Plant cellulose synthase membrane protein isolation directly from Pichia pastoris protoplasts, liposome reconstitution, and its enzymatic characterization

The most abundant renewable biopolymer on earth, viz., cellulose, acts as carbon storage reserve in plant and microbial cell walls that could potentially be converted into biofuels or other valuable bioproducts. Cellulose is synthesized by a plant cell membrane-integrated processive glycosyltransferase (GT) called cellulose synthase (CesA). Since only a few of these plant CesAs have been purified and characterized to date, there are huge gaps in our mechanistic understanding of these enzymes. Furthermore, the coordination between different CesAs involved in primary and secondary cell wall formation is yet to be unveiled. The biochemistry and structural biology studies of CesAs are currently hampered by challenges associated with their expression and extraction at high yields. To aid in understanding CesA reaction mechanisms and to provide a more efficient CesA extraction method, two putative plant CesAs - PpCesA5 from Physcomitrella patens and PttCesA8 from Populus tremula x tremuloides that are involved in primary and secondary cell wall formation in plants were expressed using Pichia pastoris as an expression host. We developed a protoplast-based membrane protein extraction approach to directly isolate both these membrane-bound enzymes for purification, as detected by immunoblotting and mass spectrometry-based analyses. Our method results in a higher purified protein yield by 3-4-fold than the standard cell homogenization protocol. Our purified CesAs were reconstituted into liposomes to yield active enzymes that gave similar biochemical characteristics (e.g., substrate utilization and cofactor requirements, no primer needed to initiate polymerization reaction) as enzymes isolated using the standard protocol. This method resulted in reconstituted CesA5 and CesA8 with similar Michaelis-Menten kinetic constants, Km = 167 M, 108 M and Vmax = 7.88x10-5 mol/min, 4.31x10-5 mol/min, respectively, in concurrence with the previous studies. Taken together, these results suggest that CesAs involved in primary and secondary cell wall formation can be expressed and purified using a simple and more efficient extraction method. This could potentially help unravel the mechanism of native and engineered cellulose synthase complexes involved in plant cell wall biosynthesis.