Functional genomic analysis reveals the utility of the I/LWEQ module as a predictor of protein:actin interaction

Huntingtin-interacting protein family members have a conserved pro-viral function from Caenorhabditis elegans to humans

Huntingtin-interacting protein family members are evolutionarily conserved from yeast to humans, and they are known to be key factors in clathrin-mediated endocytosis. Here we identified the Caenorhabditis elegans protein huntingtin-interacting protein-related 1 (HIPR-1) as a host factor essential for Orsay virus infection of C. elegans Ablation of HIPR-1 resulted in a greater than 10,000-fold reduction in viral RNA, which could be rescued by ectopic expression of HIPR-1. Viral RNA replication from an endogenous transgene replicon system was not affected by lack of HIPR-1, suggesting that HIPR-1 plays a role during an early, prereplication virus life-cycle stage. Ectopic expression of HIPR-1 mutants demonstrated that neither the clathrin light chain-binding domain nor the clathrin heavy chain-binding motif were needed for virus infection, whereas the inositol phospholipid-binding and F-actin-binding domains were essential. In human cell culture, deletion of the human HIP orthologs HIP1 and HIP1R led to decreased infection by Coxsackie B3 virus. Finally, ectopic expression of a chimeric HIPR-1 harboring the human HIP1 ANTH (AP180 N-terminal homology) domain rescued Orsay infection in C. elegans, demonstrating conservation of its function through evolution. Collectively, these findings further our knowledge of cellular factors impacting viral infection in C. elegans and humans.

Integrins in disguise - mechanosensors in Saccharomyces cerevisiae as functional integrin analogues

The ability to sense external mechanical stimuli is vital for all organisms. Integrins are transmembrane receptors that mediate bidirectional signalling between the extracellular matrix (ECM) and the cytoskeleton in animals. Thus, integrins can sense changes in ECM mechanics and can translate these into internal biochemical responses through different signalling pathways. In the model yeast species Saccharomyces cerevisiae there are no proteins with sequence similarity to mammalian integrins. However, we here emphasise that the WSC-type (Wsc1, Wsc2, and Wsc3) and the MID-type (Mid2 and Mtl1) mechanosensors in yeast act as partial functional integrin analogues. Various environmental cues recognised by these mechanosensors are transmitted by a conserved signal transduction cascade commonly referred to as the PKC1-SLT1 cell wall integrity (CWI) pathway. We exemplify the WSC- and MID-type mechanosensors functional analogy to integrins with a number of studies where they resemble the integrins in terms of both mechanistic and molecular features as well as in the overall phenotypic consequences of their activity. In addition, many important components in integrin-dependent signalling in humans are conserved in yeast; for example, Sla1 and Sla2 are homologous to different parts of human talin, and we propose that they together might be functionally similar to talin. We also propose that the yeast cell wall is a prominent cellular feature involved in sensing a number of external factors and subsequently activating different signalling pathways. In a hypothetical model, we propose that nutrient limitations modulate cell wall elasticity, which is sensed by the mechanosensors and results in filamentous growth. We believe that mechanosensing is a somewhat neglected aspect of yeast biology, and we argue that the physiological and molecular consequences of signal transduction initiated at the cell wall deserve more attention.

In vivo quantitative analysis of Talin turnover in response to force

Cell–ECM adhesion is regulated by mechanical force. Quantitative imaging and mathematical modeling are used to elucidate how the intracellular adhesion complex of integrin-based adhesions responds to force, revealing the molecular mechanisms that allow the adhesion complex to respond to force to stabilize cell–ECM adhesion over development.

Cell adhesion to the extracellular matrix (ECM) allows cells to form and maintain three-dimensional tissue architecture. Cell–ECM adhesions are stabilized upon exposure to mechanical force. In this study, we used quantitative imaging and mathematical modeling to gain mechanistic insight into how integrin-based adhesions respond to increased and decreased mechanical forces. A critical means of regulating integrin-based adhesion is provided by modulating the turnover of integrin and its adhesion complex (integrin adhesion complex [IAC]). The turnover of the IAC component Talin, a known mechanosensor, was analyzed using fluorescence recovery after photobleaching. Experiments were carried out in live, intact flies in genetic backgrounds that increased or decreased the force applied on sites of adhesion. This analysis showed that when force is elevated, the rate of assembly of new adhesions increases such that cell–ECM adhesion is stabilized. Moreover, under conditions of decreased force, the overall rate of turnover, but not the proportion of adhesion complex components undergoing turnover, increases. Using point mutations, we identify the key functional domains of Talin that mediate its response to force. Finally, by fitting a mathematical model to the data, we uncover the mechanisms that mediate the stabilization of ECM-based adhesion during development.

Dynamic regulation of the structure and functions of integrin adhesions

Integrin-mediated cell adhesions to the extracellular matrix (ECM) contribute to tissue morphogenesis and coherence and provide cells with vital environmental cues. These apparently static structures display remarkable plasticity and dynamic properties: they exist in multiple, interconvertible forms that are constantly remodeled in response to changes in ECM properties, cytoskeletal organization, cell migration, and signaling processes. Thus, integrin-mediated environmental sensing enables cells to adapt to chemical and physical properties of the surrounding matrix by modulating their proliferation, differentiation, and survival. This intriguing interplay between the apparently robust structure of matrix adhesions and their highly dynamic properties is the focus of this article.

