Neuronal Calcium Sensor-1 (Ncs1p) Is Up-regulated by Calcineurin to Promote Ca2+ Tolerance in Fission Yeast

Neuronal calcium sensor (NCS) proteins regulate signal transduction and are highly conserved from yeast to humans. NCS homolog in fission yeast (Ncs1p) is essential for cell growth under extreme Ca2+ conditions. Ncs1p expression increases similar to 100-fold when fission yeast grows in high extracellular Ca2+ (>0.1 M). Here, we show that Ca2+-induced expression of Ncs1p is controlled at the level of transcription. Transcriptional reporter assays show that ncs1 promoter activity increased 30-fold when extracellular Ca2+ was raised to 0.1 M and was highly Ca2+-specific. Ca2+-dependent transcription of ncs1 is abolished by the calcineurin inhibitor (FK506) and by knocking out the calcineurin target, prz1. Thus, Ca2+-induced expression of Ncs1p is linked to the calcineurin/prz1 stress response. The Ca2+-responsive ncs1 promoter region consists of 130 nucleotides directly upstream from the start codon and contains tandem repeats of the sequence, 5'-caact-3', that binds to Prz1p. The Ca2+-sensitive ncs1 Delta phenotype is rescued by a yam8 null mutation, suggesting a possible interaction between Ncs1p and the Ca2+ channel, Yam8p. Ca2+ uptake and Ncs1p binding to yeast membranes are both decreased in yam8 Delta, suggesting Ca2+-induced binding of Ncs1p to Yam8p results in channel closure. We propose that Ncs1p promotes Ca2+ tolerance in fission yeast, in part by cytosolic Ca2+ buffering and perhaps by negatively regulating the Yam8p Ca2+ channel.