The Israeli Journal of Aquaculture - Bamidgeh 55(4), 2003
The 7th Annual Dan Popper Symposium
ASSIMILATION OF SULFUR INTO THE CELL WALL
OF THE RED MICROALGA PORPHYRIDIUM SP.
Hadas Broshy¹,² , D. van-Moppes², M. Keidan¹,² and S.(Malis) Arad ²*
1 Department of Life Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel
2 Institute for Applied Biosciences, Ben Gurion University of the Negev, Beer Sheva, Israel
The main difference between the cell walls of land plants and those of algae lies in the presence of
sulfated polysaccharides, which are common in algae but absent in higher plants. The cells of marine
red unicellular alga Porphyridium sp.are encapsulated within such a sulfated polysaccharide. This
complex heteropolymer (3-5 kDa) is composed of 10 different sugars and contains a non-covalently
bound cell-wall glycoprotein. The polysaccharide is anionic due to the presence of glucuronic acid
and sulfate ester groups. Very little is known about sulfation pathways of polysaccharides. Current
knowledge of these pathways is based mainly on studies of higher plants and mammals where sulfur
is assimilated mainly through the N-sulfation of proteins. In some mammalian cells, the Golgi
apparatus is involved in the sulfation of proteins and proteoglycans. In experiments designed to
investigate the sulfation pathway(s) in Porphyridium sp., the uptake of organic (cysteine) and inorganic
(sodium sulfate) sources of sulfur by cells starved of sulfate was monitored. The following
results were obtained from uptake and pulse-chase experiments on sulfur-starved cells.
35S from organic and inorganic sources was incorporated into the polysaccharide of Porphyridium sp.cells.
Although the uptake of 35S into the cells was 26.5 times higher when Na2 35SO4 was used as the label source, 35S accumulation into the soluble polysaccharide was twice as high when [35S] cysteine was used.
SDS-PAGE analysis showed that the 35S label resides in the sugars of the soluble polysaccharide and not in the glycoprotein.
35S incorporation into Porphyridium sp. polysaccharide was not affected by sodium chlorate, a chemical known to inhibit sulfation reactions in mammalian cells.
35S incorporation to Porphyridium sp. polysaccharide was reduced by Brefeldin A, a fungal metabolite that disassembles the Golgi apparatus into a tubular network and causes the dissociation of coatomer proteins from Golgi membranes.
The increase in 35S labeling in the polysaccharide correlated with the decrease in 35S labeling of the low molecular weight fraction.
An unidentified precursor was detected in the low molecular weight fraction.
These findings suggest that both sources of sulfur can be used for the sulfation of the polysaccharide
in Porphyridium sp. and, hence, more than one sulfation pathway for the polysaccharide