Clathrin-mediated endocytosis in budding yeast

Clathrin-mediated endocytosis in the budding yeast Saccharomyces cerevisiae involves the ordered recruitment, activity and disassembly of nearly 60 proteins at distinct sites on the plasma membrane. Two-color live-cell fluorescence microscopy has proven to be invaluable for in vivo analysis of endocytic proteins: identifying new components, determining the order of protein arrival and dissociation, and revealing even very subtle mutant phenotypes. Yeast genetics and functional genomics facilitate identification of complex interaction networks between endocytic proteins and their regulators. Quantitative datasets produced by these various analyses have made theoretical modeling possible. Here, we discuss recent findings on budding yeast endocytosis that have advanced our knowledge of how -60 endocytic proteins are recruited, perform their functions, are regulated by lipid and protein modifications, and are disassembled, all with remarkable regularity.

Talin 1 and 2 are required for myoblast fusion, sarcomere assembly and the maintenance of myotendinous junctions

Talin 1 and 2 connect integrins to the actin cytoskeleton and regulate the affinity of integrins for ligands. In skeletal muscle, talin 1 regulates the stability of myotendinous junctions (MTJs), but the function of talin 2 in skeletal muscle is not known. Here we show that MTJ integrity is affected in talin 2-deficient mice. Concomitant ablation of talin 1 and 2 leads to defects in myoblast fusion and sarcomere assembly, resembling defects in muscle lacking beta1 integrins. Talin 1/2-deficient myoblasts express functionally active beta1 integrins, suggesting that defects in muscle development are not primarily caused by defects in ligand binding, but rather by disruptions of the interaction of integrins with the cytoskeleton. Consistent with this finding, assembly of integrin adhesion complexes is perturbed in the remaining muscle fibers of talin 1/2-deficient mice. We conclude that talin 1 and 2 are crucial for skeletal muscle development, where they regulate myoblast fusion, sarcomere assembly and the maintenance of MTJs.

Progressive myopathy and defects in the maintenance of myotendinous junctions in mice that lack talin 1 in skeletal muscle

The development and function of skeletal muscle depend on molecules that connect the muscle fiber cytoskeleton to the extracellular matrix (ECM). beta1 integrins are ECM receptors in skeletal muscle, and mutations that affect the alpha7beta1 integrin cause myopathy in humans. In mice, beta1 integrins control myoblast fusion, the assembly of the muscle fiber cytoskeleton, and the maintenance of myotendinous junctions (MTJs). The effector molecules that mediate beta1 integrin functions in muscle are not known. Previous studies have shown that talin 1 controls the force-dependent assembly of integrin adhesion complexes and regulates the affinity of integrins for ligands. Here we show that talin 1 is essential in skeletal muscle for the maintenance of integrin attachment sites at MTJs. Mice with a skeletal muscle-specific ablation of the talin 1 gene suffer from a progressive myopathy. Surprisingly, myoblast fusion and the assembly of integrin-containing adhesion complexes at costameres and MTJs advance normally in the mutants. However, with progressive ageing, the muscle fiber cytoskeleton detaches from MTJs. Mechanical measurements on isolated muscles show defects in the ability of talin 1-deficient muscle to generate force. Collectively, our findings show that talin 1 is essential for providing mechanical stability to integrin-dependent adhesion complexes at MTJs, which is crucial for optimal force generation by skeletal muscle.

Overlapping functions of the two talin homologues in Dictyostelium

Talin is a cytoskeletal protein involved in constructing and regulating focal adhesions in animal cells. The cellular slime mold Dictyostelium discoideum has two talin homologues, talA and talB, and earlier studies have characterized the single knockout mutants. talA(-) cells show reduced adhesion to the substrates and slightly impaired cytokinesis leading to a high proportion of multinucleated cells in the vegetative stage, while the development is normal. In contrast, talB(-) cells are characterized by reduced motility in the developmental stage, and they are arrested at the tight-mound stage. Here, we created and analyzed a double mutant with a disruption of both talA and talB. Defects in adhesion to the substrates, cytokinesis, and development were more severe in cells with a disruption of both talA and talB. The talA(-) talB(-) cells failed to attach to the substrates in the vegetative stage, exhibited a higher proportion of multinucleated cells than talA(-) cells, and showed more-reduced motility during the development and an earlier developmental arrest than talB(-) cells at the loose-mound stage. Moreover, overexpression of either talA or talB compensated for the loss of the other talin, respectively. The analysis of talA(-) talB(-) cells also revealed that talin was required for the formation of paxillin-rich adhesion sites and that there was another adhesion mechanism which is independent of talin in the developmental stage. This is the first study demonstrating overlapping functions of two talin homologues, and our data further indicate the importance of talin.

The integrin binding site 2 (IBS2) in the talin rod domain is essential for linking integrin beta subunits to the cytoskeleton

Talin1 is a large cytoskeletal protein that links integrins to actin filaments through two distinct integrin binding sites, one present in the talin head domain (IBS1) necessary for integrin activation and a second (IBS2) that we have previously mapped to talin residues 1984-2113 (fragment J) of the talin rod domain (1 Tremuth, L., Kreis, S., Melchior, C., Hoebeke, J., Ronde, P., Plancon, S., Takeda, K., and Kieffer, N. (2004) J. Biol. Chem. 279, 22258-22266), but whose functional role is still elusive. Using a bioinformatics and cell biology approach, we have determined the minimal structure of IBS2 and show that this integrin binding site corresponds to 23 residues located in alpha helix 50 of the talin rod domain (residues 2077-2099). Alanine mutation of 2 highly conserved residues (L2094A/I2095A) within this alpha helix, which disrupted the alpha-helical structure of IBS2 as demonstrated by infrared spectroscopy and limited trypsin proteolysis, was sufficient to prevent in vivo talin fragment J targeting to alphaIIbbeta3 integrin in focal adhesions and to inhibit in vitro this association as shown by an alphaIIbbeta3 pulldown assay. Moreover, expression of a full-length mouse green fluorescent protein-talin LI/AA mutant in mouse talin1(-/-) cells was unable to rescue the inability of these cells to assemble focal adhesions (in contrast to green fluorescent protein-talin wild type) despite the presence of IBS1. Our data provide the first direct evidence that IBS2 in the talin rod is essential to link integrins to the cytoskeleton.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

