Insect Cells Encode a Class II α-Mannosidase with Unique Properties

Abstract

Previously, we cloned and characterized an insect (Sf9) cell cDNA encoding a class II α-mannosidase with amino acid sequence and biochemical similarities to mammalian Golgi α-mannosidase II. Since then, it has been demonstrated that other mammalian class II α-mannosidases can participate in N-glycan processing. Thus, the present study was performed to evaluate the catalytic properties of the Sf9 class II α-mannosidase and to more clearly determine its relationship to mammalian Golgi α-mannosidase II. The results showed that the Sf9 enzyme is cobalt-dependent and can hydrolyze Man5GlcNAC2 to Man3GlcNAC2, but it cannot hydrolyze GlcNAcMan5GlcNAc2. These data establish that the Sf9 enzyme is distinct from Golgi α-mannosidase II. This enzyme is not a lysosomal α-mannosidase because it is not active at acidic pH and it is localized in the Golgi apparatus. In fact, its sensitivity to swainsonine distinguishes the Sf9 enzyme from all other known mammalian class II α-mannosidases that can hydrolyze Man5GlcNAc2. Based on these properties, we designated this enzyme Sf9 α-mannosidase III and concluded that it probably provides an alternate N-glycan processing pathway in Sf9 cells.