Filaments of the desiccation-tolerant cyanobacterium Nostoc commune are embedded
within, and distributed throughout, a dense glycan sheath. Analysis of the glycan of field
materials and of pure cultures of N. commune DRH1 through light and electron microscopy,
immunogold-labelling and staining with dyes, revealed changes in the pattern of differentiation
in glycan micro-structure, as well as localized shifts in pH, upon rehydration of desiccated
field material. A Ca/Si rich external (pellicular) layer of the glycan acts as a physical barrier
on the surface of N. commune colonies. A purified fraction (> 12 kDa) of an aqueous extract
of the glycan from desiccated field material contained glucose, N -acetylglucosamine,
glucosamine, mannose and galactosamine with ratios of 3.1 : 1.4 : 1 : 0.1 : 0.06, respectively.
Ethanol extracts of N. commune contained trehalose and sucrose and the levels of both
became undetectable following cell rehydration. Elemental analysis of glycan extracts showed
a flux in the concentrations of salts in the glycan matrix following rehydration of desiccated
colonies. Intracellular cyanobacterial trehalase was identified using immunoblotting and its
synthesis was detected upon rehydration of desiccated field cultures. Water-stress proteins
(Wsp; molecular masses of 33, 37, and 39 kDa are the most abundant proteins in glycan), a
water soluble UV-AlB-absorbing pigment, the lipid-soluble UV-protective pigment
scytonernim, as well as two unidentified cyanobacterial glycoproteins (75 kDa and 110 kDa),
were found within the glycan matrix. No evidence was found for either glycosylation,
phosphorylation or acylation of Wsp polypeptides. NH2-terminal sequence analysis of
the three proteins of Wsp were identical: Ala-Leu-Tyr-Gly-Tyr-Thr-Ile-Gly-Glu-Gln-X-Ile-Gln-
Asn-Pro-Ser-Asn-Pro-Ser-Asn-Gly-Lys-Gln. An unidentified 68-kDa protein, the
second most abundant protein in aqueous extracts of the glycan, was isolated and its N-terminal
sequenced was determined: Ala-Phe-lle-Phe-Gly-Thr-Ile-Ser-Pro-Asn-Asn-Leu-Ser-Gly-
Thr-Ser-Gly-Asn-Ser-Gly-Ile-Val-Gly-Ser-Ala. Gene bank searches with these
sequences, and an internal sequence ofWsp (Glu-Ala-Arg-Val-Thr-Gly-Pro-Thr-Thr-Pro-Ile-Asp),
identified homologies with various carbohydrate-modifying enzymes. Purified Wsp
polypeptides associate with 1,4-β-D-xylanxylanohydrolase activity that was inhibited
specifically by Wsp antiserum. In the absence of salt, Wsp polypeptides, and the water-soluble
UV -A/B-absorbing pigments, form multimeric complexes through strong ionic
interactions. The role of the glycan, and the protein and pigments that reside within it, in the
desiccation tolerance of N. commune is discussed with respect to structure/function
relationships.
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