Calcium dynamics in the peroxisomal lumen of living cells

We here describe the generation of novel, green fluorescent protein-based Ca2+ indicators targeted to the peroxisome lumen. We show that (i) the Ca2+ concentration of peroxisomes in living cells at rest is similar to that of the cytosol; ( ii) increases in cytosolic Ca2+ concentration ( elicited by either Ca2+ mobilization from stores or Ca2+ influx through plasma membrane Ca2+ channels) are followed by a slow rise in intraperoxisomal [ Ca2+]; ( iii) Ca2+ influx into peroxisomes is driven neither by an ATP-dependent pump nor by membrane potential nor by a H+(Na+) gradient. The peroxisomal membrane appears to play a low pass filter role, preventing the organelle from taking up shortlasting cytosolic Ca2+ transients but allowing equilibration of the peroxisomal luminal [Ca2+] with that of the cytosol during prolonged Ca2+ increases. Thus, peroxisomes appear to be an additional cytosolic Ca2+ buffer, but their influx and efflux mechanisms are unlike those of any other cellular organelle.