Role of alpha-D-mannosidases in the biosynthesis and catabolism of glycoproteins.

In mammalian cells mannose occurs predominantly in the different types of asparagine-linked oligosaccharide chains of glycoproteins. These chains are all derived from a common precursor, which is transferred from a lipid carrier to the growing polypeptide chain in the rough endoplasmic reticulum. During transport of the glycoproteins to their intracellular or extracellular destinations, the asparaginelinked oligosaccharides undergo modification in the endoplasmic reticulum and Golgi apparatus to yield the highmannose, hybrid and complex glycans found in mature glycoproteins (for a review see Kornfeld, 1982). The enzymic hydrolysis of a-( 1 4 2), a-( 1 + 3), and a-( 1 +6) mannosidic linkages occurs during this processing. The a-Dmannosidases that catalyse these steps have very high substrate specificities and precise subcellular locations. The hydrolysis of a-( 1 +2), a-( 1 + 3) and a-( 1 +6) mannosidic linkages also occurs during the catabolism of endogenous and exogenous glycoproteins in the lysosomes. In view of the diverse functions and sites of hydrolysis of a-mannosidic linkages it is not surprising that multiple forms of a-Dmannosidase are present in mammalian cells. Three structurally and genetically distinct types of mammalian a-D-mannosidase with different pH optima and subcellular locations have been described : acidic or lysosomal, intermediate or Golgi, and neutral or cytosolic aD-mannosidase (Table 1) (Phillips et al., 1974a; Opheim & Touster, 1978; Hirani & Winchester, 1979; Bischoff & Kornfeld, 1983). The three types of activity also differ in their kinetics, stability, sensitivity to activators and inhibitors, chromatographic properties and antigenicity. Multiple forms of the lysosomal and Golgi a-D-mannosidase can be separated by chromatography or centrifugation. The properties of the multiple forms of human a-D-mannosidase are reviewed and related to the properties of other mammalian a-D-mannosidases. The activity-pH profile for the soluble a-D-mannosidase activity in human liver has two optima at pH4.0 and 6.5, suggesting the presence of two enzymic components. These activities can be separated by chromatography on concanavalin A-Sepharose (Phillips et a f . , 1976). The a-D-mannosidase that does not bind to the lectin has a neutral pH optimum, whereas the activity that does bind has an acidic pH optimum. It has been shown for several species that the acidic and neutral a-D-mannosidases are located in the lysosomes and cytosol respectively (Suzuki et al., 1969; Marsh & Gourlay, 1971; Shoup & Touster, 1976). The human neutral a-D-mannosidase is labile, activated and stabilized by Co2+, and has a lower K , towards synthetic substrates than the Golgi and lysosomal a-mannosidases. It is also larger ( M , 50000&600000) than the other a-Dmannosidases and does not cross-react with antiserum raised against lysosomal a-D-mannosidase (Phillips et al., 1975). It has been localized to chromosome 15 (Champion et al., 1978), whereas lysosomal a-D-mannosidase is on chromosome 19 (Champion & Shows, 1977). Rat-liver cytosolic a-Dmannosidase is also different from the Golgi and lysosomal a-D-mannosidases (Shoup & Touster, 1976). Although purified rat-liver cytosolic a-D-mannosidase had been shown to hydrolyse the a-( 1 + 2), a-( 1 + 3) and a-( 1 4 6 ) mannosidic linkages in yeast oligosaccharides (Opheim & Touster, 1978), the function of the enzyme was not evident. However, Bischoff & Kornfeld (1983) have recently described an a-D-mannosidase that is located predominantly in the endoplasmic reticulum of rat liver, but which is also present in a soluble form. It has a pH optimum of 6.5 and very similar properties to the cytosolic enzyme. It can catalyse the hydrolysis of a-( 1 +2) mannosidic linkages in high mannose oligosaccharides and is postulated to be responsible for the removal of mannose from asparaginelinked glycans in the endoplasmic reticulum before their entry into the Golgi apparatus. Thus the cytosolic a ~ mannosidase may arise from the integral endoplasmic reticulum membrane a-D-mannosidase by proteolysis or disruption during extraction. The third type of a-D-mannosidase, which has a pH optimum intermediate between the acidic and neutral a-Dmannosidases, was first detected in human serum, where it is particularly abundant, by its sensitivity to Co2 + (Courtois & Mangeot, 1972). It was subsequently found in rat (Dewald & Touster, 1973), human (Phillips et al., 1974a) and bovine (Phillips et al., 19746) tissues. Centrifugation and extraction with detergents showed that intermediate a-D-mannosidase was located in the Golgi membranes of rat liver (Dewald & Touster, 1973) and other tissues (Tabas & Kornfeld, 1979; Forsee & Schutzbach, 1981). Human fibroblast intermediate a-D-mannosidase, which can be separated into multiple forms by chromatography, can also be shown by free-flow electrophoresis or centrifugation to be located in the Golgi-endoplasmic-reticulum-membrane fraction (S. Hirani and B. Winchester, unpublished work). Three forms of rat-liver Golgi a-D-mannosidase exist (Tulsiani et a/., 1977,1982~; Tabas & Kornfeld, 1979; Harpaz & Schachter,