SANS investigation of fungal loosenins reveal substrate dependent impacts of protein 1 action on inter-fibril distance and packing order of cellulosic substrates

TL;DR

This study uses neutron scattering to show how fungal proteins called loosenins affect the structure of cellulose in wood samples from Eucalyptus and Spruce.

Contribution

The study provides the first detailed structural and functional analysis of fungal loosenins using SANS and NMR, revealing their substrate-dependent effects on cellulose.

Findings

PcaLOOL12 increased interfibril spacing in Spruce holocellulose and reduced packing in Eucalyptus holocellulose.

PcaLOOL12's structure includes a DPBB domain and three alpha-helices, stabilized by disulfide bonds and a flexible N-terminus.

Loosenins likely function as monomers with a conserved folded C-terminal region and disordered N-terminal region.

Abstract

Background: Microbial expansin-related proteins include fungal loosenins, which have been previously shown to disrupt cellulose networks and enhance the enzymatic conversion of cellulosic substrates. Despite showing beneficial impacts to cellulose processing, detailed characterization of cellulosic materials after loosenin treatment is lacking. In this study, small-angle neutron scattering (SANS) was used to investigate the effects of three recombinantly produced loosenins that originate from Phanerochaete carnosa, PcaLOOL7, PcaLOOL9, and PcaLOOL12, on the organization of holocellulose preparations from Eucalyptus and Spruce wood samples.
Results: Whereas the SANS analysis of Spruce holocellulose revealed an increase in interfibril spacing of neighboring cellulose microfibrils following treatment with PcaLOOL12 and to a lesser extent PcaLOOL7, the analysis of Eucalyptus holocellulose revealed a reduction in packing number following treatment with PcaLOOL12 and to a lesser extent PcaLOOL9. Parallel SEC-SAXS characterization of PcaLOOL7, PcaLOOL9, and PcaLOOL12 indicated the proteins likely function as monomers; moreover, all appear to retain a flexible disordered N-terminus and folded C-terminal region. The comparatively high impact of PcaLOOL12 motivated its NMR structural characterization, revealing a double-psi b-barrel (DPBB) domain surrounded by three alpha-helices - the largest nestled against the DPBB core and the other two part of loops extending from the core.
Conclusions: The SANS analysis of PcaLOOL action on holocellulose samples confirms their ability to disrupt cellulose fiber networks and suggests a progression from reducing microfibril packing to increasing interfibril distance. The most impactful PcaLOOL, PcaLOOL12, was previously observed to be the most highly expressed loosenin in P. carnosa. Its structural characterization herein reveals its stabilization through two disulfide linkages, and an extended N-terminal region distal to a negatively charged and surface accessible polysaccharide binding groove.