The mechanism for thermal-enhanced chaperone-like activity of α-crystallin against UV irradiation-induced aggregation of γD-crystallin

IR Fingerprint of the Intermolecular Hydrogen Bond on Amino Acids and Its Relevance to Chaperone Activity of αB-Crystallin

Intermolecular hydrogen bonds between carboxyl (COO-) and amino groups are a common weak interaction in proteins. Infrared (IR) spectral assignment of such an intermolecular hydrogen bond provides a fingerprint for studying protein-protein interactions as its absorption frequency is affected by the molecular electrostatic environment. Temperature-dependent FTIR and temperature-jump time-resolved IR absorbance difference spectra of several typical amino acids and those of wild type and single-site mutated αB-crystallin were performed. It was found that the antisymmetric vibrational frequency of the COO- groups in amino acids decreases from approximately 1626 to 1610 cm-1 upon the formation of intermolecular hydrogen bonds, which was further supported by DFT calculations, while the IR frequency of the intermolecular hydrogen bonds on the formation of intermolecular hydrogen bonds, which was further supported by DFT calculations, while the IR frequency of the intermolecular hydrogen bonded COO- groups at the αB-crystallin dimeric interface was also observed around 1610 cm-1. With this spectral label, the active site of αB-crystallin, a heat shock molecular chaperone against the UV-light-damaged γD-crystallin was investigated. The active site was found to be localized at an arch loop structure connecting the two β-strands locked by intermolecular hydrogen bonds at the dimeric interface. It is the liberated arch loop after breaking of the intermolecular hydrogen bond locks that binds the damaged γD-crystallin, leading to the observed chaperone-like activity of αB-crystallin.

Investigation of Transient Temperature Rising of Light-Harvesting Complex II by Nonradiative Heat Dissipation at the Protein Level

Light-harvesting complex II (LHCII), the most abundant membrane protein in photosystem II, plays dual roles, i.e., efficient light harvesting and energy transfer to the reaction center under low light conditions and dissipating excess energy as heat to prevent photodamage under high irradiation conditions. The latter process is known as nonphotochemical quenching (NPQ). It has been established that both the pH gradient and temperature rise can trigger NPQ, while the transient heat release via nonradiative decay of the excess energy, as well as the accompanying transient temperature rising of LHCII at room temperature, have not been observed yet. Here we conducted femtosecond and nanosecond time-resolved visible pump and mid-infrared probe measurements on the LHCII trimer, respectively. We detected an excited-state heat dissipation-induced transient temperature rise in the LHCII trimer. The results show that the LHCII gets thermal equilibrium with D2O medium with a temperature rise of 7 °C under 480 nm excitation (mainly absorbed by Chlb and carotenoid) at a power of 0.4 mJ and a pulse duration of 10 ns, fairly consistent with the theoretical estimation of a temperature increase of 9.3 °C. Furthermore, we observed the conformational changes of LHCII in response to the raised temperature, i.e., from 310-helix/random coil to α-helix. Combining the femtosecond time-resolved visible pump and mid-infrared probe spectra, the light-induced temperature jump of LHCII is determined to take place around 60 ns.

A neutral invertase controls cell division besides hydrolysis of sucrose for nutrition during germination and seed setting in rice

Sucrose is the transport form of carbohydrate in plants serving as signal molecule besides nutrition, but the signaling is elusive. Here, neutral invertase 8 (OsNIN8) mutated at G461R into OsNIN8m, which increased its charge and hydrophobicity, decreased hydrolysis of sucrose to 13% and firmer binding to sucrose than the wildtype. This caused downstream metabolites and energy accumulation forming overnutrition. Paradoxically, division of subinitials in longitudinal cell lineages was only about 15 times but more than 100 times in wildtype, resulting in short radicle. Further, mutation of OsNIN8 into deficiency of hydrolysis but maintenance of sucrose binding allowed cell division until ran out of energy showing the association but not hydrolysis gave the signal. Chemically, sucrose binding to OsNIN8 was exothermic but to OsNIN8m was endothermic. Therefore, OsNIN8m lost the signal function owing to change of thermodynamic state. So, OsNIN8 sensed sucrose for cell division besides hydrolyzed sucrose.

Cold-induced calreticulin OsCRT3 conformational changes promote OsCIPK7 binding and temperature sensing in rice

Unusually low temperatures caused by global climate change adversely affect rice production. Sensing cold to trigger signal network is a key base for improvement of chilling tolerance trait.  Here, we report that Oryza sativa Calreticulin 3 (OsCRT3) localized at the endoplasmic reticulum (ER) exhibits conformational changes under cold stress, thereby enhancing its interaction with CBL-interacting protein kinase 7 (OsCIPK7) to sense cold. Phenotypic analyses of OsCRT3 knock-out mutants and transgenic overexpression lines demonstrate that OsCRT3 is a positive regulator in chilling tolerance. OsCRT3 localizes at the ER and mediates increases in cytosolic calcium levels under cold stress. Notably, cold stress triggers secondary structural changes of OsCRT3 and enhances its binding affinity with OsCIPK7, which finally boosts its kinase activity. Moreover, Calcineurin B-like protein 7 (OsCBL7) and OsCBL8 interact with OsCIPK7 specifically on the plasma membrane. Taken together, our results thus identify a cold-sensing mechanism that simultaneously conveys cold-induced protein conformational change, enhances kinase activity, and Ca²⁺ signal generation to facilitate chilling tolerance in rice.