Hsp70 proteins are central components of the cellular network of molecular chaperones and folding catalysts. They assist a large variety of protein folding processes in the cell by transient association of their substrate binding domain with short hydrophobic peptide segments within their substrate proteins. The substrate binding and release cycle is driven by the switching of Hsp70 between the low-affinity ATP bound state and the high-affinity ADP bound state. Thus, ATP binding and hydrolysis are essential in vitro and in vivo for the chaperone activity of Hsp70 proteins. This ATPase cycle is controlled by co-chaperones of the family of J-domain proteins, which target Hsp70s to their substrates, and by nucleotide exchange factors, which determine the lifetime of the Hsp70-substrate complex. Additional co-chaperones fine-tune this chaperone cycle. For specific tasks the Hsp70 cycle is coupled to the action of other chaperones, such as Hsp90 and Hsp100.

/pdf/hsp70-chaperones-cellular-functions-and-molecular-mechanism-lfkzg6qhex.pdf

Hsp70 chaperones: cellular functions and molecular mechanism.

Cells are faced with the task of folding thousands of different polypeptides into a wide range of conformations. For many proteins, the folding process requires the action of molecular chaperones. In the cytosol of prokaryotic and eukaryotic cells, molecular chaperones of different structural classes form a network of pathways that can handle substrate polypeptides from the point of initial synthesis on ribosomes to the final stages of folding.

Pathways of chaperone-mediated protein folding in the cytosol

Responses of the rate of net CO2 assimilation (A) to the intercellular partial pressure of CO2 (p

 i
 ) were measured on intact spinach (Spinacia oleracea L.) leaves at different irradiances. These responses were analysed to find the value of p

 i
 at which the rate of photosynthetic CO2 uptake equalled that of photorespiratory CO2 evolution. At this CO2 partial pressure (denoted Г), net rate of CO2 assimilation was negative, indicating that there was non-photorespiratory CO2 evolution in the light. Hence Г was lower than the CO2 compensation point, Γ. Estimates of Г were obtained at leaf temperatures from 15 to 30°C, and the CO2/O2 specificity of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (E.C. 4.1.1.39) was calculated from these data, taking into account changes in CO2 and O2 solubilities with temperature. The CO2/O2 specificity decreased with increasing temperature. Therefore we concluded that temperature effects on the ratio of photorespiration to photosynthesis were not solely the consequence of differential effects of temperature on the solubilities of CO2 and O2. Our estimates of the CO2/O2 specificity of RuBP carboxylase/oxygenase are compared with in-vitro measurements by other authors. The rate of nonphotorespiratory CO2 evolution in the light (R

 d
 ) was obtained from the value of A at Г. At this low CO2 partial pressure, R

 d
 was always less than the rate of CO2 evolution in darkness and appeared to decrease with increasing irradiance. The decline was most marked up to about 100 μmol quanta m-2 s-1 and less marked at higher irradiances. At one particular irradiance, however, R

 d
 as a proportion of the rate of CO2 evolution in darkness was similar in different leaves and this proportion was unaffected by leaf temperature or by [O2] (ambient and greater). After conditions of high [CO2] and high irradiance for several hours, the rate of CO2 evolution in darkness increased and R

 d
 also increased.

Effect of temperature on the CO2/O 2 specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the light : Estimates from gas-exchange measurements on spinach.

We review the current understanding of the temperature responses of C(3) and C(4) photosynthesis across thermal ranges that do not harm the photosynthetic apparatus. In C(3) species, photosynthesis is classically considered to be limited by the capacities of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), ribulose bisphosphate (RuBP) regeneration or P(i) regeneration. Using both theoretical and empirical evidence, we describe the temperature response of instantaneous net CO(2) assimilation rate (A) in terms of these limitations, and evaluate possible limitations on A at elevated temperatures arising from heat-induced lability of Rubisco activase. In C(3) plants, Rubisco capacity is the predominant limitation on A across a wide range of temperatures at low CO(2) (<300 microbar), while at elevated CO(2), the limitation shifts to P(i) regeneration capacity at suboptimal temperatures, and either electron transport capacity or Rubisco activase capacity at supraoptimal temperatures. In C(4) plants, Rubisco capacity limits A below 20 degrees C in chilling-tolerant species, but the control over A at elevated temperature remains uncertain. Acclimation of C(3) photosynthesis to suboptimal growth temperature is commonly associated with a disproportional enhancement of the P(i) regeneration capacity. Above the thermal optimum, acclimation of A to increasing growth temperature is associated with increased electron transport capacity and/or greater heat stability of Rubisco activase. In many C(4) species from warm habitats, acclimation to cooler growth conditions increases levels of Rubisco and C(4) cycle enzymes which then enhance A below the thermal optimum. By contrast, few C(4) species adapted to cooler habitats increase Rubisco content during acclimation to reduced growth temperature; as a result, A changes little at suboptimal temperatures. Global change is likely to cause a widespread shift in patterns of photosynthetic limitation in higher plants. Limitations in electron transport and Rubisco activase capacity should be more common in the warmer, high CO(2) conditions expected by the end of the century.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01682.x

The temperature response of C(3) and C(4) photosynthesis.

