Loosenin-like proteins from Phanerochaete carnosa impact both cellulose and chitin fiber networks

Microbial expansin-related proteins are ubiquitous across bacterial and fungal organisms, and reportedly play a role in the modification and deconstruction of cell wall polysaccharides including lignocellulose. So far, very few microbial expansin related proteins, including loosenins and loosenin-like (LOOL) proteins, have been functionally characterized. Herein, four LOOLs encoded by Phanerochaete carnosa and belonging to different subfamilies (i.e., PcaLOOL7 and PcaLOOL9 from subfamily A; PcaLOOL2 and PcaLOOL12 from subfamily B) were recombinantly produced and the purified proteins were characterized using diverse cellulose and chitin substrates. Whereas all of the purified PcaLOOLs weakened cellulose filter paper and cellulose nanofibril networks (CNF), none significantly boosted cellulase activity on the selected cellulose substrates (Avicel and Whatman paper). Binding of PcaLOOLs to alpha-chitin was higher than to cellulose (Avicel), and highest at pH 5.0. Notably, whereas PcaLOOL9 reduced the yield strain of chitin nanofibrils (ChNF) in a protein-dose dependent manner, the reverse pattern was observed for PcaLOOL7 despite belonging to the same LOOL subfamily. The current study reveals the potential of microbial expansin-related proteins to impact both cellulose and chitin networks, and provides further evidence pointing to a non-lytic mode of action.