Aquatic invertebrate’s Carbohydrate-binding module assists environmental cellulase to immobilize in wetland sediments

Abstract

Carbohydrate-binding modules (CBMs) are non-catalytic protein domains that bind to carbohydrates, and have been well studied in microorganisms. Endogenous CBMs in aquatic invertebrates, however, have not yet been identified, and little is known about their ecological significance to wetland environments. Using an approach of characterizing a recombinant CBM (CjCel9A) from a brackish bivalve, Corbicula japonica, this work identified CjCel9A-CBM’s cellulose-binding activity. Scatchard plot analysis in the study of CjCel9A-CBM binding to α-cellulose showed a high corresponding partitioning coefficient (Kr) of 20.33, indicating CjCel9A-CBM’s high affinity for cellulose. In addition, this affinity tolerated a high ion concentration buffer system, consistent with C. japonica’s adaption to brackish wetland environments. Moreover, immuno-scanning electron microscopy (immuno-SEM) suggested that CjCel9A-CBM binds to α-cellulose unevenly, which was further determined to be caused by its higher affinity for crystalline cellulose (Cellulose I, mostly seen in plant leaves). Together, these findings suggest that CjCel9A-CBM is capable of immobilizing its associated catalytic domain on environmental crystalline cellulose (i.e., fallen leaves) in wetland sediments. Most importantly, they could provide a reasonable answer to a question recognized broadly in wetland ecologists, namely, why many wetland sediments have constant cellulase activities, although the sediments are being washed almost every day.