Acidic water stress polypeptides (Wsp) with molecular masses of 33, 37 and 39 kDa are the most abundant soluble proteins in the cyanobacterium Nostoc commune. Wsp polypeptides and UV-A/B-absorbing pigments are secreted by cells, accumulate in the extracellular glycan sheath, and are released from desiccated colonies upon rehydration. No evidence was obtained for either glycosylation, phosphorylation, or acylation of Wsp polypeptides. NH2-terminal amino acid sequences of the 33-, 37-, and 39-kDa polypeptides 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. This consensus NH2-terminal sequence and an internal sequence (Glu-Ala-Arg-Val-Thr-Gly-Pro-Thr-Thr-Pro-Ile-Asp) showed homologies with the sequences of carbohydrate-modifying enzymes. Purified Wsp polypeptides associate with a 1,4-beta-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. A possible role is suggested for Wsp polypeptides in the synthesis and/or modification of a xylose-containing UV-A/B-absorbing pigment.