The mechanism of guard cell movement

Leaves of higher terrestrial plants have small pores — stomata — responsible for gas exchange. The opening of each stoma results from the osmotic uptake of water by two specialised cells — the guard cells. Because of the involvement in this mechanism of ATPase-proton pumps and active transport of ions across membranes, we have designed an Exploring Guard Cell Movement activity for biology teachers to provide a practical approach to some relevant aspects of bioenergetics and plant physiology.     

The mechanism of adenosine production in rat polymorphonuclear leucocytes.

Adenosine production in intact rat polymorphonuclear leucocytes was studied during 2-deoxyglucose-induced ATP catabolism. A cell-free system containing the cytosolic 5'-nucleotidase (EC 3.1.3.5) as the only phosphohydrolase was also studied. The rate of adenosine formation in both intact cells and the cell-free system showed a similar dependence on energy charge (([ATP] + 1/2 [ADP]/([ATP] + [ADP] + [AMP])), being maximal only at values close to 0.8. Sufficient cytosolic 5'-nucleotidase was present in intact cells to explain the observed rate of adenosine formation. We conclude that the cytosolic 5'-nucleotidase is responsible for adenosine production in rat polymorphonuclear leucocytes. This mechanism provides a direct biochemical link between the energy status of a cell and the rate of adenosine formation.     

The mechanism of potassium movement across the liposomal membrane

Addition of potassium to sodium-loaded asolectin liposomes induces an internal alkalinization even in the absence of ionophores. Most of the K+ entry is electrogenic, as shown by fluorescent changes in the potential-sensitive probe Oxonol V. The major part of the proton efflux observed must therefore be electrophoretic. However, in the presence of high concentrations of membrane permeable n-butyltriphenylphosphonium, potassium addition induces a residual alkalinization under conditions where no membrane potential can be observed with Oxonol V. This suggests that liposomes also catalyze direct electroneutral K+/H+ exchange, as has been theoretically predicted for cytochrome oxidase proteoliposomes (Wrigglesworth, J.M., Cooper, C.E., Sharpe, M.A. and Nicholls, P. (1990) Biochem. J. 270, 109-118). Free fatty acids present in the soybean phospholipid mixture may be responsible for such activity.     

The role of peristomatal transpiration in the mechanism of stomatal movement

Abstract. Peristomatal transpiration is defined as the relative high local rate of cuticular water loss from external and internal surfaces around the stomatal pore and its decisive role in the control of stomatal movement is re-emphasized. As the resistance towards changes in air humidity is low in the pore surroundings, the state of turgor is particularly unsteady there. Due to the inherent instability the guard cell 'senses' fluctuations in the supply-demand relationship of water and is thus the control unit proper. The environmental variables (supply and demand) are cross-correlated within the subsidiary cell and the information is transmitted to the guard cell through the water potential gradient between the two cells. A conceptual segregation of a 'humidity response' by 'passive' stomatal movements is rejected.
As ions always accumulate at the most distant point of the liquid path and as this point varies with pore width according to the prevailing water potential gradients, it is felt that the water stream is causing the characteristic pattern of ion distribution within the epidermis. Passive import of ions is attributed to local concentration gradients which are steepened by continuous supply and by water uptake into the guard cell in response to starch hydrolysis. A mechanistic model supplements the discussion.     

The mechanism of stomatal movement in Allium cepa L.

In the guard cells of Allium cepa leaves, no starch was found either when the stomata were open or closed. The lack of other soluble polysaccharides that could be hydrolyzed during the opening reaction of the stomata (Schnabl, Planta 1977, in press) leads to the question, how is the osmotic effect, which is the basis of the stomatal movement, achieved in Allium? It is shown in this paper, by histochemical and microprobe analyses, that in Allium — as in other plant species—the K⁺ concentration of the guard cells increases during stomatal opening. The charges of the K⁺ ions in the guard cells seem to be fully compensated by imported Cl^- ions. This could mean that if starch is present in the guard cells, as in the majority of plant species, its major role in the mechanism of stomatal movement is to deliver the cuunteranions for the imported K⁺ ions.     

The ribosome as a molecular machine: the mechanism of tRNA-mRNA movement in translocation

Translocation of tRNA and mRNA through the ribosome is one of the most dynamic events during protein synthesis. In the cell, translocation is catalysed by EF-G (elongation factor G) and driven by GTP hydrolysis. Major unresolved questions are: how the movement is induced and what the moving parts of the ribosome are. Recent progress in time-resolved cryoelectron microscopy revealed trajectories of tRNA movement through the ribosome. Driven by thermal fluctuations, the ribosome spontaneously samples a large number of conformational states. The spontaneous movement of tRNAs through the ribosome is loosely coupled to the motions within the ribosome. EF-G stabilizes conformational states prone to translocation and promotes a conformational rearrangement of the ribosome (unlocking) that accelerates the rate-limiting step of translocation: the movement of the tRNA anticodons on the small ribosomal subunit. EF-G acts as a Brownian ratchet providing directional bias for movement at the cost of GTP hydrolysis.     

