Endocytosis is an essential process for the internalization of plasma membrane proteins, lipids and extracellular molecules into the cells. The mechanisms underlying endocytosis in plant cells ...involve several endosomal organelles whose origins and specific role needs still to be clarified. In this study we compare the internalization events of a GFP-tagged polygalacturonase-inhibiting protein of
(PGIP2-GFP) to that of a GFP-tagged subunit of cellulose synthase complex of
(secGFP-CesA6). Through the use of endocytic traffic chemical inhibitors (tyrphostin A23, salicylic acid, wortmannin, concanamycin A, Sortin 2, Endosidin 5 and BFA) it was evidenced that the two protein fusions were endocytosed through distinct endosomes with different mechanisms. PGIP2-GFP endocytosis is specifically sensitive to tyrphostin A23, salicylic acid and Sortin 2; furthermore, SYP51, a tSNARE with interfering effect on late steps of vacuolar traffic, affects its arrival in the central vacuole. SecGFP-CesA6, specifically sensitive to Endosidin 5, likely reaches the plasma membrane passing through the
Golgi network (TGN), since the BFA treatment leads to the formation of BFA bodies, compatible with the aggregation of TGNs. BFA treatments determine the accumulation and tethering of the intracellular compartments labeled by both proteins, but PGIP2-GFP aggregated compartments overlap with those labeled by the endocytic dye FM4-64 while secGFP-CesA6 fills different compartments. Furthermore, secGFP-CesA6 co-localization with RFP-NIP1.1, marker of the direct ER-to-Vacuole traffic, in small compartments separated from ER suggests that secGFP-CesA6 is sorted through TGNs in which the direct contribution from the ER plays an important role. All together the data indicate the existence of a heterogeneous population of Golgi-independent TGNs.
Summary
The xyloglucan endotransglucosylase/hydrolases (XTHs) are enzymes involved in cell wall assembly and growth regulation, cleaving and re‐joining hemicellulose chains in the ...xyloglucan–cellulose network. Here, in a homologous system, we compare the secretion patterns of XTH11, XTH33 and XTH29, three members of the Arabidopsis thaliana XTH family, selected for the presence (XTH11 and XTH33) or absence (XTH29) of a signal peptide, and the presence of a transmembrane domain (XTH33). We show that XTH11 and XTH33 reached, respectively, the cell wall and plasma membrane through a conventional protein secretion (CPS) pathway, whereas XTH29 moves towards the apoplast following an unconventional protein secretion (UPS) mediated by exocyst‐positive organelles (EXPOs). All XTHs share a common C‐terminal functional domain (XET‐C) that, for XTH29 and a restricted number of other XTHs (27, 28 and 30), continues with an extraterminal region (ETR) of 45 amino acids. We suggest that this region is necessary for the correct cell wall targeting of XTH29, as the ETR‐truncated protein never reaches its final destination and is not recruited by EXPOs. Furthermore, quantitative real‐time polymerase chain reaction analyses performed on 4‐week‐old Arabidopsis seedlings exposed to drought and heat stress suggest a different involvement of the three XTHs in cell wall remodeling under abiotic stress, evidencing stress‐, organ‐ and time‐dependent variations in the expression levels. Significantly, XTH29, codifying the only XTH that follows a UPS pathway, is highly upregulated with respect to XTH11 and XTH33, which code for CPS‐secreted proteins.
Significance Statement
The cellular distribution of three xyloglucan endotransglucosylase/hydrolases (XTHs) was monitored comparatively while expressing respective fluorescent constructs in Arabidopsis cotyledons. We show that XTH11 and XTH33 follow conventional protein secretion towards cell wall and plasma membrane, respectively, whereas XTH29, the only XTH predicted to lack a signal peptide, is secreted into the wall via exocyst‐positive organelle‐mediated unconventional protein secretion (UPS). Significantly, XTH29, poorly expressed in physiological conditions, demonstrates upregulation under drought and high temperature conditions, suggesting a specific possible role for UPS‐secreted XTH29 in stress responses.
Little heterotic strings Faraggi, Alon E.; Manno, Elisa
The European physical journal. C, Particles and fields,
04/2010, Letnik:
66, Številka:
3-4
Journal Article
Recenzirano
Odprti dostop
We discuss toroidal orbifolds of the
E
8
×
E
8
heterotic string, in which the free-fermionic Higgs–matter splitting is implemented by a shift in the internal lattice coupled with the fermion numbers ...of the gauge degrees of freedom. We consider models in which some choices of the orbifold result in the projection of the graviton. In the models that we consider the projection also results in flipping the spin–statistics assignments in the massive string spectrum, whereas the massless spectrum retains the conventional spin–statistics assignments. We argue that the partition functions are mathematically consistent for one- and multi-loop amplitudes, owning to the existence of supersymmetry in the spectrum. A duality between different models at non-zero temperature is briefly discussed.
