The spliceosome is the complex macromolecular machine responsible for removing introns from precursors to mRNAs (pre-mRNAs). We combined yeast genetic engineering, chemical biology, and multi-wavelength fluoresence microscopy to follow assembly of single spliceosomes in real time in whole cell extracts. We find that individual spliceosomal subcomplexes associate with pre-mRNA sequentially via an ordered pathway to yield functional spliceosomes, and that association of every subcomplex is reversible. Further, early subcomplex binding events do not fully commit a pre-mRNA to splicing; rather commitment increases as assembly proceeds. These findings have important implications for the regulation of alternative splicing. This experimental strategy should prove widely useful for mechanistic analysis of other macromolecular machines in environments approaching the complexity of living cells.

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Dissociation of non-productively bound cellulolytic enzymes from cellulose is hypothesized to be a key rate-limiting factor impeding cost-effective biomass conversion to fermentable sugars. However, the role of carbohydrate-binding modules (CBMs) in enabling non-productive enzyme binding is not well understood. Here, we examine the subtle interplay of CBM binding and cellulose hydrolysis activity for three model Type-A CBMs (families 1, 3a, and 64) tethered to a multifunctional endoglucanase (CelE) on two distinct cellulose allomorphs (i.e., cellulose I and III). We generated a small-library of mutant CBMs with varying cellulose affinity, as determined by equilibrium binding assays, followed by monitoring cellulose hydrolysis activity of CelE-CBM fusion constructs. Finally, kinetic binding assays using quartz crystal microbalance with dissipation (QCM-D) were employed to measure CBM adsorption and desorption rate constants and , respectively, towards nanocrystalline cellulose derived from both allomorphs. Overall, our results indicate that reduced CBM equilibrium binding affinity towards cellulose I alone, resulting from increased desorption rates () and reduced effective adsorption rates (), is correlated to overall improved endocellulase activity. Future studies could employ similar approaches to unravel the role of CBMs in non-productive enzyme binding and develop improved cellulolytic enzymes for industrial applications.

AbstractDissociation of non-productively bound cellulolytic enzymes from cellulose is hypothesized to be a key rate-limiting factor impeding cost-effective biomass conversion to fermentable sugars. However, the role of carbohydrate-binding modules (CBMs) in enabling non-productive enzyme binding is not well understood. Here, we examine the subtle interplay of CBM binding and cellulose hydrolysis activity for three model Type-A CBMs (families 1, 3a, and 64) tethered to a multifunctional endoglucanase (CelE) on two distinct cellulose allomorphs (i.e., cellulose I and III). We generated a small-library of mutant CBMs with varying cellulose affinity, as determined by equilibrium binding assays, followed by monitoring cellulose hydrolysis activity of CelE-CBM fusion constructs. Finally, kinetic binding assays using quartz crystal microbalance with dissipation (QCM-D) were employed to measure CBM adsorption and desorption rate constants Kon and Koff, respectively, towards nanocrystalline cellulose derived from both allomorphs. Overall, our results indicate that reduced CBM equilibrium binding affinity towards cellulose I alone, resulting from increased desorption rates (Koff) and reduced effective adsorption rates (nKon), is correlated to overall improved endocellulase activity. Future studies could employ similar approaches to unravel the role of CBMs in non-productive enzyme binding and develop improved cellulolytic enzymes for industrial applications.