The light-sensitive photoprotein berovin from the bioluminescent ctenophoreBeroe abyssicola: a novel type of Ca2+-regulated photoprotein

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Abstract

Light-sensitive Ca2+-regulated photoproteins are responsible for the bright bioluminescence of ctenophores. Using functional screening, four full-size cDNA genes encoding the same 208-amino-acid polypeptide were isolated from two independent cDNA libraries prepared from two Beroe abyssicola specimens. Sequence analysis revealed three canonical EF-hand calcium-binding sites characteristic of Ca2+-regulated photoproteins, but a very low degree of sequence identity (27–29%) with aequorin-type photoproteins, despite functional similarities. Recombinant berovin was expressed in Escherichia coli cells, purified, converted to active photoprotein and characterized. Active berovin has absorption maxima at 280 and 437 nm. The Ca2+-discharged protein loses visible absorption, but exhibits a new absorption maximum at 335 nm. The berovin bioluminescence is blue (λmax = 491 nm) and a change in pH over the range 6.0–9.5 has no significant effect on the light emission spectrum. By contrast, the fluorescence of Ca2+-discharged protein (λex = 350 nm) is pH sensitive: at neutral pH the maximum is at 420 nm and at alkaline pH there are two maxima at 410 and 485 nm. Like native ctenophore photoproteins, recombinant berovin is also inactivated by light. The Ca2+ concentration–effect curve is a sigmoid with a slope on a log–log plot of ˜ 2.5. Although this curve for berovin is very similar to those obtained for obelin and aequorin, there are evident distinctions: berovin responds to calcium changes at lower concentrations than jellyfish photoproteins and its Ca2+-independent luminescence is low. Recombinant berovin was successfully expressed in mammalian cells, thereby demonstrating potential for monitoring intracellular calcium.

Database

The nucleotide sequences have been deposited in the GenBankTM/EBI Data Bank with accession numbers: apoberovin cDNA genes, JN673813 (BA1), JN673814 (BA2), JN673815 (BA3), JN673816 (BA4); fragment 18S rRNA, JN673817 (BA-rRNA5).

Bright bioluminescence of ctenophores is determined by light-sensitive Ca2+-regulated photoproteins. Here we describe cloning, expression, and properties of the recombinant Ca2+-regulated photoprotein berovin from ctenophore Beroe abyssicola. Although berovin contains three Ca2+-binding sites like jellyfish photoproteins, the highest degree of identity between their amino acid sequences is only 29.4%. It assumes that most probably ctenophore photoproteins independently arose during evolution.

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