Role of the linker region in the expression of Rhizopus oryzae glucoamylase

Abstract

Abstract Background Rhizopus oryzae glucoamylase (Ro GA) consists of three domains: an amino (N)-terminal raw starch-binding domain (SBD), a glycosylated linker domain, and a carboxy (C)-terminal catalytic domain. The 36-amino-acid linker region (residues 132–167) connects the two functional domains, but its structural and functional roles are unclear. Results To characterize the linker sequences of Ro GA and its involvement in protein expression, a number of Ro GA variants containing deletions and mutations were constructed and expressed in Saccharomyces cerevisiae. Deletion analyses demonstrate that the linker region, especially within residues 161 to 167, is required for protein expression. In addition, site-directed mutagenesis and deglycosylation studies reveal that the linker region of Ro GA contains both N- and O-linked carbohydrate moieties, and the N-linked oligosaccharides play a major role in the formation of active enzyme. Although the linker segment itself appears to have no ordered secondary structural conformation, the flexible region indeed contributes to the stabilization of functional N- and C-terminal domains. Conclusion Our data provide direct evidence that the length, composition, and glycosylation of the interdomain linker play a central role in the structure and function of Ro GA.