Crystal structure at 2.8 A of the DLLRKN-containing coiled-coil domain of huntingtin-interacting protein 1 (HIP1) reveals a surface suitable for clathrin light chain binding

Huntingtin interacting protein 1 (HIP1) is a member of a family of proteins whose interaction with Huntingtin is critical to prevent cells from initiating apoptosis. HIP1, and related protein HIP12/1R, can also bind to clathrin and membrane phospholipids, and HIP12/1R links the CCV to the actin cytoskeleton. HIP1 and HIP12/1R interact with the clathrin light chain EED regulatory site and stimulate clathrin lattice assembly. Here, we report the X-ray structure of the coiled-coil domain of HIP1 (residues 482-586) that includes residues crucial for binding clathrin light chain. The dimeric HIP1 crystal structure is partially splayed open. The comparison of the HIP1 model with coiled-coil predictions revealed the heptad repeat in the dimeric trunk (S2 path) is offset relative to the register of the heptad repeat from the N-terminal portion (S1 path) of the molecule. Furthermore, surface analysis showed there is a third hydrophobic path (S3) running parallel with S1 and S2. We present structural evidence supporting a role for the S3 path as an interaction surface for clathrin light chain. Finally, comparative analysis suggests the mode of binding between sla2p and clathrin light chain may be different in yeast.

Evidence that talin alternative splice variants from Ciona intestinalis have different roles in cell adhesion

Background

Talins are large, modular cytoskeletal proteins found in animals and amoebozoans such as Dictyostelium discoideum. Since the identification of a second talin gene in vertebrates, it has become increasingly clear that vertebrate Talin1 and Talin2 have non-redundant roles as essential links between integrins and the actin cytoskeleton in distinct plasma membrane-associated adhesion complexes. The conserved C-terminal I/LWEQ module is important for talin function. This structural element mediates the interaction of talins with F-actin. The I/LWEQ module also targets mammalian Talin1 to focal adhesion complexes, which are dynamic multicomponent assemblies required for cell adhesion and cell motility. Although Talin1 is essential for focal adhesion function, Talin2 is not targeted to focal adhesions. The nonvertebrate chordate Ciona intestinalis has only one talin gene, but alternative splicing of the talin mRNA produces two proteins with different C-terminal I/LWEQ modules. Thus, C. intestinalis contains two talins, Talin-a and Talin-b, with potentially different activities, despite having only one talin gene.

Results

We show here that, based on their distribution in cDNA libraries, Talin-a and Talin-b are differentially expressed during C. intestinalis development. The I/LWEQ modules of the two proteins also have different affinities for F-actin. Consistent with the hypothesis that Talin-a and Talin-b have different roles in cell adhesion, the distinct I/LWEQ modules of Talin-a and Talin-b possess different subcellular targeting determinants. The I/LWEQ module of Talin-a is targeted to focal adhesions, where it most likely serves as the link between integrin and the actin cytoskeleton. The Talin-b I/LWEQ module is not targeted to focal adhesions, but instead preferentially labels F-actin stress fibers. These different properties of C. intestinalis the Talin-a and Talin-b I/LWEQ modules mimic the differences between mammalian Talin1 and Talin2.

Conclusion

Vertebrates and D. discoideum contain two talin genes that encode proteins with different functions. The urochordate C. intestinalis has a single talin gene but produces two separate talins by alternative splicing that vary in a domain crucial for talin function. This suggests that multicellular organisms require multiple talins as components of adhesion complexes. In C. intestinalis, alternative splicing, rather than gene duplication followed by neo-functionalization, accounts for the presence of multiple talins with different properties. Given that C. intestinalis is an excellent model system for chordate biology, the study of Talin-a and Talin-b will lead to a deeper understanding of cell adhesion in the chordate lineage and how talin functions have been parceled out to multiple proteins during metazoan evolution.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

The conserved C-terminal I/LWEQ module targets Talin1 to focal adhesions

The cytoskeletal protein Talin1 is a critical link between integrins and the actin cytoskeleton, where it is required for the structural and signaling functions of integrin-containing adhesion complexes. However, the elements in Talin1 that are responsible for localizing it to adhesion complexes are not known. In this report we have used a series of constructs based on the modular structure of Talin1 to determine the structural elements that specify the subcellular localization of Talin1. We show that the conserved actin-binding I/LWEQ module at the C-terminus of Talin1 is necessary and sufficient for targeting to focal adhesion complexes. We also used truncation and site-directed mutagenesis to demonstrate that this novel targeting function correlates with, but is separable from, the actin-binding properties of the Talin1 I/LWEQ module. In addition, we have shown that focal adhesion targeting, unlike actin binding, is not conserved among I/LWEQ module proteins. Finally, we have demonstrated that the subcellular localization of the Talin1 I/LWEQ module is regulated by an intrasteric interaction with an upstream alpha-helix, suggesting that both the actin binding and adhesion-targeting elements are masked in full-length Talin1. Our results define a novel role for the I/LWEQ module as the primary adhesion-complex targeting determinant of Talin1 and suggest that pathways that can relieve inhibition of I/LWEQ module function will be important for regulating the structural and signaling properties of adhesion complexes.

