike Ace2, is restricted to the daughter cell. Using both in vivo and in vitro assays, we found that two critical Cbk1 phosphorylations block Ace2's interaction with nuclear export machinery, while a third distal modification most likely acts to increase the transcription factor's activity. Our findings show that Cbk1 directly controls Ace2, regulating the transcription factor's activity and interaction with nuclear export machinery through three phosphorylation sites. Furthermore, Cbk1 exhibits a novel specificity that is likely conserved among related kinases from yeast to metazoans. Cbk1 is functionally restricted to the daughter cell, and cannot diffuse from the daughter to the mother. In addition to providing a mechanism for Ace2 segregation, these findings show that an isotropically distributed cell fate determinant can be asymmetrically partitioned in cytoplasmically contiguous cells through spatial segregation of a regulating protein kinase.

Natural-like function in artificial WW domains.

Protein sequences evolve through random mutagenesis with selection for optimal fitness. Cooperative folding into a stable tertiary structure is one aspect of fitness, but evolutionary selection ultimately operates on function, not on structure. In the accompanying paper, we proposed a model for the evolutionary constraint on a small protein interaction module (the WW domain) through application of the SCA, a statistical analysis of multiple sequence alignments. Construction of artificial protein sequences directed only by the SCA showed that the information extracted by this analysis is sufficient to engineer the WW fold at atomic resolution. Here, we demonstrate that these artificial WW sequences function like their natural counterparts, showing class-specific recognition of proline-containing target peptides. Consistent with SCA predictions, a distributed network of residues mediates functional specificity in WW domains. The ability to recapitulate natural-like function in designed sequences shows that a relatively small quantity of sequence information is sufficient to specify the global energetics of amino acid interactions.

Structure and mechanism of BRCA1 BRCT domain recognition of phosphorylated BACH1 with implications for cancer.

Germline mutations in the BRCA1 tumor suppressor gene often result in a significant increase in susceptibility to breast and ovarian cancers. Although the molecular basis of their effects remains largely obscure, many mutations are known to target the highly conserved C-terminal BRCT repeats that function as a phosphoserine/phosphothreonine-binding module. We report the X-ray crystal structure at a resolution of 1.85 A of the BRCA1 tandem BRCT domains in complex with a phosphorylated peptide representing the minimal interacting region of the DEAH-box helicase BACH1. The structure reveals the determinants of this novel class of BRCA1 binding events. We show that a subset of disease-linked mutations act through specific disruption of phospho-dependent BRCA1 interactions rather than through gross structural perturbation of the tandem BRCT domains.

A coupled chemical-genetic and bioinformatic approach to Polo-like kinase pathway exploration.

Protein phosphorylation is a ubiquitous mechanism for cellular signal propagation, and signaling network complexity presents a challenge to protein kinase substrate identification. Few targets of Polo-like kinases are known, despite their significant role in coordinating cell-cycle progression. Here, we combine chemical-genetic, bioinformatic, and proteomic tools for Polo-like kinase substrate identification. Specific pharmacological inhibition of budding yeast Polo-like kinase, Cdc5, resulted in a misaligned preanaphase spindle and subsequently delayed anaphase nuclear migration, revealing a Cdc5 function. A cellular screen for Cdc5 substrates identified Spc72, a spindle pole body (SPB) component and microtubule anchor required for nuclear positioning. Spc72 bound to the Cdc5 PBD in a mitosis-specific manner, was phosphorylated by Cdc5 in vitro, and demonstrated a loss of mitotic phosphorylation in vivo upon Cdc5 inhibition. Finally, an examination of Cdc5 binding by SPB-localized proteins expanded our knowledge of Cdc5 function at the SPB.

Structure and function of Polo-like kinases.

Polo-like kinases play critical roles during multiple stages of cell cycle progression. All Polo-like kinases contain an N-terminal Ser/Thr kinase catalytic domain and a C-terminal region that contains one or two Polo-boxes. For Polo-like kinase 1, 2, and 3, and their homologs, the entire C-terminal region, including both Polo-boxes, functions as a single modular phosphoserine/threonine-binding domain known as the Polo-box domain (PBD). In the absence of a bound substrate, the PBD inhibits the basal activity of the kinase domain. Phosphorylation-dependent binding of the PBD to its ligands releases the kinase domain, while simultaneously localizing Polo-like kinases to specific subcellular structures. These observations suggest two different models for how the PBD integrates signals arising from other mitotic kinases to target the activated kinase towards distinct substrates. The recent X-ray crystal structures of the PBD provide insights into the structural basis for PBD function and kinase regulation. Molecular modelling of the structure of the isolated kinase domain reveals a potential basis for motif-dependent substrate specificity.

