Dr. Hill et al., EXTRACELLULAR POLYSACCHARIDE OF NOSTOC COMMUNE (CYANOBACTERIA) INHIBITS FUSION OF MEMBRANE-VESICLES DURING DESICCATION, Journal of applied phycology, 9(3), 1997, pp. 237-248
Cells of the cyanobacterium Nostoc commune secrete a complex, high mol
ecular weight, extracellular polysaccharide (EPS) which accumulates to
more than 60% of the dry weight of colonies, The EPS was purified fro
m the clonal isolate N. commune DRH1. The midpoint of the membrane pha
se transition (T-m) of desiccated cells of N. commune CHEN was low (T-
m dry = 8 degrees C) and was comparable to the T-m of rehydrated cells
((T-m)(wet) = 6 degrees C). The EPS was not responsible for the depre
ssion of T-m. However, the EPS, at low concentrations, inhibited speci
fically the fusion of phosphatidylcholine membrane vesicles when they
were dried in vitro at 0% relative humidity (-400 MPa). Low concentrat
ions of a trehalose:sucrose mixture, in a molar ratio which correspond
ed with that present in cells in vivo, together with small amounts of
the EPS, were efficient in preventing leakage of carboxyfloroscein (CF
) from membrane vesicles. Freeze-fracture electron microscopy resolved
complex changes in the structure of the EPS and the outer membrane in
response to rehydration of desiccated cells. The capacity of the EPS
to prevent membrane fusion, the maintenance of a low T-m dry in desicc
ated cells, and the changes in rheological properties of the EPS in re
sponse to water availability, constitute what are likely important mec
hanisms for desiccation tolerance in this cyanobacterium.