PcExl1 a Novel Acid Expansin-Like Protein from the Plant Pathogen Pectobacterium carotovorum, Binds Cell Walls Differently to BsEXLX1

Abstract

Microbial expansins act on plant cell walls similarly to plant expansins, albeit their loosening activity levels are tenfold lesser compared to plant expansins. We report the characterization of an expansin-like gene from the plant pathogen Pectobacterium carotovorum, named exl1. PcExl1 is an acidic protein that binds cellulose (Avicel), and weakens filter paper. The acidic nature of PcExl1 confers different binding properties when compared to Bacillus subtilis BsEXLX1, which is a basic protein. PcExl1 binding to wheat cell wall increased when acidic components were depleted, reaching a similar level to the binding to Avicel, indicating that cellulose is the target of PcExl1.

Bacterial expansins and related proteins from the world of microbes

“…Tests of individual D1 and D2 domains showed that D1 binding to cellulose and to plant cell walls was too low to be measured, whereas binding of D2 could account for the binding of the BsEXLX1 protein. Binding of BsEXLX1 to cellulose has an apparent K d of ~1 µM (Georgelis et al 2011; Kim et al 2013a) and similar values were reported for other bacterial expansins (Bunterngsook et al 2015; Olarte-Lozano et al 2014). In contrast, native α-expansin from cucumber binds more tightly (McQueen-Mason and Cosgrove 1995), with a K d of ~0.2 µM (this value is based on a recalculation of the published data).…”

“…These findings suggest that microbial expansins bind to and possibly modify plant cell walls in the course of plant infection. These inferences gain support from studies showing that bacterial expansins bind to cellulose in vitro (Georgelis et al 2014; Georgelis et al 2011; Georgelis et al 2012; Kim et al 2013a; Kim et al 2013b; Lee et al 2010; Olarte-Lozano et al 2014). In species not known to interact with plants, these proteins may have been adopted for other cellular roles, as in Aspergillus nidulans (Bouzarelou et al 2008) and Dictyostelium (Ogasawara et al 2009).…”

“…Figure 3 shows a plant cell wall surface decorated with a representative density of markers for expansin binding sites. It is obvious from this image that expansins bind only a small fraction of microfibril surface, even when most matrix polysaccharides have been removed (McQueen-Mason and Cosgrove 1995; Olarte-Lozano et al 2014). …”

Plant expansins: diversity and interactions with plant cell walls

“…Additional studies have confirmed that other bacterial expansins have the ability to bind cellulose and cell walls [39–43]. The binding determinants generally appear to be similar to the situation described above for BsEXLX1, except for the expansin of Pectobacterium carotovorum, which has an acidic pI (~4.8); as a result, this protein binds to cellulose but not pectin [43].…”

“…The binding determinants generally appear to be similar to the situation described above for BsEXLX1, except for the expansin of Pectobacterium carotovorum, which has an acidic pI (~4.8); as a result, this protein binds to cellulose but not pectin [43]. …”