The instantaneous rate of photosynthetic CO2 assimilation in C3 plants has generally been studied in model systems such as isolated chloroplasts and algae. From these studies and from theoretical analyses of gas exchange behavior it is now possible to study the biochemistry of photosynthesis in intact leaves using a combination of methods, most of which are nondestructive. The limitations to the rate of photosynthesis can be divided among three general classes: (1) the supply or utilization of CO2, (2) the supply or utilization of light, and (3) the supply or utilization of phosphate. The first limitation is most readily studied by determining how the CO2 assimilation rate varies with the partial pressure of CO2 inside the leaf. The second limitation can be studied by determining the quantum requirement of photosynthesis. The third limitation is most easily detected as a loss of O2 sensitivity of photosynthesis. Measurement of fluorescence from intact leaves can give additional information about the various limitations. These methods are all non-destructive and so can be observed repeatedly as the environment of a leaf is changed. In addition, leaves can be quick-frozen and metabolite concentrations then measured to give more information about the limitations to intact leaf photosynthesis rates. In this review the physics and biochemistry of photosynthesis in intact C3 leaves, and the interface between physiology and photosynthesis—triose phosphate utilization—are discussed.

Photosynthesis in intact leaves of C3 plants: Physics, physiology and rate limitations

In limiting light the activation of ribulose-1,5-bisphosphate (RuP2) carboxylase [3-phospho-D-glycerate carboxylyase (dimerizing), EC 4.1.1.39] in leaf extracts of 7- to 8-day-old wheat seedlings changed proportionally with the photosynthetic rate of the intact plants. Higher rates of photosynthesis, induced by increasing irradiances, were accompanied by an increase in activation of the leaf RuP2 carboxylase, while RuP2 levels remained unchanged. The degree of activation varied from 20% to 60% of full activation at irradiances of 225-1650 μE/m2·s (photosynthetically active radiation; E = einstein, 1 mol of photons). Between 225 μE/m2·s and darkness, activation approached 50% while RuP2 levels dropped more than 90%. During steady-state photosynthesis, levels of the substrate RuP2 were 250-300 nmol/mg of chlorophyll in the leaves and were similar at all irradiances above 225 μE/m2·s (25% of light saturation). When velocities of the carboxylase in leaf extracts were corrected for CO2 levels estimated to exist within the leaf, they compared favorably with the photosynthetic rates of the intact seedlings. Comparison of CO2 exchange rate, RuP2 level, and activation of the carboxylase indicates that light limitation of photosynthesis can be due to two factors: the availability of RuP2 in dark to dim light and activation of the RuP2 carboxylase in dim light and higher irradiances.

https://www.pnas.org/content/pnas/78/5/2985.full.pdf

Light limitation of photosynthesis and activation of ribulose bisphosphate carboxylase in wheat seedlings

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/S0014-5793%2802%2903570-6

HspBP1, a homologue of the yeast Fes1 and Sls1 proteins, is an Hsc70 nucleotide exchange factor

Inhibition of Hsp70 ATPase Activity and Protein Renaturation by a Novel Hsp70-binding Protein

Cultured bovine, equine, ovine and chicken lymphocytes responded to heat stress by the increased synthesis of a specific set of proteins known as heat stress proteins (HSP). Proteins with molecular weights of 70 and 90 kDa were synthesized in all species. Additional proteins were found in bovine, ovine and chicken lymphocytes. A time course of induction showed an increased synthesis of some of these proteins with only 30 min of heat stress and of several proteins with 60 min of heat stress. A specifk monoclonal antibody was used to identify HSWO as one of the stress proteins in bovine lymphocytes. Cells from these animals are capable of responding to heat stress by the production of heat stress proteins. These may be of physiological importance. (Key Words: Proteins, Heat Stress, Lymphocytes, Synthesis.)

Synthesis of heat stress proteins in lymphocytes from livestock

Photosynthetic carbon fixation is regulated in the chloroplast by the amount of ribulose 1,5-bisphosphate carboxylase which is activated. The activated carboxylase was preserved in detached leaves (barley, maize, soybean, spinach, wheat) for 90 min when stored on ice. With leaf extracts stored at 2°C, the amount of activated enzyme, representing that originally in the leaf, as well as the fully activated enzyme, formed by incubation of leaf extracts with Mg 2+ and bicarbonate, both slowly declined in activity. However, for each activity this decline was proportional such that the ratio (percent activation) appeared constant. No change was observed in activation of the enzyme during the brief time of leaf homogenization. Optimal conditions (Mg 2+ , incubation time) for measurement of leaf activation of ribulose bisphosphate carboxylase vary depending on the plant. 3-Phosphonoproprionate, a positive effector of the purified ribulose bisphosphate carboxylase and a metabolically inert analog of 2-phosphoglycolate, was used to examine what metabolic effectors might do to enzyme activation during leaf homogenization and preparation of the extract at 2° C. Activation under these conditions was not altered by 3-phosphonoproprionate. When 3-phosphonoproprionate was brushed on attached leaves or taken up by the transpiration stream of detached leaves, a considerable increase in activation of the carboxylase was measured.

Deborah A. Raynes

Papers

Light limitation of photosynthesis and activation of ribulose bisphosphate carboxylase in wheat seedlings

HspBP1, a homologue of the yeast Fes1 and Sls1 proteins, is an Hsc70 nucleotide exchange factor

Inhibition of Hsp70 ATPase Activity and Protein Renaturation by a Novel Hsp70-binding Protein

Synthesis of heat stress proteins in lymphocytes from livestock

Measurement and preservation of the in vivo activation of ribulose 1,5-bisphosphate carboxylase in leaf extracts.