Authors: Z. Ren, B. Perman, V. Srajer, T.-Y. Teng, C. Pradervand, D. Bourgeois, F. Schotte, T. Ursby, R. Kort, M. Wulff and K. Moffat
Affilation: University of Chicago, United States
Pages: 21 - 24
Keywords: photocycle, yellow protein, nanoseconds, seconds
Photoactive yellow protein (PYP; Meyer, 1985; Meyer et al., 1987) from the photoautotrophic purple eubacterium Ectothiorhodospira halophila is a 14-kD, water-soluble, cytoplasmic blue-light receptor, in which a 4-hydroxycinnamic acid chromophore is covalently linked by a thioester bond to the g sulfur of Cys69 (Hoff et al., 1994; Baca et al., 1994). The visible absorption spectrum of PYP with a maximum at 446 nm roughly matches the action spectrum of E. halophila for negative phototaxis (Sprenger et al., 1993). Therefore, PYP is proposed to be the primary photoreceptor for this biological process. PYP also serves as the structural prototype (Pellequer et al., 1998) for the widely-distributed Per-Arnt-Sim (PAS) domain class of signal transduction proteins (Gu et al., 2000). The 1.4 Å crystal structure (Borgstahl et al., 1995) of the resting, dark state of PYP (denoted pG) showed that the chromophore of PYP is completely buried in a hydrophobic pocket with no atom exposed to solvent. The chromophore is stabilized in the trans configuration as the phenolate anion (Baca et al., 1994; Kim et al., 1995), in which the phenolate oxygen O4' is H-bonded to Tyr42 and the protonated side chain of Glu46 (Xie et al., 1996; Imamoto et al., 1997). Upon absorption of light by the pG state, PYP enters a fully reversible photocycle with a quantum yield of 0.35 (van Brederode et al., 1995). In aqueous solution at neutral pH, relaxation occurs from the initial electronic excited state (denoted P*) to short-lived intermediates (I0 and I0‡) whose absorption maxima are highly red-shifted to around 510 nm (Ujj et al., 1998; Devanathan et al., 1999). Further thermal relaxation to the next spectrally-distinct intermediate state I1 (also named pR) occurs within 10 ns, and is accompanied by a shift of the absorption maximum to 465 nm (Meyer et al., 1989; Hoff et al., 1994). The system subsequently relaxes into a relatively long-lived, presumed signaling intermediate denoted I2 (also named pB) with a strongly blue-shifted absorbance maximum at 355 nm, maximally occupied 1 ms after photon absorption (Meyer et al., 1989; Hoff et al., 1994). I2 is presumed to be the signaling intermediate whose conformation is sensed by unidentified downstream partner(s). The photocycle is completed by full recovery of the dark state pG in a few seconds (Meyer et al., 1989; Hoff et al., 1994).