**19. Conclusions**

zation typical of expansins, have a number of divergent sequence features that separate

Another group included in the expansin superfamily is the *expansin-like X family* (EXLX), comprising proteins that exhibit weak sequence homology with the domains of EXPA and EXPB members, and identified in organisms other than plants, such as the mucilaginous

The denomination of expansin or expansin-like is reserved for proteins exhibiting both do‐ main I and domain II. Proteins with only one of these domains are not classified as expan‐ sins [161]. However other proteins with similar disrupting activity of the cell wall have been

Expansins and expansin-like proteins have been detected in angiosperms such as *Arabidopsis thaliana, Oryza sativa, Zea mays* and *Triticum aestivum,* gymnosperms such as pine and pop‐ lar, ferns such as *Regnellidium diphyllum* and *Marsilea quadrifolia* and the moss *Physcomitrella patens.* Some members of the expansin superfamily have been found even in a potato-infect‐ ing nematode, *Globodera rostochiensis*, where they are hypothesized to promote the infection

Proteins with expansin-like activity called swollenins and loosenins have been identified in ascomycete and basidiomycete fungi such as *T. reesei, A. fumigatus* and *B. adusta* [6-8, 170].

In [7], a swollenin gene from *T. reesei* denominated *swo1,* was cloned and expressed in *Sac‐ charomyces cerevisiae,* coding for a protein that modifies the structure of cellulose in swollen regions of cotton fibres (hence the name) without releasing reducing sugars. Swo1 is a fun‐ gal expansin-like protein, containing a pollen allergen domain and a cellulose-binding do‐

Proteins with expansin activity could be used to improve the efficiency of cellulose biocon‐ version processes. For example, a swollenin purified from *A. fumigatus* has been used in combination with cellulases to facilitate the saccharification of microcrystalline cellulose (Avicel) [8]. In [163] also is described the synergism of an EXLX from *B. subtilis* in the enzy‐ matic hydrolysis of cellulose and recently, and a new protein with expansin activity from the basidiomycete fungus *B. adusta,* denominated loosenin (LOOS1] was cloned and charac‐

Not only expansins, but also swollenins and loosenin represent good candidate as pretreat‐ ment to enhance sugar production from plant biomass. For example, loosenin activity was efficient to release reducing sugars (after cellulase treatment) from *Agave tequilana,* a crop ex‐ tensively grown in some areas of Mexico, which shredded fibrous waste is usually burnt or left to decompose. Indeed, *A. tequilana* fiber became a susceptible substrate for a cocktail of commercial cellulases and xylanases in the presence of LOOS1. Loosenin shows optimum activity at the same pH as most cellulolytic enzymes, opening the possibility to use them as a mixture. This protein is able to relax the structure of cotton, enhancing up to 7.5-fold the

fungus *Dictyostelium* and the bacteria *Bacillus subtilis,* and *Hahella chejuensis* [161-164].

140 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

them from the EXPA and EXPB families [161].

**18.2. Novel proteins with expansin-like activity**

reported.

main.

terized [6].

process [165-169].

Cellulose biodegradation represents the major carbon flow from fixed carbon sinks to at‐ mospheric CO2, this process is very important in several agricultural and waste treatment processes. Also, cellulose contained in plant wastes could be used as a raw material to pro‐ duce sustainable products and bioenergy to replace depleting fossil fuels. However, one of the most important and difficult technological challenges is to overcome the recalcitrance of natural cellulosic materials, which must be enzymatically hydrolysed to produce fermenta‐ ble sugars. In order to achieve this goal, new enzymes with cellulolytic activities are being improved and organisms with novel properties have been found. Although the efforts are being directed to improve cellulolytic activity, proteins capable to relax plant cell structure (expansins, swollenins and loosenin) could be used as a biological pretreatment since they would be disrupting crystalline structure of cellulose making it more accessible to the en‐ zymes and enhancing sugar releasing.
