Cellulose-binding modules from extracellular matrix proteins of Dictyostelium discoideum stalk and sheath

Cellulose-binding modules (CBMs) of two extracellular matrix proteins, St15 and ShD. from the slime mold Dictyostelium discoideum were expressed in Es cherichia coli. The expressed proteins were purified to >98% purity by extr acting inclusion bodies at pH 11.5 and refolding proteins at pH 7.5. The tw o refolded CBMs bound tightly to amorphous phosphoric acid swollen cellulos e (PASC), but had a low affinity toward xylan. Neither protein exhibited ce llulase activity. St15, the stalk-specific protein. had fourfold higher bin ding affinity toward microcrystalline cellulose (Avicel) than the sheath-sp ecific ShD CBM. St15 is unusual in that it consists of a solitary CBM homol ogous to family IIa CBMs. Sequence analysis of SD reveals three putative do mains containing: (a) a C-terminal CBM homologous to family IIb CBMs; (b) a Pro/Thr-rich linker domain; and (c) a N-terminal Cys-rich domain. The biol ogical functions and potential role of St15 and ShD in building extracellul ar matrices during D. discoideum development are discussed.