Modular biosynthesis of plant hemicellulose and its impact on yeast cells

Background The carbohydrate polymers that encapsulate plants cells have benefited humans for centuries and have valuable biotechnological uses. In the past 5 years, exciting possibilities have emerged in the engineering of polysaccharide-based biomaterials. Despite impressive advances on bacterial cellulose-based hydrogels, comparatively little is known about how plant hemicelluloses can be reconstituted and modulated in cells suitable for biotechnological purposes. Results Here, we assembled cellulose synthase-like A (CSLA) enzymes using an optimized Pichia pastoris platform to produce tunable heteromannan (HM) polysaccharides in yeast. By swapping the domains of plant mannan and glucomannan synthases, we engineered chimeric CSLA proteins that made β-1,4-linked mannan in quantities surpassing those of the native enzymes while minimizing the burden on yeast growth. Prolonged expression of a glucomannan synthase from Amorphophallus konjac was toxic to yeast cells: reducing biomass accumulation and ultimately leading to compromised cell viability. However, an engineered glucomannan synthase as well as CSLA pure mannan synthases and a CSLC glucan synthase did not inhibit growth. Interestingly, Pichia cell size could be increased or decreased depending on the composition of the CSLA protein sequence. HM yield and glucose incorporation could be further increased by co-expressing chimeric CSLA proteins with a MANNAN-SYNTHESIS-RELATED (MSR) co-factor from Arabidopsis thaliana. Conclusion The results provide novel routes for the engineering of polysaccharide-based biomaterials that are needed for a sustainable bioeconomy. The characterization of chimeric cellulose synthase-like enzymes in yeast offers an exciting avenue to produce plant polysaccharides in a tunable manner. Furthermore, cells modified with non-toxic plant polysaccharides such as β-mannan offer a modular chassis to produce and encapsulate sensitive cargo such as therapeutic proteins. Graphic abstract

Rapid, modular biosynthesis of plant hemicellulose and its impact on yeast cells

Interest in the engineering of polysaccharide-based biomaterials has emerged in recent years. Despite impressive advances on bacterial cellulose, comparatively little is known about how plant hemicelluloses can be reconstituted and modulated in cells suitable for biotechnological purposes. Here, we optimized the cultivation of the yeast Pichia pastoris for the orthogonal production of plant polysaccharides, and enhanced heteromannan (HM) production by assembling modular cellulose synthase-like A (CSLA) enzymes. Chimeric proteins swapping the domains of a plant mannan synthase and a glucomannan synthase led, in three cases, to higher yields or improved growth compared to the parental CSLA enzymes. Prolonged expression of a glucomannan synthase from Amorphophallus konjac (AkCSLA3) was toxic to yeast cells, as demonstrated by reduced biomass accumulation and elevated uptake of dyes that are normally restricted to the extracellular matrix. However, no growth inhibition was observed for CSLA variants producing relatively pure mannan or a CSLC glucan synthase. The toxicity of AkCSLA3 was reduced by swapping its C-terminal region with that of a mannan synthase. HM production was further boosted by co-expressing chimeric CSLA proteins with the MANNAN-SYNTHESIS-RELATED1 (MSR1) putative glycosyltransferase. Interestingly, Pichia cells either increased or decreased in size depending on the CSLA variant expressed, and most of them remained viable even producing copious amounts of hemicellulose. Therefore, yeast modified with non-toxic plant polysaccharides could represent a modular chassis to produce and protect sensitive cargo such as therapeutic proteins.