Ca2+-transport in sea urchin unfertilized eggs: Regulation by endogenous sulfated polysaccharides and K+

Previous data from our laboratory showed that the reticulum of the sea cucumber smooth muscle body wall retains both a sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and a sulfated polysaccharide. In this invertebrate, the transport of Ca2+ by the SERCA is naturally inhibited by these endogenous sulfated polysaccharides. The inhibition is reverted by K+ leading to an enhancement of the Ca2+ transport rate. We now show that vesicles derived from the endoplasmic reticulum of unfertilized eggs from the sea urchin Arbacia lixula retain a SERCA that is able to transport Ca2+ at the expense of ATP hydrolysis. As described for the sea cucumber SERCA isoform, the enzyme from the sea urchin is activated by K+ but not by Li+ and is inhibited by thapsigargin, a specific inhibitor of SERCA. A new sulfated polysaccharide was identified in the sea urchin eggs reticulum composed mainly by galactose, glucose, hexosamine and manose. After extraction and purification, this sulfated polysaccharide was able to inhibit the mammal SERCA isoform found in rabbit skeletal muscle and the inhibition is reversed by K+. These data suggest that the regulation of the SERCA pump by K+ and sulfated polysaccharides is not restricted to few marine invertebrates but is widespread.