Superstring phenomenology explores classes of vacua which can reproduce the low energy data provided by the Standard Model. We consider the heterotic E8 x E8 string theory, which gives rise to ...four-dimensional Standard-like Models and allows for their S0(10) embedding. The exploration of realistic vacua consists of finding compactifications of the heterotic string from ten to four dimensions. We investigate two different schemes of compactification: the free fermionic formulation and the orbifold construction. The relation of free fermion models to Z2 x Z2 orbifold compactifications implies that they produce three pairs of untwisted Higgs multiplets. In the examples presented in this dissertation we explore the removal of the extra Higgs representations by using the free fermion boundary conditions directly at the string level, rather than in the effective low energy field theory. Moreover, by employing the standard analysis of flat directions we present a quasi-realistic three generation string model in which stringent F - and D- flat solutions do not appear to exist to all orders in the superpotential. We speculate that this result is indicative of the non-existence of supersymmetric F - and D- flat solutions in this model and discuss its potential implications. By continuing our search of semi-realistic models in different string compactifications we present a simple, yet rich, set up: the orbifold. The simplest examples of orbifold compactifications generally produce a large number of families, which are clearly unappealing for experimental reasons. We show that, by choosing a non-factorisable compactification lattice, defined by skewing its standard simple roots, we decrease the total number of generations. Although we do not provide a semi-realistic model in this framework, the method represents an intermediate step to the final realisation of phenomenologically viable three generation models. Moreover, we mention other possible tools which may be applied in the search of Standard Model-like solutions. Finally, the construction of modular invariant partition functions for E8 x E8 orbifold compactifications is presented. Several interesting examples are derived with this formalism, such as the case of a Z2 x Z2 shift orbifold model, in order to provide a more technical approach in the construction of consistent string models.
We discuss toroidal orbifolds of the E8XE8 heterotic string, in which the free fermionic Higgs-matter splitting is implemented by a shift in the internal lattice coupled with the fermion numbers of ...the gauge degrees of freedom. We consider models in which some choices of the orbifold result in the projection of the graviton. In the models that we consider the projection also results in flipping the spin-statistics assignments in the massive string spectrum, whereas the massless spectrum retains the conventional spin-statistics assignments. We argue that the partition functions are mathematically consistent for one- and multi-loop amplitudes, owning to the existence of supersymmetry in the spectrum. A duality between different models at nonzero temperature is briefly discussed.
Eur.Phys.J.C50:701-710,2007 Three generation heterotic-string vacua in the free fermionic formulation
gave rise to models with solely the MSSM states in the observable Standard
Model charged sector. ...The relation of these models to Z_2 x Z_2 orbifold
compactifications dictates that they produce three pairs of untwisted Higgs
multiplets. The reduction to one pair relies on the analysis of supersymmetric
flat directions, that give superheavy mass to the dispensable Higgs states. We
explore the removal of the extra Higgs representations by using the free
fermion boundary conditions and hence directly at the string level, rather than
in the effective low energy field theory. We present a general mechanism that
achieves this reduction by using asymmetric boundary conditions between the
left- and right-moving internal fermions. We incorporate this mechanism in
explicit string models containing three twisted generations and a single
untwisted Higgs doublet pair. We further demonstrate that an additional effect
of the asymmetric boundary conditions is to substantially reduce the
supersymmetric moduli space.
Quasi-realistic string models in the free fermionic formulation typically contain an anomalous U(1), which gives rise to a Fayet-Iliopoulos D-term that breaks supersymmetry at the one--loop level in ...string perturbation theory. Supersymmetry is traditionally restored by imposing F- and D-flatness on the vacuum. By employing the standard analysis of flat directions we present a quasi--realistic three generation string model in which stringent F- and D-flat solution do not appear to exist to all orders in the superpotential. We speculate that this result is indicative of the non-existence of supersymmetric flat F- and D-solutions in this model. We provide some arguments in support of this scenario and discuss its potential implications. Bose-Fermi degeneracy of the string spectrum implies that the one--loop partition function and hence the one-loop cosmological constant vanishes in the model. If our assertion is correct, this model may represent the first known example with vanishing cosmological constant and perturbatively broken supersymmetry. We discuss the distinctive properties of the internal free fermion boundary conditions that may correspond to a large set of models that share these properties. The geometrical moduli in this class of models are fixed due to asymmetric boundary conditions, whereas absence of supersymmetric flat directions would imply that the supersymmetric moduli are fixed as well and the dilaton may be fixed by hidden sector nonperturbative effects.
Three generation heterotic-string vacua in the free fermionic formulation gave rise to models with solely the MSSM states in the observable Standard Model charged sector. The relation of these models ...to Z_2 x Z_2 orbifold compactifications dictates that they produce three pairs of untwisted Higgs multiplets. The reduction to one pair relies on the analysis of supersymmetric flat directions, that give superheavy mass to the dispensable Higgs states. We explore the removal of the extra Higgs representations by using the free fermion boundary conditions and hence directly at the string level, rather than in the effective low energy field theory. We present a general mechanism that achieves this reduction by using asymmetric boundary conditions between the left- and right-moving internal fermions. We incorporate this mechanism in explicit string models containing three twisted generations and a single untwisted Higgs doublet pair. We further demonstrate that an additional effect of the asymmetric boundary conditions is to substantially reduce the supersymmetric moduli space.
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