The blue-light-regulated phosphodiesterase BlrP1 from Klebsiella pneumoniae was found by time-resolved transient grating spectroscopy to undergo its functional, quaternary conformational change — and to activate c-di-GMP hydrolysis with a turnover number proportional to the square of the photoconverted fraction — only when both protomers of its homodimer are photoexcited, making it a molecular AND gate and a nonlinear sensor of light intensity. The kinetic and enzymological origin of this nonlinearity is established, but its molecular mechanism is not: no existing structure or simulation explains how two BLUF-domain photoconversions are integrated across the dimer interface into the millisecond quaternary change and the quadratic activity law, and the conjectured generality of multi-protomer-excitation gating among oligomeric BLUF photosensors is unverified. The open problem is to determine the structural and kinetic mechanism of dual-excitation gating in BlrP1 and to establish whether such gating is a general design principle of oligomeric BLUF photosensors.

Background

BlrP1 (blue-light-regulated phosphodiesterase 1) from Klebsiella pneumoniae is a dimeric bacterial photosensor in which each protomer consists of an N-terminal BLUF domain (blue light sensor using flavin adenine dinucleotide, FAD — a flavin-binding light-sensing module whose photoactivation rearranges the hydrogen-bond network around the FAD chromophore and red-shifts its visible absorption by roughly 10 nm, producing a long-lived signaling state) fused to a C-terminal EAL domain (named after a conserved Glu-Ala-Leu sequence motif; an enzyme domain that hydrolyzes cyclic dimeric guanosine monophosphate, c-di-GMP, a bacterial second messenger controlling motility, biofilm formation, and virulence). Blue light activates BlrP1's c-di-GMP phosphodiesterase activity several-fold. Crystallography established that BlrP1 is an antiparallel homodimer whose EAL domains form the dimer interface, and that light-induced changes originating at the BLUF domain of one protomer are propagated to the EAL domain of the opposing protomer through the dimer interface (Barends et al. 2009); hydrogen-deuterium exchange and small-angle X-ray scattering subsequently mapped the participating allosteric elements (Winkler et al. 2014).

The dynamic evidence for the nonlinear response comes from the time-resolved transient grating (TG) diffusion method: two interfering picosecond laser pulses create a sinusoidal refractive-index grating in the protein solution, and the grating's decay reports the translational diffusion coefficient D of transient photoinduced species on microsecond-to-second timescales. Because D depends on molecular shape and friction, a light-induced change in D — a diffusion-sensitive conformational change (DSCC) — detects conformational and association changes, including subtle quaternary rearrangements that static spectroscopies can miss (Terazima 2006). Applying this method to BlrP1, Shibata, Nakasone and Terazima found that the isolated BLUF domain undergoes only a minor conformational change upon blue light, whereas the full-length dimer shows a pronounced D decrease with a time constant of about 21 ms, attributable to a quaternary structural change; strikingly, the amplitude of this change depends nonlinearly on the excitation light intensity, indicating that the conformational change requires photoexcitation of both monomer units of the dimer (Shibata et al. 2018). A follow-up study that measured the turnover number while independently monitoring the fraction fredf_{red} of photoconverted (red-shifted) protein showed that kcatk_{cat} is proportional to fred2f_{red}^2, so the enzymatic output implements a molecular AND gate that responds only when both protomers are lit (Shibata et al. 2021).

The same laboratory had earlier found related intensity-gating in other oligomeric BLUF proteins: the TePixD decamer undergoes its global light-induced conformational change only when exactly one subunit is excited (multiple excitation suppresses it), and the Synechocystis PixD-PixE complex disassembles only after photoactivation of two PixD subunits (Tanaka et al. 2011, 2012). Identical dual-protomer gating was later demonstrated for the BLUF-regulated adenylate cyclase OaPAC, whose conformational change and enzymatic activation also require both protomers of the dimer to be photoconverted, leading those authors to conjecture that a significant number of BLUF proteins function as nonlinear light-intensity sensors by forming oligomers (Nakasone et al. 2023).

What the established results do not provide is a molecular-level account of the gate. The structural studies characterize dark-adapted and fully light-adapted ensembles and cannot distinguish a singly-lit dimer from a doubly-lit one; no structure or simulation explains how the two BLUF photoconversion events are integrated across the dimer interface to produce the roughly 21 ms quaternary change and the quadratic activity law; and the structural determinants that assign different oligomeric BLUF proteins different intensity-gating logic (AND-gating in BlrP1, the PixD-PixE complex, and OaPAC; suppressed response under multiple excitation in TePixD) are unknown.

Problem Statement

Explain, at the molecular (structural and kinetic) level, the nonlinear light-intensity response of the BlrP1 photosensor: why the light-induced quaternary conformational change (the roughly 21 ms diffusion-coefficient decrease of the full-length dimer detected by transient grating spectroscopy) and the activation of c-di-GMP hydrolysis (kcat∝fred2k_{cat}\propto f_{red}^2) arise only when both protomers of the homodimer are photoconverted — and determine whether multi-protomer-excitation gating is a general design principle of oligomeric BLUF photosensors. The two questions serve one overarching objective: a mechanistic account of how oligomeric photosensors encode light intensity, not merely light presence.

The open problem comprises two linked questions:

  1. Provide a structurally resolved mechanism for the dual-excitation gate of BlrP1: identify the interfaces, residues, and conformational intermediates that distinguish singly-lit from doubly-lit dimers, quantitatively consistent with the published transient-grating kinetics, the requirement of two excited protomers, the quadratic activity law, the minor conformational change of the isolated BLUF domain, and the residual dark-state activity.

  2. Establish what determines the gating logic across oligomeric BLUF proteins — why BlrP1, the PixD-PixE complex, and OaPAC require two excitations while TePixD suppresses its conformational change under multiple excitation. The four named systems are the experimentally established cases to be explained; beyond them, the question is what general principle for the class holds, what its domain of validity is, and what it implies for other oligomeric BLUF photosensors.

References

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