Multiple factors contribute to integrin-talin interactions in vivo

The cytoplasmic protein talin is an essential part of the integrin-cytoskeleton link. We characterized the interaction between integrin and two conserved regions of talin, the N-terminal ;head' domain and the C-terminus, which includes the I/LWEQ domain, within the living organism. Green-fluorescent-protein-tagged head and C-terminal domains were recruited to integrin adhesion sites. Both required integrins for recruitment, but the C-terminal domain also required endogenous talin, showing it was not recruited directly by integrins. We used chimeric transmembrane proteins containing the cytoplasmic domain of the integrin beta subunit to examine the integrin-talin head interaction. Monomeric chimeric proteins did not recruit talin head, whereas dimeric chimeras efficiently recruited it and caused a strong inhibition of integrin-mediated adhesion. These chimeras recruited surprisingly few integrin-associated proteins, indicating that recruitment of talin did not initiate a cascade of recruitment. Mutagenesis of the integrin cytoplasmic domain, within the chimera, showed the dominant-negative inhibition was not due to talin sequestration alone and that additional interactions are required.

Structural definition of the F-actin–binding THATCH domain from HIP1R

Huntingtin-interacting protein-1 related (HIP1R) has a crucial protein-trafficking role, mediating associations between actin and clathrin-coated structures at the plasma membrane and trans-Golgi network. Here, we characterize the F-actin-binding region of HIP1R, termed the talin-HIP1/R/Sla2p actin-tethering C-terminal homology (THATCH) domain. The 1.9-A crystal structure of the human HIP1R THATCH core reveals a large sequence-conserved surface patch created primarily by residues from the third and fourth helices of a unique five-helix bundle. Point mutations of seven contiguous patch residues produced significant decreases in F-actin binding. We also show that THATCH domains have a conserved C-terminal latch capable of oligomerizing the core, thereby modulating F-actin engagement. Collectively, these results establish a framework for investigating the links between endocytosis and actin dynamics mediated by THATCH domain-containing proteins.

Gene duplication and functional divergence during evolution of the cytoskeletal linker protein talin

The animal talins are large, modular proteins that link the actin cytoskeleton to the extracellular environment through interactions with beta-integrins and actin. Dictyostelium discoideum has two talins, TalA and TalB, which have distinct physiological roles in cell adhesion, cell differentiation, and cytokinesis. We previously identified a second talin gene in vertebrates. Thus, talin function in vertebrates is also due to the action of multiple proteins. Using a phylogenomic approach we have determined that D. discoideum TalA/B and the animal talins are related by descent from a common ancestral talin and that duplication of TLN2 early in the chordate lineage produced TLN1. An additional duplication subsequently produced a second Talin-2 in teleost fishes and a second Talin-1 in Xenopus laevis. We also show that vertebrate Talin-2 mRNA is alternatively processed. In the invertebrate Drosophila melanogaster and in the non-vertebrate chordate Ciona intestinalis, which each have only one talin gene, alternative processing of talin mRNA also produces multiple talin species. Thus, in these organisms, talin function may be due to the action of more than one protein. To identify isoform-specific functions of vertebrate talins we have shown through proteomic analysis that mammalian Talin-1 and Talin-2 bind to different protein partners. Further characterization of the differences between animal talins, especially the direct comparison of talins in the model urochordate C. intestinalis, which has one talin gene that produces two talins through alternative mRNA splicing, with Talin-1 and Talin-2 in model vertebrates, will provide an experimental system for studying neofunctionalization or subfunctionalization of talin following the vertebrate talin gene duplication.

Genetic, biochemical and structural approaches to talin function

The cytoskeletal protein talin plays a key role in coupling the integrin family of cell adhesion molecules to the actin cytoskeleton. In this paper I present a brief review on talin and summarize our recent studies, in which we have taken both genetic and structural approaches to further elucidate the function of the protein.

Huntingtin interacting protein 1 modulates the transcriptional activity of nuclear hormone receptors

Internalization of activated receptors regulates signaling, and endocytic adaptor proteins are well-characterized in clathrin-mediated uptake. One of these adaptor proteins, huntingtin interacting protein 1 (HIP1), induces cellular transformation and is overexpressed in some prostate cancers. We have discovered that HIP1 associates with the androgen receptor through a central coiled coil domain and is recruited to DNA response elements upon androgen stimulation. HIP1 is a novel androgen receptor regulator, significantly repressing transcription when knocked down using a silencing RNA approach and activating transcription when overexpressed. We have also identified a functional nuclear localization signal at the COOH terminus of HIP1, which contributes to the nuclear translocation of the protein. In conclusion, we have discovered that HIP1 is a nucleocytoplasmic protein capable of associating with membranes and DNA response elements and regulating transcription.

Mapping and consensus sequence identification for multiple vinculin binding sites within the talin rod

The interaction between the cytoskeletal proteins talin and vinculin plays a key role in integrin-mediated cell adhesion and migration. Three vinculin binding sites (VBS1-3) have previously been identified in the talin rod using a yeast two-hybrid assay. To extend these studies, we spot-synthesized a series of peptides spanning all the alpha-helical regions predicted for the talin rod and identified eight additional VBSs, two of which overlap key functional regions of the rod, including the integrin binding site and C-terminal actin binding site. The talin VBS alpha-helices bind to a hydrophobic cleft in the N-terminal vinculin Vd1 domain. We have defined the specificity of this interaction by spot-synthesizing a series of 25-mer talin VBS1 peptides containing substitutions with all the commonly occurring amino acids. The consensus for recognition is LXXAAXXVAXX- VXXLIXXA with distinct classes of hydrophobic side chains at positions 1, 4, 5, 8, 9, 12, 15, and 16 required for vinculin binding. Positions 1, 8, 12, 15, and 16 require an aliphatic residue and will not tolerate alanine, whereas positions 4, 5, and 9 are less restrictive. These preferences are common to all 11 VBS sequences with a minor variation occurring in one case. A crystal structure of this variant VBS peptide in complex with the vinculin Vd1 domain reveals a subtly different mode of vinculin binding.

