HSP70; CHARACTERISTICS AND FUNCTION S 582 HSP70 Diversity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 582 Cytop lasmic HSP70 Genes and Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . .. 583 Cytoplasmic HSP70 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 586 HSP70 in the Endoplasmic Reticulum . . . . .. . . ..... . . .... . . . . . . . . ... . 588 Mitochondrial and Chloroplast HSP70s . . . .. . .. . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588 H SP60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 Mitochondrial HSP60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 Chloroplast Chaperonin 60 . . . . . . . .... . . . . . .. . . . ....... ........ . ......... . ....... . . . . .. . . . ...... 591 LMW HSPs . . . . . . . . " . . ... .... ,... ....... . . . ........ . ...... . ...... 593 The LMW HSP Gene Supeifamily . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . 593 LMW HSP Structure . . . . . . . . . . . . . . . . . . . . . . . . . , .... , , 597 P lastid LMW HSPs . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . , ..... .... , 599 Endomembrane LMW HSPs . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 601 LMW HSP Function . .. . . . .. . . . . . . .... . . . . . .... . . . .. . . . . . . . . . . . . . . . . . . . ... . . . . . . . .... . . . . . . . . . . . . 602 OTHER H SPs . . 604 HSP90 and HSPlJO. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 604 Ubiquitin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , ... ,......... 605 Additional Heat-Regulated Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 606 RELEV ANCE OF HSPS TO PLANT G ROWTH AND DEVELOPMENT 607 Expression of HSPs in the Natural Environment . . .. . . . . . . . ..... . . . . . .. . . . . . . . . . . ....... .. . 607 HSP Expression During Development 608 Role of HSPs in Thermotolerance . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . 609 SUMMARY AND FUTURE PROSPECTS....... ....... ..... . .... .. . .. ....... ..... 611

The Roles of Heat Shock Proteins in Plants

Representative samples of mitochondrial and chloroplast targeting peptides have been analyzed in terms of amino acid composition, positional amino acid preferences and amphiphilic character No highly conserved 'homology blocks' are found in either class of topogenic sequence Mitochondrial-matrix-targeting peptides are composed of two domains with different amphiphilic properties Arginine is frequently found either at position -10 or -2 relative to the cleavage site, suggesting that some targeting peptides may be cleaved twice in succession by two different matrix proteases In stroma-targeting chloroplast transit peptides three distinct regions are evident: an uncharged amino-terminal domain, a central domain lacking acidic residues and a carboxy-terminal domain with the potential to form an amphiphilic beta-strand Targeting peptides that route proteins to the mitochondrial intermembrane space or the lumen of chloroplast thylakoids have a mosaic design with an amino-terminal matrix- or stroma-targeting part attached to a carboxy-terminal extension that shares many characteristics with secretory signal peptides

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1989.tb14679.x

Domain structure of mitochondrial and chloroplast targeting peptides.

The entire chloroplast genome of the monocot rice (Oryza sativa) has been sequenced and comprises 134525 bp. Predicted genes have been identified along with open reading frames (ORFs) conserved between rice and the previously sequenced chloroplast genomes, a dicot, tobacco (Nicotiana tabacum), and a liverwort (Marchantia polymorpha). The same complement of 30 tRNA and 4 rRNA genes has been conserved between rice and tobacco. Most ORFs extensively conserved betweenN. tabacum andM. polymorpha are also conserved intact in rice. However, several such ORFs are entirely absent in rice, or present only in severely truncated form. Structural changes are also apparent in the genome relative to tobacco. The inverted repeats, characteristic of chloroplast genome structure, have expanded outward to include several genes present only once per genome in tobacco and liverwort and the large single copy region has undergone a series of inversions which predate the divergence of the cereals. A chimeric tRNA pseudogene overlaps an apparent endpoint of the largest inversion, and a model invoking illegitimate recombination between tRNA genes is proposed which accounts simultaneously for the origin of this pseudogene, the large inversion and the creation of repeated sequences near the inversion endpoints.

The complete sequence of the rice (Oryza sativa) chloroplast genome: Intermolecular recombination between distinct tRNA genes accounts for a major plastid DNA inversion during the evolution of the cereals

Complexity of the heat stress response in plants.

To investigate the importance of different processes to heat stress tolerance, 45 Arabidopsis (Arabidopsis thaliana) mutants and one transgenic line were tested for basal and acquired thermotolerance at different stages of growth. Plants tested were defective in signaling pathways (abscisic acid, salicylic acid, ethylene, and oxidative burst signaling) and in reactive oxygen metabolism (ascorbic acid or glutathione production, catalase) or had previously been found to have temperature-related phenotypes (e.g. fatty acid desaturase mutants, uvh6). Mutants were assessed for thermotolerance defects in seed germination, hypocotyl elongation, root growth, and seedling survival. To assess oxidative damage and alterations in the heat shock response, thiobarbituric acid reactive substances, heat shock protein 101, and small heat shock protein levels were determined. Fifteen mutants showed significant phenotypes. Abscisic acid (ABA) signaling mutants (abi1 and abi2) and the UV-sensitive mutant, uvh6, showed the strongest defects in acquired thermotolerance of root growth and seedling survival. Mutations in nicotinamide adenine dinucleotide phosphate oxidase homolog genes (atrbohB and D), ABA biosynthesis mutants (aba1, aba2, and aba3), and NahG transgenic lines (salicylic acid deficient) showed weaker defects. Ethylene signaling mutants (ein2 and etr1) and reactive oxygen metabolism mutants (vtc1, vtc2, npq1, and cad2) were more defective in basal than acquired thermotolerance, especially under high light. All mutants accumulated wild-type levels of heat shock protein 101 and small heat shock proteins. These data indicate that, separate from heat shock protein induction, ABA, active oxygen species, and salicylic acid pathways are involved in acquired thermotolerance and that UVH6 plays a significant role in temperature responses in addition to its role in UV stress.

