Functional Green-Tuned Proteorhodopsin from Modern Stromatolites

VIRGINIA HELENA ALBARRACÍN; PHILLIP G. WOOD; WOLFGANG GÄRTNER; IVANA KRAISELBURD; ERNST BAMBERG; CHRISTIAN BAMANN; FARÍAS, MARÍA EUGENIA

PLOS ONE

PUBLIC LIBRARY SCIENCE

Lugar: San Francisco; Año: 2016

1932-6203

AbstractThe sequenced genome of the poly-extremophile Exiguobacterium sp. S17, isolated frommodern stromatolites at Laguna Socompa (3,570 m), a High-Altitude Andean Lake (HAAL)in Argentinean Puna revealed a putative proteorhodopsin-encoding gene. The HAAL areais exposed to the highest UV irradiation on Earth, making the microbial community living inthe stromatolites test cases for survival strategies under extreme conditions. The heterologousexpressed protein E17R from Exiguobacterium (248 amino acids, 85% sequenceidentity to its ortholog ESR from E. sibiricum) was assembled with retinal displaying anabsorbance maximum at 524 nm, which makes it a member of the green-absorbing PR-subfamily.Titration down to low pH values (eventually causing partial protein denaturation) indicateda pK value between two and three. Global fitting of data from laser flash-inducedabsorption changes gave evidence for an early red-shifted intermediate (its formation beingbelow the experimental resolution) that decayed (τ1 = 3.5 μs) into another red-shifted intermediate.This species decayed in a two-step process (τ2 = 84 μs, τ3 = 11 ms), to which theinitial state of E17-PR was reformed with a kinetics of 2 ms. Proton transport capability ofthe HAAL protein was determined by BLM measurements. Additional blue light irradiationreduced the proton current, clearly identifying a blue light absorbing, M-like intermediate.The apparent absence of this intermediate is explained by closely matching formation anddecay kinetics.