In Vivo Dynamics of Clathrin and Its Adaptor-Dependent Recruitment to the Actin-Based Endocytic Machinery in Yeast

Clathrin-mediated transport is a major pathway for endocytosis. However, in yeast, where cortical actin patches are essential for endocytosis, plasma membrane-associated clathrin has never been observed. Using live cell imaging, we demonstrate cortical clathrin in association with the actin-based endocytic machinery in yeast. Fluorescently tagged clathrin is found in highly mobile internal trans-Golgi/endosomal structures and in smaller cortical patches. Total internal reflection fluorescence microscopy showed that cortical patches are likely endocytic sites, as clathrin is recruited prior to a burst of intensity of the actin patch/endocytic marker, Abp1. Clathrin also accumulates at the cortex with internalizing alpha factor receptor, Ste2p. Cortical clathrin localizes with epsins Ent1/2p and AP180s, and its recruitment to the surface is dependent upon these adaptors. In contrast, Sla2p, End3p, Pan1p, and a dynamic actin cytoskeleton are not required for clathrin assembly or exchange but are required for the mobility, maturation, and/or turnover of clathrin-containing endocytic structures.

The phosphoinositide phosphatase Sjl2 is recruited to cortical actin patches in the control of vesicle formation and fission during endocytosis

The Saccharomyces cerevisiae synaptojanin-like proteins (Sjl1, Sjl2, and Sjl3) are phosphoinositide (PI) phosphatases that regulate PI metabolism in the control of actin organization and membrane trafficking. However, the primary sites of action for each of the yeast synaptojanin-like proteins remain unclear. In this study, we show that Sjl2 is localized to cortical actin patches, sites of endocytosis. Cortical recruitment of Sjl2 requires the actin patch component Abp1. Consistent with this, the SH3 domain-containing protein Abp1 physically associates with Sjl2 through its proline-rich domain. Furthermore, abp1Delta mutations confer defects resembling loss of SJL2; sjl1Delta abp1Delta double-mutant cells exhibit invaginated plasma membranes and impaired endocytosis, findings similar to those for sjl1Delta sjl2Delta mutant cells. Thus, Abp1 acts as an adaptor protein in the localization or concentration of Sjl2 during late stages of endocytic vesicle formation. Overexpression of the Hip1-related protein Sla2 delayed the formation of extended plasma membrane invaginations in sjl2ts cells, indicating that Sla2 may become limiting or misregulated in cells with impaired PI phosphatase activity. Consistent with this, the cortical actin patch protein Sla2 is mislocalized in sjl1Delta sjl2Delta mutant cells. Together, our studies suggest that PI metabolism by the synaptojanin-like proteins coordinately directs actin dynamics and membrane invagination, in part by regulation of Sla2.

End4/Sla2 is involved in establishment of a new growth zone inSchizosaccharomyces pombe

The rod-shaped Schizosaccharomyces pombe cell grows in a polarized fashion from opposing ends. Correct positioning of the growth zones is directed by the polarity marker Tea1 located at the cell ends where actin patches accumulate and cell growth takes place. We show that the S. pombe homologue of Saccharomyces cerevisiae SLA2, a protein involved in cortical actin organization and endocytosis, provides a link between the polarity marker and the growth machinery. In wild-type fission yeast cells, this homologue End4/Sla2 is enriched at cell ends during interphase and localizes to a medial ring at cell division, mirroring the actin localization pattern throughout the cell cycle. Proper localization relies on membrane trafficking and is independent of both the actin and microtubule cytoskeletons. End4/Sla2 is required for the establishment of new polarised growth zones, and deletion of its C-terminal talin-like domain prevents the establishment of a new growth zone after cell fission. We propose that End4/Sla2 acts downstream of the polarity marker Tea1 and is implicated in the recruitment of the actin cytoskeleton to bring about polarised cell growth.

Cytoskeletal proteins talin and vinculin in integrin-mediated adhesion

The cytoskeletal proteins talin and vinculin form part of a macromolecular complex on the cytoplasmic face of integrin-mediated cellular junctions with the extracellular matrix. Recent genetic, biochemical and structural data show that talin is essential for the assembly of such junctions, whereas vinculin appears to be important in regulating adhesion dynamics and cell migration.

Characterization of an actin-binding site within the talin FERM domain

Talin is a large cytoskeletal protein that couples integrins to F-actin. Three actin-binding sites (ABS1-3) have been reported: one in the N-terminal head, and two in the C-terminal rod domain. Although the C-terminal ABS3 has been partially characterized, the presence and properties of ABS1 within the talin head are less well defined. We show here that the talin head binds F-actin in vitro and in vivo at a specific site within the actin filament. Thus, purified talin head liberated from gizzard talin by calpain cleavage cosediments with F-actin in a low salt buffer at pH 6.4 (conditions that are optimal for binding intact talin), and using recombinant polypeptides, we have mapped ABS1 to the FERM domain within the talin head. Both the F2 and F3 FERM subdomains contribute to binding, and EGFP-tagged FERM subdomains colocalize with actin stress fibers when expressed in COS cells. High-resolution electron microscopy of actin filaments decorated with F2F3 localizes binding to a site that is distinct from that recognized by members of the calponin-homology superfamily. Finally, we show that the FERM domain can couple F-actin to PIPkin, and by inference to integrins, since they bind to the same pocket in the F3 subdomain. This suggests that the talin FERM domain functions as a linker between PIPkin or integrins and F-actin at sites of cell-matrix adhesions.

