A New Spiropyran-Based Sensor for Colorimetric and Fluorescent Detection of Divalent Cu2+ and Hg2+ Ions and Trivalent Ce3+, Cr3+ and Al3+ Ions.

A new spiropyran derivative acts as colorimetric and fluorescent sensor for metal ions. Addition of Cu2+, Al3+, Cr3+ and Ce3+ to the sensor solution brings about marked increase in the absorption intensities while addition of other metal ions (Na+, K+, Li+, Sr2+, Hg2+, Zn2+, Co2+, Ni2+ and Ag+) fails to cause substantial change in the UV-Vis spectra. The sensor can be used for detection of Hg2+, Cu2+, Ce3+, Cr3+ and Al3+ ions by the naked eye and by fluorescence enhancement. The detection limit of Cu2+ and Hg2+ ions by the sensor is estimated to be 10 and 14 µM, respectively. Job's plot analysis on the sensor-Cu2+ complexation reveals the formation of a 2:1 stoichiometric complex between the sensor and Cu2+ ion.

Recognition and Cleavage of Human tRNA Methyltransferase TRMT1 by the SARS-CoV-2 Main Protease.

The SARS-CoV-2 main protease (Mpro) is critical for the production of functional viral proteins during infection and, like many viral proteases, can also target host proteins to subvert their cellular functions. Here, we show that the human tRNA methyltransferase TRMT1 can be recognized and cleaved by SARS-CoV-2 Mpro. TRMT1 installs the N2,N2-dimethylguanosine (m2,2G) modification on mammalian tRNAs, which promotes global protein synthesis and cellular redox homeostasis. We find that Mpro can cleave endogenous TRMT1 in human cell lysate, resulting in removal of the TRMT1 zinc finger domain required for tRNA modification activity in cells. Evolutionary analysis shows that the TRMT1 cleavage site is highly conserved in mammals, except in Muroidea, where TRMT1 may be resistant to cleavage. In primates, regions outside the cleavage site with rapid evolution could indicate adaptation to ancient viral pathogens. We determined the structure of a TRMT1 peptide in complex with Mpro, revealing a substrate binding conformation distinct from the majority of available Mpro-peptide complexes. Kinetic parameters for peptide cleavage showed that the TRMT1(526-536) sequence is cleaved with comparable efficiency to the Mpro-targeted nsp8/9 viral cleavage site. Mutagenesis studies and molecular dynamics simulations together indicate that kinetic discrimination occurs during a later step of Mpro-mediated proteolysis that follows substrate binding. Our results provide new information about the structural basis for Mpro substrate recognition and cleavage that could help inform future therapeutic design and raise the possibility that proteolysis of human TRMT1 during SARS-CoV-2 infection suppresses protein translation and oxidative stress response to impact viral pathogenesis.