Heat stress phenotypes of Arabidopsis mutants implicate multiple signaling pathways in the acquisition of thermotolerance.

PERSPECTIVES AND SUMMARy            879 OVERVIEW OF TRANSPORT OF PROTEINS THROUGH MEMBRANES 880 Cotranslational Transport 880 Postt ranslational Transport 881 CHLOROPLAST TOPOLOGY: DIVERGENT PATHWAYS FOR PROTEIN IMPORT AND SORTING 882 Proteins of the Stromal Compartment 882 Transit Sequences of Stromal Protein Precursors 886 Proteins of the Thylakoid Compartment 889 Transit Sequences of Thylakoid Membrane Proteins  893 Envelope Membrane Proteins 894 ENVELOPE RECEPTORS                              895 ENERGETICS OF TRANSPORT                                                     897 TRANSIT PEPTIDASES  899 DIRECTIONS AND PROSPECTS                   903 Translocation to Inner Compartments 903 Transit Sequences                                                   905 Import Factors 906 Transport Mutants                                                                                908

The Transport of Proteins into Chloroplasts

We have detected a proteolytic mechanism in chloroplasts that selectively and rapidly degrades the imported small subunit of ribulose 1,5-bisphosphate carboxylase when pools of the chloroplast-synthesized large subunit are depleted. This degradation system is constitutively present and appears to be responsible for precise stoichiometric accumulation of the two subunits of the enzyme. We believe similar proteolytic mechanisms participate in regulating the accumulation of other photosynthetic proteins during chloroplast biogenesis.

Rapid Degradation of Unassembled Ribulose 1,5-bisphosphate Carboxylase Small Subunits in Chloroplasts

We demonstrate that in three plant species—soybean, pea, and corn—certain nuclear-encoded heat shock proteins are transported into chloroplasts. In vitro translation products of poly(A)-RNA from control or heat-shocked plants were incubated with isolated intact pea chloroplasts and differences in the profile of imported proteins were analyzed. In all three species, abundant polypeptides between 21 and 27 kDa are present in the heat shock sample and absent in the controls. These polypeptides are protected from trypsin and chymotrypsin digestion after their import into chloroplasts and are recovered primarily with the soluble chloroplast protein fraction. Chloroplasts isolated from pea or corn leaves labeled in vivo at heat shock temperatures, but not at normal growth temperatures, contain the same polypeptides observed in vitro. Synthesis of the heat shock polypeptides can be inhibited in vivo by cycloheximide but not by chloramphenicol, further indicating they are products of cytoplasmic protein synthesis. The in vitro transport experiments demonstrate that synthesis of the chloroplast-localized heat shock proteins results from heat-induced accumulation of the corresponding poly(A)-RNAs. The same mRNAs are also produced in response to heat shock by a nonphotosynthetic tissue, the etiolated soybean hypocotyl.

Specific heat shock proteins are transported into chloroplasts

Plant Responses to High Temperature

Immunocytological techniques were developed to localize the plant lectin, wheat germ agglutinin (WGA), in the tissues and cells of wheat plants. In a previous study we demonstrated with a radioimmunoassay that the lectin is present in wheat embryos and adult plants both in the roots and at the base of the stem. We have now found, using rhodamine, peroxidase, and ferritin-labeled secondary antibodies, that WGA is located in cells and tissues that establish direct contact with the soil during germination and growth of the plant In the embryo, WGA is found in the surface layer of the radicle, the first adventitious roots, the coleoptile, and the scutellum. Although found throughout the coleorhiza and epiblast, it is at its highest levels within the cells at the surface of these organs. In adult plants, WGA is located only in the caps and tips of adventitious roots. Reaction product for WGA was not visualized in embryonic or adult leaves or in other tissues of adult plants. At the subcellular level, WGA is located at the periphery of protein bodies, within electron-translucent regions of the cytoplasm, and at the cell wall-protoplast interface. Since WGA is found at potential infection sites and is known to have fungicidal properties, it may function in the defense against fungal pathogens.

https://rupress.org/jcb/article-pdf/92/3/753/1075803/753.pdf

Michael L. Mishkind

Papers

The Transport of Proteins into Chloroplasts

Rapid Degradation of Unassembled Ribulose 1,5-bisphosphate Carboxylase Small Subunits in Chloroplasts

Specific heat shock proteins are transported into chloroplasts

Plant Responses to High Temperature

Immunocytochemical localization of wheat germ agglutinin in wheat.