Identification of the actin-binding domain of Ins(1,4,5)P3 3-kinase isoform B (IP3K-B)

Dewaste et al. [Dewaste, Moreau, De Smedt, Bex, De Smedt, Wuytaack, Missiaen and Erneux (2003) Biochem. J. 374, 41-49] showed that over-expressed EGFP (enhanced green fluorescent protein) fused to Ins(1,4,5)P3 3-kinase B (IP3K-B) co-localizes with the cytoskeleton, as well as with the endoplasmic reticulum and the plasma membrane. The domains responsible for these subcellular localizations are not yet identified. For the endogenous enzyme, we confirmed both actin and endoplasmic reticulum localization by employing a high affinity antibody against IP3K-B. F-actin targeting is exclusively dependent on the non-catalytic N-terminal region of IP3K-B. By expressing fragments of this N-terminal domain as EGFP-fusion proteins and inspecting transfected cells by confocal microscopy, we characterized a distinct 63-amino-acid domain comprising amino acids 108-170 of the enzyme which is responsible for F-actin targeting. A truncation of this fragment from both sides revealed that the full size of this segment is essential for this function. Deletion of this segment in a full-length over-expressed IP3K-B-EGFP-fusion protein completely abolished F-actin interaction. Direct interaction of this actin-binding segment with only F-actin, but not with G-actin, was observed in vitro using a bacterially expressed, affinity-purified GST (glutathione S-transferase)-Rattus norvegicus IP3K (aa 108-170) fusion protein. Helix-breaking mutations within this isolated segment abolished the F-actin binding properties both in vitro and when over-expressed in cells, indicating that an intact secondary structure is essential for actin targeting. The segment shows sequence similarities to the actin-binding region in IP3K-A, but no similarity to other actin-binding domains.

A Fluorescence Cell Biology Approach to Map the Second Integrin-binding Site of Talin to a 130-Amino Acid Sequence within the Rod Domain

The cytoskeletal protein talin, which provides a direct link between integrins and actin filaments, has been shown to contain two distinct binding sites for integrin beta subunits. Here, we report the precise delimitation and a first functional analysis of the talin rod domain integrin-binding site. Partially overlapping cDNAs covering the entire human talin gene were transiently expressed as DsRed fusion proteins in Chinese hamster ovary cells expressing alpha(IIb)beta(3), linked to green fluorescent protein (GFP). Two-color fluorescence analysis of the transfected cells, spread on fibrinogen, revealed distinct subcellular staining patterns including focal adhesion, actin filament, and granular labeling for different talin fragments. The rod domain fragment G (residues 1984-2344), devoid of any known actin- or vinculin-binding sites, colocalized with beta(3)-GFP in focal adhesions. Direct in vitro interaction of fragment G with native platelet integrin alpha(IIb)beta(3) or with the recombinant wild type, but not the Y747A mutant beta(3) cytoplasmic tail, linked to glutathione S-transferase, was demonstrated by surface plasmon resonance analysis and pull-down assays, respectively. Here, we demonstrate for the first time the in vivo relevance of this interaction by fluorescence resonance energy transfer between beta(3)-GFP and DsRed-talin fragment G. Further in vitro pull-down studies allowed us to map out the integrin-binding site within fragment G to a stretch of 130 residues (fragment J, residues 1984-2113) that also localized to focal adhesions. Finally, we show by a cell biology approach that this integrin-binding site within the talin rod domain is important for beta(3)-cytoskeletal interactions but does not participate in alpha(IIb)beta(3) activation.

Actin assembly and endocytosis: from yeast to mammals

Internalization of receptors, lipids, pathogens, and other cargo at the plasma membrane involves several different pathways and requires coordinated interactions between a variety of protein and lipid molecules. The actin cytoskeleton is an integral part of the cell cortex, and there is growing evidence that F-actin plays a direct role in these endocytic events. Genetic studies in yeast have firmly established a functional connection between actin and endocytosis. Identification of several proteins that may function at the interface between actin and the endocytic machinery has provided further evidence for this association in both yeast and mammalian cells. Several of these proteins are directly involved in regulating actin assembly and could thus harness forces produced during actin polymerization to facilitate specific steps in the endocytic process. Recent microscopy studies in mammalian cells provide powerful evidence that localized recruitment and polymerization of actin occurs at endocytic sites. In this review, we focus on progress made in elucidating the functions of the actin cytoskeleton in endocytosis.

The Sla2p Talin Domain Plays a Role in Endocytosis in Saccharomyces cerevisiae

Clathrin-binding adaptors play critical roles for endocytosis in multicellular organisms, but their roles in budding yeast have remained unclear. To address this question, we created a quadruple mutant yeast strain lacking the genes encoding the candidate clathrin adaptors Yap1801p, Yap1802p, and Ent2p and containing a truncated version of Ent1p, Ent1ΔCBMp, missing its clathrin-binding motif. This strain was viable and competent for endocytosis, suggesting the existence of other redundant adaptor-like factors. To identify these factors, we mutagenized the quadruple clathrin adaptor mutant strain and selected cells that were viable in the presence of full-length Ent1p, but inviable with only Ent1ΔCBMp; these strains were named Rcb (requires clathrin binding). One mutant strain, rcb432, contained a mutation in SLA2 that resulted in lower levels of a truncated protein lacking the F-actin binding talin homology domain. Analyses of this sla2 mutant showed that the talin homology domain is required for endocytosis at elevated temperature, that SLA2 exhibits genetic interactions with both ENT1 and ENT2, and that the clathrin adaptors and Sla2p together regulate the actin cytoskeleton and revealed conditions under which Yap1801p and Yap1802p contribute to viability. Together, our data support the view that Sla2p is an adaptor that links actin to clathrin and endocytosis.

