Modular biosynthesis of plant hemicellulose and its impact on yeast cells

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.

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.

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.