Signal transduction in light-oxygen-voltage receptors lacking the active-site glutamine.

In nature as in biotechnology, light-oxygen-voltage photoreceptors perceive blue light to elicit spatiotemporally defined cellular responses. Photon absorption drives thioadduct formation between a conserved cysteine and the flavin chromophore. An equally conserved, proximal glutamine processes the resultant flavin protonation into downstream hydrogen-bond rearrangements. Here, we report that this glutamine, long deemed essential, is generally dispensable. In its absence, several light-oxygen-voltage receptors invariably retained productive, if often attenuated, signaling responses. Structures of a light-oxygen-voltage paradigm at around 1 Å resolution revealed highly similar light-induced conformational changes, irrespective of whether the glutamine is present. Naturally occurring, glutamine-deficient light-oxygen-voltage receptors likely serve as bona fide photoreceptors, as we showcase for a diguanylate cyclase. We propose that without the glutamine, water molecules transiently approach the chromophore and thus propagate flavin protonation downstream. Signaling without glutamine appears intrinsic to light-oxygen-voltage receptors, which pertains to biotechnological applications and suggests evolutionary descendance from redox-active flavoproteins.

Mechanistic insights revealed by a UBE2A mutation linked to intellectual disability.

Ubiquitin-conjugating enzymes (E2) enable protein ubiquitination by conjugating ubiquitin to their catalytic cysteine for subsequent transfer to a target lysine side chain. Deprotonation of the incoming lysine enables its nucleophilicity, but determinants of lysine activation remain poorly understood. We report a novel pathogenic mutation in the E2 UBE2A, identified in two brothers with mild intellectual disability. The pathogenic Q93E mutation yields UBE2A with impaired aminolysis activity but no loss of the ability to be conjugated with ubiquitin. Importantly, the low intrinsic reactivity of UBE2A Q93E was not overcome by a cognate ubiquitin E3 ligase, RAD18, with the UBE2A target PCNA. However, UBE2A Q93E was reactive at high pH or with a low-pKa amine as the nucleophile, thus providing the first evidence of reversion of a defective UBE2A mutation. We propose that Q93E substitution perturbs the UBE2A catalytic microenvironment essential for lysine deprotonation during ubiquitin transfer, thus generating an enzyme that is disabled but not dead.