Phosphatidylinositol phosphate kinase type 1gamma and beta1-integrin cytoplasmic domain bind to the same region in the talin FERM domain

Talin is an essential component of focal adhesions that couples beta-integrin cytodomains to F-actin and provides a scaffold for signaling proteins. Recently, the integrin beta3 cytodomain and phosphatidylinositol phosphate (PIP) kinase type 1gamma (a phosphatidylinositol 4,5-bisphosphate-synthesizing enzyme) were shown to bind to the talin FERM domain (subdomain F3). We have characterized the PIP kinase-binding site by NMR using a 15N-labeled talin F2F3 polypeptide. A PIP kinase peptide containing the minimal talin-binding site formed a 1:1 complex with F2F3, causing a substantial number of chemical shift changes. In particular, two of the three Arg residues (Arg339 and Arg358), four of eight Ile residues, and one of seven Val residues in F3 were affected. Although a R339A mutation did not affect the exchange kinetics, R358A or R358K mutations markedly weakened binding. The Kd for the interaction determined by Trp fluorescence was 6 microm, and the R358A mutation increased the Kd to 35 microm. Comparison of these results with those of the crystal structure of a beta3-integrin cytodomain talin F2F3 chimera shows that both PIP kinase and integrins bind to the same surface of the talin F3 subdomain. Indeed, binding of talin present in rat brain extracts to a glutathione S-transferase integrin beta1-cytodomain polypeptide was inhibited by the PIP kinase peptide. The results suggest that ternary complex formation with a single talin FERM domain is unlikely, although both integrins and PIP kinase may bind simultaneously to the talin anti-parallel dimer.

The hunt for huntingtin function: interaction partners tell many different stories

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormally elongated polyglutamine (polyQ) tract in the large protein huntingtin (htt). Currently, both the normal function of htt in neurons and the molecular mechanism by which the expanded polyQ sequence in htt causes selective neurodegeneration remain elusive. Research in past years has identified several htt-interacting proteins such as htt-interacting protein 1, Src homology region 3-containing Grb2-like protein 3, protein kinase C and casein kinase substrate in neurons 1, htt-associated protein 1, postsynaptic density-95, FIP-2 (for 14.7K-interacting protein), specificity protein 1 and nuclear receptor co-repressor. These proteins play roles in clathrin-mediated endocytosis, apoptosis, vesicle transport, cell signalling, morphogenesis and transcriptional regulation, suggesting that htt is also involved in these processes.

Huntingtin-interacting protein-1-related protein of rat (rHIP1R) is localized in the postsynaptic regions

We cloned a rHIP1R (GenBank Accession No., AB005052) encoding a Sla2/huntingtin-interacting protein (HIP1) family protein from a rat brain cDNA library. Localization of rHIP1R was investigated in the rat brain using an antibody specific to the HIP1R antibody. The rHIP1R protein was enriched in the synaptic plasma membrane fraction along with huntingtin, a synaptic protein and a causal protein for Huntington's disease. The electron microscopic examination revealed that HIP1R was localized at postsynaptic spines. Localization of HIP1R in the small vesicular structures in the spine, possible sites of vesicular transport of synaptic proteins, together with the structure-based analysis, suggested a role of HIP1R for vesicle trafficking through interaction with F-actin and working together with huntingtin and HIP1 at the synaptic sites.

Further characterization of the interaction between the cytoskeletal proteins talin and vinculin

The cytoskeletal protein talin, which is thought to couple integrins to F-actin, contains three binding sites (VBS1-VBS3) for vinculin, a protein implicated in the negative regulation of cell motility and whose activity is modulated by an intramolecular interaction between the vinculin head (Vh) and vinculin tail (Vt) domains. In the present study we show that recombinant talin polypeptides containing the three VBSs (VBS1, residues 498-636; VBS2, residues 727-965; and VBS3, residues 1943-2157) each bind tightly to the same or overlapping sites within vinculin(1-258). A short synthetic talin VBS3 peptide (residues 1944-1969) was sufficient to inhibit binding of a (125)I-labelled talin VBS3 polypeptide to vinculin(1-258), and NMR spectroscopy confirmed that this peptide forms a 1:1 complex in slow exchange with vinculin(1-258). Binding of the (125)I-labelled VBS3 polypeptide was markedly temperature dependent, but was not inhibited by 1 M salt or 10% (v/v) 2-methyl-2-propanol. Attempts to further define the talin-binding site within vinculin(1-258) using a gel-blot assay were unsuccessful, but near maximal talin-binding activity was retained by a construct spanning vinculin residues 1-131 in a yeast two-hybrid assay. Interestingly, the talin VBS3 polypeptide was a potent inhibitor of the Vh-Vt interaction, and the VBS3 synthetic peptide was able to expose the actin-binding site in intact vinculin, which is otherwise masked by the Vh-Vt interaction. The results suggest that under certain conditions, talin may be an effective activator of vinculin.

The Abl-related gene (Arg) nonreceptor tyrosine kinase uses two F-actin-binding domains to bundle F-actin

Abl family nonreceptor tyrosine kinases regulate cellular morphogenesis and motility through functional interactions with the actin cytoskeleton. Although Abl family kinases are known to contain filamentous (F)-actin-binding domains at their C termini, it is unclear how Abl family kinases regulate the structure and/or function of the actin cytoskeleton. We show here that the Abl-related kinase Arg binds with positive cooperativity to F-actin in vitro with binding saturating at a ratio of one Arg/two actin molecules. Measurements of the F-actin-binding properties of Arg deletion mutants led to the identification of a second, previously uncharacterized internal F-actin-binding domain in Arg. Purified Arg can bundle F-actin in vitro, and this bundling activity requires both F-actin-binding domains. An Arg-yellow fluorescent protein fusion protein can induce the formation of actin-rich structures at the lamellipodia of Swiss 3T3 fibroblasts. Both of Arg's F-actin-binding domains are necessary and sufficient for the formation of these actin-rich structures. Together, our data suggest that Arg can use its F-actin-bundling activity to directly regulate actin cytoskeletal structure in vivo.