Proteomic screen defines the Polo-box domain interactome and identifies Rock2 as a Plk1 substrate.

Polo-like kinase-1 (Plk1) phosphorylates a number of mitotic substrates, but the diversity of Plk1-dependent processes suggests the existence of additional targets. Plk1 contains a specialized phosphoserine-threonine binding domain, the Polo-box domain (PBD), postulated to target the kinase to its substrates. Using the specialized PBD of Plk1 as an affinity capture agent, we performed a screen to define the mitotic Plk1-PBD interactome by mass spectrometry. We identified 622 proteins that showed phosphorylation-dependent mitosis-specific interactions, including proteins involved in well-established Plk1-regulated processes, and in processes not previously linked to Plk1 such as translational control, RNA processing, and vesicle transport. Many proteins identified in our screen play important roles in cytokinesis, where, in mammalian cells, the detailed mechanistic role of Plk1 remains poorly defined. We go on to characterize the mitosis-specific interaction of the Plk1-PBD with the cytokinesis effector kinase Rho-associated coiled-coil domain-containing protein kinase 2 (Rock2), demonstrate that Rock2 is a Plk1 substrate, and show that Rock2 colocalizes with Plk1 during cytokinesis. Finally, we show that Plk1 and RhoA function together to maximally enhance Rock2 kinase activity in vitro and within cells, and implicate Plk1 as a central regulator of multiple pathways that synergistically converge to regulate actomyosin ring contraction during cleavage furrow ingression.

Polo-like kinase Cdc5 controls the local activation of Rho1 to promote cytokinesis.

The links between the cell cycle machinery and the cytoskeletal proteins controlling cytokinesis are poorly understood. The small guanine nucleotide triphosphate (GTP)-binding protein RhoA stimulates type II myosin contractility and formin-dependent assembly of the cytokinetic actin contractile ring. We found that budding yeast Polo-like kinase Cdc5 controls the targeting and activation of Rho1 (RhoA) at the division site via Rho1 guanine nucleotide exchange factors. This role of Cdc5 (Polo-like kinase) in regulating Rho1 is likely to be relevant to cytokinesis and asymmetric cell division in other organisms.

BRCT repeats as phosphopeptide-binding modules involved in protein targeting.

We used a proteomic approach to identify phosphopeptide-binding modules mediating signal transduction events in the DNA damage response pathway. Using a library of partially degenerate phosphopeptides, we identified tandem BRCT (BRCA1 carboxyl-terminal) domains in PTIP (Pax transactivation domain-interacting protein) and in BRCA1 as phosphoserine- or phosphothreonine-specific binding modules that recognize substrates phosphorylated by the kinases ATM (ataxia telangiectasia-mutated) and ATR (ataxia telangiectasia- and RAD3-related) in response to gamma-irradiation. PTIP tandem BRCT domains are responsible for phosphorylation-dependent protein localization into 53BP1- and phospho-H2AX (gamma-H2AX)-containing nuclear foci, a marker of DNA damage. These findings provide a molecular basis for BRCT domain function in the DNA damage response and may help to explain why the BRCA1 BRCT domain mutation Met1775 --> Arg, which fails to bind phosphopeptides, predisposes women to breast and ovarian cancer.

Inhibition of TATA binding protein dimerization by RNA polymerase III transcription initiation factor Brf1.

The Brf1 subunit of TFIIIB plays an important role in recruiting the TATA-binding protein (TBP) to the up-stream region of genes transcribed by RNA polymerase III. When TBP is not bound to promoters, it sequesters its DNA binding domain through dimerization. Promoter assembly factors therefore might be required to dissociate TBP into productively binding monomers. Here we show that Saccharomyces cerevisiae Brf1 induces TBP dimers to dissociate. The high affinity TBP binding domain of Brf1 is not sufficient to promote TBP dimer dissociation but in addition requires the TFIIB homology domain of Brf1. A model is proposed to explain how two distinct functional domains of Brf1 work in concert to dissociate TBP into monomers.