Amoeboid movement in human leucocytes: basic mechanisms, cytobiological and clinical significance.

The present paper is an analytical review of the information available on amoeboid movement in human leucocytes. The reported evidence suggests that leucocyte locomotion is due to pressure developed in the cell cortex in the middle and posterior parts of the moving cell, that 4 nm fibrils may provide at least part of the ultrastructural basis of locomotion, that actin-like and myosin-like proteins may be involved in the mechanism of movement and that ATP may serve as an energy source. Leucocyte motility appears to be governed mainly by factors produced in the external medium. Neutrophil chemotaxis is the most antitubulin-susceptible cell mechanism known; from this observation an essential role of microtubule redistribution in chemotaxis is inferred. In contrast, the random movement of neutrophils is not appreciably affected by antimitotic concentrations of antitubulins. Amoeboid movement seems to be an important mechanism in the short-distance locomotion and immunological functions of leucocytes.     

A unique mechanism for the processive movement of single-headed myosin-IX

It has been puzzled that in spite of its single-headed structure, myosin-IX shows the typical character of processive motor in multi-molecule in vitro motility assay, because this cannot be explained by hand-over-hand mechanism of the two-headed processive myosins. Here, we show direct evidence of the processive movement of myosin-IX using two different single molecule techniques. Using optical trap nanometry, we found that myosin-IX takes several large ( approximately 20nm) steps before detaching from an actin filament. Furthermore, we directly visualized the single myosin-IX molecules moving on actin filaments for several hundred nanometers without dissociating from actin filament. Since myosin-IX processively moves without anchoring the neck domain, the result suggests that the neck tilting is not involved for the processive movement of myosin-IX. We propose that the myosin-IX head moves processively along an actin filament like an inchworm via a unique long and positively charged insertion in the loop 2 region of the head.     

A kinetic mechanism for the fast movement of Chara myosin

Endoplasmic streaming of characean cells of Nitella or Chara is known to be in the range 30-100 microm/second. The Chara myosin extracted from the cells and fixed onto a glass surface was found to move muscle actin filaments at a velocity of 60 microm/second. This is ten times faster than that of skeletal muscle myosin (myosin II). In this study, the displacement caused by single Chara myosin molecules was measured using optical trapping nanometry. The step size of Chara myosin was approximately 19nm. This step size is longer than that of skeletal muscle myosin but shorter than that of myosin V. The dwell time of the steps was relatively long, and this most likely resulted from two rate-limiting steps, the dissociation of ADP and the binding of ATP. The rate of ADP release from Chara myosin after the completion of the force-generation step was similar to that of myosin V, but was considerably slower than that of skeletal muscle myosin. The 19nm step size and the dwell time obtained could not explain the fast movement. The fast movement could be explained by the load-dependent release of ADP. As the load imposed on the myosin decreased, the rate of ADP release increased. We propose that the interaction of Chara myosin with an actin filament resulted in a negative load being imposed on other myosin molecules interacting with the same actin filament. This resulted in an accelerated release of ADP and the fast sliding movement.     

Signalling mechanism of phototropin-mediated chloroplast movement in Arabidopsis

Journal of Plant Biochemistry and Biotechnology - To efficiently use light for photosynthesis, chloroplasts move to the appropriate location according to ambient light conditions. Chloroplasts...     

Recent advances in understanding the molecular mechanism of chloroplast photorelocation movement

Plants are photosynthetic organisms that have evolved unique systems to adapt fluctuating environmental light conditions. In addition to well-known movement responses such as phototropism, stomatal opening, and nastic leaf movements, chloroplast photorelocation movement is one of the essential cellular responses to optimize photosynthetic ability and avoid photodamage. For these adaptations, chloroplasts accumulate at the areas of cells illuminated with low light (called accumulation response), while they scatter from the area illuminated with strong light (called avoidance response). Plant-specific photoreceptors (phototropin, phytochrome, and/or neochrome) mediate these dynamic directional movements in response to incident light position and intensity. Several factors involved in the mechanisms underlying the processes from light perception to actin-based movements have also been identified through molecular genetic approach. This review aims to discuss recent findings in the field relating to how chloroplasts move at molecular levels. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.     