Clathrin- and AP-2-binding sites in HIP1 uncover a general assembly role for endocytic accessory proteins

Clathrin-mediated endocytosis is a major pathway for the internalization of macromolecules into the cytoplasm of eukaryotic cells. The principle coat components, clathrin and the AP-2 adaptor complex, assemble a polyhedral lattice at plasma membrane bud sites with the aid of several endocytic accessory proteins. Here, we show that huntingtin-interacting protein 1 (HIP1), a binding partner of huntingtin, copurifies with brain clathrin-coated vesicles and associates directly with both AP-2 and clathrin. The discrete interaction sequences within HIP1 that facilitate binding are analogous to motifs present in other accessory proteins, including AP180, amphiphysin, and epsin. Bound to a phosphoinositide-containing membrane surface via an epsin N-terminal homology (ENTH) domain, HIP1 associates with AP-2 to provide coincident clathrin-binding sites that together efficiently recruit clathrin to the bilayer. Our data implicate HIP1 in endocytosis, and the similar modular architecture and function of HIP1, epsin, and AP180 suggest a common role in lipid-regulated clathrin lattice biogenesis.

The actin-binding protein Hip1R associates with clathrin during early stages of endocytosis and promotes clathrin assembly in vitro

Huntingtin-interacting protein 1 related (Hip1R) is a novel component of clathrin-coated pits and vesicles and is a mammalian homologue of Sla2p, an actin-binding protein important for both actin organization and endocytosis in yeast. Here, we demonstrate that Hip1R binds via its putative central coiled-coil domain to clathrin, and provide evidence that Hip1R and clathrin are associated in vivo at sites of endocytosis. First, real-time analysis of Hip1R-YFP and DsRed-clathrin light chain (LC) in live cells revealed that these proteins show almost identical temporal and spatial regulation at the cell cortex. Second, at the ultrastructure level, immunogold labeling of 'unroofed' cells showed that Hip1R localizes to clathrin-coated pits. Third, overexpression of Hip1R affected the subcellular distribution of clathrin LC. Consistent with a functional role for Hip1R in endocytosis, we also demonstrated that it promotes clathrin cage assembly in vitro. Finally, we showed that Hip1R is a rod-shaped apparent dimer with globular heads at either end, and that it can assemble clathrin-coated vesicles and F-actin into higher order structures. In total, Hip1R's properties suggest an early endocytic function at the interface between clathrin, F-actin, and lipids.

Analysis of the mammalian talin2 gene TLN2

We have utilised genomic and EST databases to assemble the sequence of the human talin2 (TLN2) gene. Talin2 protein is similar in size and sequence to talin1 throughout its length (74% identity, 86% similarity). The major differences are in (i) the size of the genes, the TLN2 gene is >200 kb compared with approximately 30 kb for TLN1 due to a difference in intron size, although intron/exon boundaries, with the exception of two, are strictly conserved; (ii) the expression patterns, TLN1 gives rise to an approximately 8-kb mRNA which is observed in all tissues, whereas TLN2 gives rise to multiple transcripts with the highest levels in heart.

Candida albicans INT1-induced filamentation in Saccharomyces cerevisiae depends on Sla2p

The Candida albicans INT1 gene is important for hyphal morphogenesis, adherence, and virulence (C. Gale, C. Bendel, M. McClellan, M. Hauser, J. M. Becker, J. Berman, and M. Hostetter, Science 279:1355-1358, 1998). The ability to switch between yeast and hyphal morphologies is an important virulence factor in this fungal pathogen. When INT1 is expressed in Saccharomyces cerevisiae, cells grow with a filamentous morphology that we exploited to gain insights into how C. albicans regulates hyphal growth. In S. cerevisiae, INT1-induced filamentous growth was affected by a small subset of actin mutations and a limited set of actin-interacting proteins including Sla2p, an S. cerevisiae protein with similarity in its C terminus to mouse talin. Interestingly, while SLA2 was required for INT1-induced filamentous growth, it was not required for polarized growth in response to several other conditions, suggesting that Sla2p is not required for polarized growth per se. The morphogenesis checkpoint, mediated by Swe1p, contributes to INT1-induced filamentous growth; however, epistasis analysis suggests that Sla2p and Swe1p contribute to INT1-induced filamentous growth through independent pathways. The C. albicans SLA2 homolog (CaSLA2) complements S. cerevisiae sla2Delta mutants for growth at 37 degrees C and INT1-induced filamentous growth. Furthermore, in a C. albicans Casla2/Casla2 strain, hyphal growth did not occur in response to either nutrient deprivation or to potent stimuli, such as mammalian serum. Thus, through analysis of INT1-induced filamentous growth in S. cerevisiae, we have identified a C. albicans gene, SLA2, that is required for hyphal growth in C. albicans.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly-published material on comparative and functional genomics. Each bibliography is divided into 16 sections. 1 Reviews & symposia; 2 General; 3 Large-scale sequencing and mapping; 4 Genome evolution; 5 Comparative genomics; 6 Gene families and regulons; 7 Pharmacogenomics; 8 Large-scale mutagenesis programmes; 9 Functional complementation; 10 Transcriptomics; 11 Proteomics; 12 Protein structural genomics; 13 Metabolomics; 14 Genomic approaches to development; 15 Technological advances; 16 Bioinformatics. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted