A calmodulin-related light chain from fission yeast that functions with myosin-I and PI 4-kinase

Characterization of Pik1 function in fission yeast reveals its conserved role in lipid synthesis and not cytokinesis

Phosphatidylinositol (PI)-4-phosphate (PI4P) is a lipid found at the plasma membrane (PM) and Golgi in cells from yeast to humans. PI4P is generated from PI by PI4-kinases and can be converted into PI-4,5-bisphosphate [PI(4,5)P2]. Schizosaccharomyces pombe have two essential PI4-kinases - Stt4 and Pik1. Stt4 localizes to the PM, and its loss from the PM results in a decrease of PM PI4P and PI(4,5)P2. As a result, cells divide non-medially due to disrupted cytokinetic ring-PM anchoring. However, the localization and function of S. pombe Pik1 has not been thoroughly examined. Here, we found that Pik1 localizes exclusively to the trans-Golgi and is required for Golgi PI4P production. We determined that Ncs1 regulates Pik1, but unlike in other organisms, it is not required for Pik1 Golgi localization. When Pik1 function was disrupted, PM PI4P but not PI(4,5)P2 levels were reduced, a major difference compared with Stt4. We conclude that Stt4 is the chief enzyme responsible for producing the PI4P that generates PI(4,5)P2. Also, that cells with disrupted Pik1 do not divide asymmetrically highlights the specific importance of PM PI(4,5)P2 for cytokinetic ring-PM anchoring.

Membrane binding of endocytic myosin-1s is inhibited by a class of ankyrin repeat proteins

Myosin-1s are monomeric actin-based motors that function at membranes. Myo1 is the single myosin-1 isoform in Schizosaccharomyces pombe that works redundantly with Wsp1-Vrp1 to activate the Arp2/3 complex for endocytosis. Here, we identified Ank1 as an uncharacterized cytoplasmic Myo1 binding partner. We found that in ank1Δ cells, Myo1 dramatically redistributed from endocytic patches to decorate the entire plasma membrane and endocytosis was defective. Biochemical analysis and structural predictions suggested that the Ank1 ankyrin repeats bind the Myo1 lever arm and the Ank1 acidic tail binds the Myo1 TH1 domain to prevent TH1-dependent Myo1 membrane binding. Indeed, Ank1 overexpression precluded Myo1 membrane localization and recombinant Ank1 reduced purified Myo1 liposome binding in vitro. Based on biochemical and cell biological analyses, we propose budding yeast Ank1 and human OSTF1 are functional Ank1 orthologs and that cytoplasmic sequestration by small ankyrin repeat proteins is a conserved mechanism regulating myosin-1s in endocytosis.

Characterization of Pik1 function in fission yeast reveals its conserved role in lipid synthesis and not cytokinesis

Phosphatidylinositol (PI)-4-phosphate (PI4P) is a lipid found at the plasma membrane (PM) and Golgi in cells from yeast to humans. PI4P is generated from PI by PI4-kinases and can be converted to PI-4,5-bisphosphate [PI(4,5)P 2 ]. Schizosaccharomyces pombe have 2 essential PI4-kinases: Stt4 and Pik1. Stt4 localizes to the PM and its loss from the PM results in a decrease of PM PI4P and PI(4,5)P 2 . As a result, cells divide non-medially due to disrupted cytokinetic ring-PM anchoring. However, the localization and function of S. pombe Pik1 has not been thoroughly examined. Here, we found that Pik1 localizes exclusively to the trans-Golgi and is required for Golgi PI4P production. We determined that Ncs1 regulates Pik1, but unlike in other organisms, it is not required for Pik1 Golgi localization. When Pik1 function was disrupted, PM PI4P but not PI(4,5)P 2 levels were reduced, a major difference with Stt4. We conclude that Stt4 is the chief enzyme responsible for producing the PI4P that generates PI(4,5)P 2 . Also, that cells with disrupted Pik1 do not divide asymmetrically highlights the specific importance of PM PI(4,5)P 2 for cytokinetic ring-PM anchoring.

Summary statement

Fission yeast Pik1 localizes exclusively to the trans-Golgi independently of Ncs1, where it contributes to PI4P but not PI(4,5)P 2 synthesis. Pik1 does not affect cytokinesis.

Membrane binding of endocytic myosin-1s is inhibited by a class of ankyrin repeat proteins

Myosin-1s are monomeric actin-based motors that function at membranes. Myo1 is the single myosin-1 isoform in Schizosaccharomyces pombe that works redundantly with Wsp1-Vrp1 to activate the Arp2/3 complex for endocytosis. Here, we identified Ank1 as an uncharacterized cytoplasmic Myo1 binding partner. We found that in ank1Δ cells, Myo1 dramatically redistributed from endocytic patches to decorate the entire plasma membrane and endocytosis was defective. Biochemical analysis and structural predictions suggested that the Ank1 ankyrin repeats bind the Myo1 lever arm and the Ank1 acidic tail binds the Myo1 TH1 domain to prevent TH1-dependent Myo1 membrane binding. Indeed, Ank1 over-expression precluded Myo1 membrane localization and recombinant Ank1 blocked purified Myo1 liposome binding in vitro. Based on biochemical and cell biology analyses, we propose budding yeast Ank1 and human OSTF1 are functional Ank1 orthologs and that cytoplasmic sequestration by small ankyrin repeat proteins is a conserved mechanism regulating myosin-1s in endocytosis.

Summary

Fission yeast long-tailed myosin-1 binds Ank1. Ank1 ankyrin repeats associate with the Myo1 lever arm and Ank1 acidic tail binds the Myo1 TH1 domain to inhibit Myo1 membrane binding. Ank1 orthologs exists in budding yeast (Ank1) and humans (OSTF1).

Acetylation stabilises calmodulin-regulated calcium signalling

Calmodulin is a conserved calcium signalling protein that regulates a wide range of cellular functions. Amino‐terminal acetylation is a ubiquitous post‐translational modification that affects the majority of human proteins, to stabilise structure, as well as regulate function and proteolytic degradation. Here, we present data on the impact of amino‐terminal acetylation upon structure and calcium signalling function of fission yeast calmodulin. We show that NatA‐dependent acetylation stabilises the helical structure of the Schizosaccharomyces pombe calmodulin, impacting its ability to associate with myosin at endocytic foci. We go on to show that this conserved modification impacts both the calcium‐binding capacity of yeast and human calmodulins. These findings have significant implications for research undertaken into this highly conserved essential protein.

Effects of Mutations in the Phenamacril-Binding Site of Fusarium Myosin-1 on Its Motor Function and Phenamacril Sensitivity

Phenamacril is a Fusarium-specific fungicide used for Fusarium head blight management. The target of phenamacril is FgMyo1, the sole class I myosin in Fusarium graminearum. The point mutation S217L in FgMyo1 is responsible for the high resistance of F. graminearum to phenamacril. Recent structural studies have shown that phenamacril binds to the 50 kDa cleft of the FgMyo1 motor domain, forming extensive interactions, including a hydrogen bond between the cyano group of phenamacril and the hydroxyl group of S217. Here, we produced FgMyo1_IQ2, a truncated FgMyo1 composed of the motor domain and two IQ motifs complexed with the F. graminearum calmodulin in insect Sf9 cells. Phenamacril potently inhibited both the basal and the actin-activated ATPase activities of FgMyo1_IQ2, with an IC₅₀ in a micromolar range. S217 mutations of FgMyo1_IQ2 substantially increased the IC₅₀ of phenamacril. S217T or S217L each increased the IC₅₀ of phenamacril for ∼60-fold, while S217A only increased the IC₅₀ for ∼4-fold. These results indicate that the hydroxyl group of S217 plays an important, but nonessential role in phenamacril binding and that the bulky side chain at the position 217 sterically hinders phenamacril binding. On the other hand, S217P, which might alter the local conformation of the phenamacril-binding site, completely abolished the phenamacril inhibition. Because the cyano group of phenamacril does not form discernible interactions with FgMyo1 other than the nonessential hydrogen bond with the S217 hydroxyl group, we propose the cyano group of phenamacril as a key modification site for the development of novel fungicides.

Human myosin 1e tail but not motor domain replaces fission yeast Myo1 domains to support myosin-I function during endocytosis

In both unicellular and multicellular organisms, long-tailed class I myosins function in clathrin-mediated endocytosis. Myosin 1e (Myo1e) in vertebrates and Myo1 in fission yeast have similar domain organization, yet whether these proteins or their individual protein domains are functionally interchangeable remains unknown. In an effort to assess functional conservation of class I myosins, we tested whether human Myo1e could replace Myo1 in fission yeast Schizosaccharomyces pombe and found that it was unable to substitute for yeast Myo1. To determine if any individual protein domain is responsible for the inability of Myo1e to function in yeast, we created human-yeast myosin-I chimeras. By functionally testing these chimeric myosins in vivo, we concluded that the Myo1e motor domain is unable to function in yeast, even when combined with the yeast Myo1 tail and a full complement of yeast regulatory light chains. Conversely, the Myo1e tail, when attached to the yeast Myo1 motor domain, supports localization to endocytic actin patches and partially rescues the endocytosis defect in myo1Δ cells. Further dissection showed that both the TH1 and TH2-SH3 domains in the human Myo1e tail are required for localization and function of chimeric myosin-I at endocytic sites. Overall, this study provides insights into the role of individual myosin-I domains, expands the utility of fission yeast as a simple model system to study the effects of disease-associated MYO1E mutations, and supports a model of co-evolution between a myosin motor and its actin track.

TORC2-Gad8-dependent myosin phosphorylation modulates regulation by calcium

Cells respond to changes in their environment through signaling networks that modulate cytoskeleton and membrane organization to coordinate cell-cycle progression, polarized cell growth and multicellular development. Here, we define a novel regulatory mechanism by which the motor activity and function of the fission yeast type one myosin, Myo1, is modulated by TORC2-signalling-dependent phosphorylation. Phosphorylation of the conserved serine at position 742 (S742) within the neck region changes both the conformation of the neck region and the interactions between Myo1 and its associating calmodulin light chains. S742 phosphorylation thereby couples the calcium and TOR signaling networks that are involved in the modulation of myosin-1 dynamics to co-ordinate actin polymerization and membrane reorganization at sites of endocytosis and polarised cell growth in response to environmental and cell-cycle cues.

The Great Escape: how phosphatidylinositol 4-kinases and PI4P promote vesicle exit from the Golgi (and drive cancer)

Phosphatidylinositol 4-phosphate (PI4P) is a membrane glycerophospholipid and a major regulator of the characteristic appearance of the Golgi complex as well as its vesicular trafficking, signalling and metabolic functions. Phosphatidylinositol 4-kinases, and in particular the PI4KIIIβ isoform, act in concert with PI4P to recruit macromolecular complexes to initiate the biogenesis of trafficking vesicles for several Golgi exit routes. Dysregulation of Golgi PI4P metabolism and the PI4P protein interactome features in many cancers and is often associated with tumour progression and a poor prognosis. Increased expression of PI4P-binding proteins, such as GOLPH3 or PITPNC1, induces a malignant secretory phenotype and the release of proteins that can remodel the extracellular matrix, promote angiogenesis and enhance cell motility. Aberrant Golgi PI4P metabolism can also result in the impaired post-translational modification of proteins required for focal adhesion formation and cell–matrix interactions, thereby potentiating the development of aggressive metastatic and invasive tumours. Altered expression of the Golgi-targeted PI 4-kinases, PI4KIIIβ, PI4KIIα and PI4KIIβ, or the PI4P phosphate Sac1, can also modulate oncogenic signalling through effects on TGN-endosomal trafficking. A Golgi trafficking role for a PIP 5-kinase has been recently described, which indicates that PI4P is not the only functionally important phosphoinositide at this subcellular location. This review charts new developments in our understanding of phosphatidylinositol 4-kinase function at the Golgi and how PI4P-dependent trafficking can be deregulated in malignant disease.

Fission yeast myosin I facilitates PI(4,5)P2-mediated anchoring of cytoplasmic dynein to the cortex

Several key processes in the cell, such as vesicle transport and spindle positioning, are mediated by the motor protein cytoplasmic dynein, which produces force on the microtubule. For the functions that require movement of the centrosome and the associated nuclear material, dynein needs to have a stable attachment at the cell cortex. In fission yeast, Mcp5 is the anchor protein of dynein and is required for the oscillations of the horsetail nucleus during meiotic prophase. Although the role of Mcp5 in anchoring dynein to the cortex has been identified, it is unknown how Mcp5 associates with the membrane as well as the importance of the underlying attachment to the nuclear oscillations. Here, we set out to quantify Mcp5 organization and identify the binding partner of Mcp5 at the membrane. We used confocal and total internal reflection fluorescence microscopy to count the number of Mcp5 foci and the number of Mcp5 molecules in an individual focus. Further, we quantified the localization pattern of Mcp5 in fission yeast zygotes and show by perturbation of phosphatidylinositol 4-phosphate 5-kinase that Mcp5 binds to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. Remarkably, we discovered that the myosin I protein in fission yeast, Myo1, which is required for organization of sterol-rich domains in the cell membrane, facilitates the localization of Mcp5 and that of cytoplasmic dynein on the membrane. Finally, we demonstrate that Myo1-facilitated association of Mcp5 and dynein to the membrane determines the dynamics of nuclear oscillations and, in essence, dynein activity.

Effects of FSGS-associated mutations on the stability and function of myosin-1 in fission yeast

Point mutations in the human MYO1E gene, encoding class I myosin Myo1e, are associated with focal segmental glomerulosclerosis (FSGS), a primary kidney disorder that leads to end-stage kidney disease. In this study, we used a simple model organism, fission yeast Schizosaccharomyces pombe, to test the effects of FSGS-associated mutations on myosin activity. Fission yeast has only one class I myosin, Myo1, which is involved in actin patch assembly at the sites of endocytosis. The amino acid residues mutated in individuals with FSGS are conserved between human Myo1e and yeast Myo1, which allowed us to introduce equivalent mutations into yeast myosin and use the resulting mutant strains for functional analysis. Yeast strains expressing mutant Myo1 exhibited defects in growth and endocytosis similar to those observed in the myo1 deletion strain. These mutations also disrupted Myo1 localization to endocytic actin patches and resulted in mis-localization of Myo1 to eisosomes, linear membrane microdomains found in yeast cells. Although both mutants examined in this study exhibited loss of function, one of these mutants was also characterized by the decreased protein stability. Thus, using the yeast model system, we were able to determine that the kidney-disease-associated mutations impair myosin functional activity and have differential effects on protein stability.

Summary: In the fission yeast S. pombe, kidney disease-associated mutations in Myo1, a homolog of human Myo1e, disrupt myosin localization and function.

Contractile ring stability in S. pombe depends on F-BAR protein Cdc15p and Bgs1p transport from the Golgi complex

Cdc15p is known to contribute to cytokinesis in fission yeast; however, the protein is not required to assemble the contractile ring of actin and myosin, but it helps to anchor the ring to the plasma membrane. Cdc15p has a lipid-binding F-BAR domain, suggesting that it provides a physical link between the plasma membrane and contractile ring proteins. However, we find that a more important function of Cdc15p during cytokinesis is to help deliver a transmembrane enzyme, Bgs1p (also called Cps1p), from the Golgi apparatus to the plasma membrane, where it appears to anchor the contractile ring. Bgs1p synthesizes the cell wall in the cleavage furrow, but its enzyme activity is not required to anchor the contractile ring. We estimate that ∼ 2,000 Bgs1p molecules are required to anchor the ring. Without Bgs1p anchors, contractile rings slide along the plasma membrane, a phenomenon that depends on an unconventional type II myosin called Myp2p.

Regulation and control of myosin-I by the motor and light chain-binding domains

Members of the myosin-I family of molecular motors are expressed in many eukaryotes, where they are involved in a multitude of critical processes. Humans express eight distinct members of the myosin-I family, making it the second largest family of myosins expressed in humans. Despite the high degree of sequence conservation in the motor and light chain-binding domains (LCBDs) of these myosins, recent studies have revealed surprising diversity of function and regulation arising from isoform-specific differences in these domains. Here we review the regulation of myosin-I function and localization by the motor and LCBDs.

Phosphoinositides and cytokinesis: the "PIP" of the iceberg

Phosphoinositides [Phosphatidylinositol (PtdIns), phosphatidylinositol 3-monophosphate (PtdIns3P), phosphatidylinositol 4-monophosphate (PtdIns4P), phosphatidylinositol 5-monophosphate (PtdIns5P), phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2) ), phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2) ), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2) ), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3) )] are lowly abundant acidic lipids found at the cytosolic leaflet of the plasma membrane and intracellular membranes. Initially discovered as precursors of second messengers in signal transduction, phosphoinositides are now known to directly or indirectly control key cellular functions, such as cell polarity, cell migration, cell survival, cytoskeletal dynamics, and vesicular traffic. Phosphoinositides actually play a central role at the interface between membranes and cytoskeletons and contribute to the identity of the cellular compartments by recruiting specific proteins. Increasing evidence indicates that several phosphoinositides, particularly PtdIns(4,5)P(2) , are essential for cytokinesis, notably after furrow ingression. The present knowledge about the specific phosphoinositides and phosphoinositide modifying-enzymes involved in cytokinesis will be first presented. The review of the current data will then show that furrow stability and cytokinesis abscission require that both phosphoinositide production and hydrolysis are regulated in space and time. Finally, I will further discuss recent mechanistic insights on how phosphoinositides regulate membrane trafficking and cytoskeletal remodeling for successful furrow ingression and intercellular bridge abscission. This will highlight unanticipated connections between cytokinesis and enzymes implicated in human diseases, such as the Lowe syndrome.

Roles for actin assembly in endocytosis

Endocytosis includes a number of processes by which cells internalize segments of their plasma membrane, enclosing a wide variety of material from outside the cell. Endocytosis can contribute to uptake of nutrients, regulation of signaling molecules, control of osmotic pressure, and function of synapses. The actin cytoskeleton plays an essential role in several of these processes. Actin assembly can create protrusions that encompass extracellular materials. Actin can also support the processes of invagination of a membrane segment into the cytoplasm, elongation of the invagination, scission of the new vesicle from the plasma membrane, and movement of the vesicle away from the membrane. We briefly discuss various types of endocytosis, including phagocytosis, macropinocytosis, and clathrin-independent endocytosis. We focus mainly on new findings on the relative importance of actin in clathrin-mediated endocytosis (CME) in yeast versus mammalian cells.