The leucocytes of fish: A review

The data available in the literature concerning fish thrombocytes, lymphocytes, monocytes, macrophages and granulocytes is discussed and the dearth of reliable information concerning the functional identity of most fish leucocytes is underlined. The methods of applying nomenclatures to fish leucocytes encountered in the literature is discussed in some detail and found to be unsatisfactory. An attempt is made to simplify the confused and conflicting reports by applying the concepts which have recently arisen in the field of mammalian immunology. Nomenclatures are assigned to cells on a functional and morphological basis where sufficient data exists.
Thrombocytes are responsible for clot formation and are considered to be distinct, separate and unrelated to lymphocytes. Lymphocytes are recognised as immuno-competent cells though as yet little information exists on their role in immune mechanisms. Recent work is furnishing interesting results in this direction, especially in the origin of lymphocyte populations and their co-operative responses in immunity. Previous references to monocytes and macrophages in fish are confused and the literature is discussed in the light of the new classification of the Mononuclear Phagocyte System.
Very few experimental data exist concerning granulocytic leucocytes in fish, indeed the presence of such cells with granulocytic functions still requires proof. Cells with morphological similarity to mammalian granulocytes do exist in fish but information essential for an understanding of their role in defence mechanisms is lacking. The state of the literature concerning the eosinophil, basophil and mast cell in fish is so confused that an entirely new approach to a study of these cells is warranted. It is hoped that this review will supply guidelines for future research.     

Insight into the mechanism of fast movement of myosin from Chara corallina

Chara myosin, two-headed plant myosin belonging to class XI, slides F-actin at maximally 60 microm s(-1). To elucidate the mechanism of this fast sliding, we extensively investigated its mechanochemical properties. The maximum actin activated ATPase activity, Vmax, was 21.3 s(-1) head(-1) in a solution, but when myosin was immobilized on the surface, its activity was 57.6 s(-1) head(-1) at 2 mg ml(-1) of F-actin. The sliding velocity and the actin activated ATPase activity were greatly inhibited by ADP, suggesting that ADP dissociation was the rate limiting step. With the extensive assay of motility by varying the surface density, the duty ratio of Chara myosin was found to be 0.49-0.44 from velocity measurements and 0.34 from the landing rate analysis. At the surface density of 10 molecules microm(-2), Chara myosin exhibited pivot movement under physiological conditions. Based on the results obtained, we will discuss the sliding mechanism of Chara myosin according to the working stroke model in terms of its physiological aspects. aspects.     

Computational studies of the domain movement and the catalytic mechanism of thymidine phosphorylase

Thymidine phosphorylase (TP) is a dual substrate enzyme with two domains. Each domain binds a substrate. In the crystal structure of Escherichia coli TP, the two domains are arranged so that the two substrate binding sites are too far away for the two substrates to directly react. Molecular dynamics simulations reveal a different structure of the enzyme in which the two domains have moved to place the two substrates in close contact. This structure has a root-mean-square deviation from the crystal structure of 4.1 A. Quantum mechanical calculations using this structure find that the reaction can proceed by a direct nucleophilic attack with a low barrier. This mechanism is not feasible in the crystal structure environment and is consistent with the mechanism observed for other N-glycosidic enzymes. Important catalytic roles are found for the three highly conserved residues His 85, Arg 171, and Lys 190.     

A mechanism for movement of eggs along insect ovipositors

This paper aims to describe in detail the mechanism by which eggs are moved along the length of the ovipositor of insects. A series of posteriorly orientated scales, located along the inside of the ovipositor valves, catch the surface of the egg as it emerges from the oviduct and move it along the ovipositor as the valves oscillate back and forth. The morphology of ovipositor scales is examined by scanning electron microscopy, and is compared for 22 species in 20 Families of insects. The mechanism is also confirmed by direct manipulation of the ovipositor of an anaesthetized insect. Ovipositor scales vary in length from 1 to 30 μm and can be spine-like, comb-like or scale-like in structure in different species. Caddis-flies (Trichoptera) were the only group examined which did not possess ovipositor scales. One species, Philanisus plebeius Walker, has longitudinal ridges along the inside surface of its ovipositor valves. The lack of ovipositor scales in the caddisfly species examined in this study is discussed in relation to their behaviour and possible oviposition sites.     

The catalytic mechanism of dye-decolorizing peroxidase DyP may require the swinging movement of an aspartic acid residue

The dye-decolorizing peroxidase (DyP)-type peroxidase family is a unique heme peroxidase family. The primary and tertiary structures of this family are obviously different from those of other heme peroxidases. However, the details of the structure-function relationships of this family remain poorly understood. We show four high-resolution structures of DyP (EC1.11.1.19), which is representative of this family: the native DyP (1.40 ?), the D171N mutant DyP (1.42 ?), the native DyP complexed with cyanide (1.45 ?), and the D171N mutant DyP associated with cyanide (1.40 ?). These structures contain four amino acids forming the binding pocket for hydrogen peroxide, and they are remarkably conserved in this family. Moreover, these structures show that OD2 of Asp171 accepts a proton from hydrogen peroxide in compound I formation, and that OD2 can swing to the appropriate position in response to the ligand for heme iron. On the basis of these results, we propose a swing mechanism in compound I formation. When DyP reacts with hydrogen peroxide, OD2 swings towards an optimal position to accept the proton from hydrogen peroxide bound to the heme iron.