ALPHA-MANNOSIDASE-CATALYZED TRIMMING OF HIGH-MANNOSE GLYCANS IN NONINFECTED AND BACULOVIRUS-INFECTED SPODOPTERA-FRUGIPERDA CELLS (IPLB-SF-21AE) - A POSSIBLE CONTRIBUTING REGULATORY MECHANISM FOR ASSEMBLY OF COMPLEX-TYPE OLIGOSACCHARIDES IN INFECTED-CELLS

Incubation of a Spodoptera frugiperda (IPLB-SF-21AE) cell extract with the oligosaccharide Man9GlcNAc2, the aglucosyl derivative of the glycan that is normally transferred from the dolichol carrier to the relevant Asn residue in the nascent protein, results in its trimming to Man6GlcNAc2, an intermediate that is relatively stable to further alpha-D-mannosidase action in these cells. On the other hand, incubation of a similar extract from cells that had been infected for various times with a wild-type baculovirus (Autographa californica nuclear polyhedrosis virus) or a recombinant baculovirus (r-BAC)/human plasminogen (HPg) construct employed for expression of HPg led to rapid trimming of Man6GlcNAc2 to Man5GlcNAc2 and Man3GlcNAc2. These latter reactions displayed temporal effects, in that an enhancement of this latter trimming process occurred as a function of the time of infection of the cells with the wild-type and recombinant viral constructs. We have previously demonstrated that the nature of the oligosaccharide assembled on Asn289 of HPg expressed in several lepidopteran insect cell lines was dependent on the time of infection of the cells with r-BAC/HPg and that the amount of complex glycan found on this recombinant protein increased with an increase in infection times [Davidson, D. J., & Castellino, F. J. (1991) Biochemistry 30, 6167-61741. A significant contributing explanation for these findings is provided herein, in that the infective process leads to an enhancement of an alpha-D-mannosidase activity that catalyzes trimming of the Man6GlcNAc2 that accumulates in noninfected cells, to Man5GlcNAC2, the preferred substrate for GlcNAc transferase I. This latter process is requisite for further processing of high-mannose oligosaccharide to complex and hybrid glycans. The control of Man6GIcNAc2 trimming is one potentially important determinant as to whether complex and/or hybrid glycans will be assembled on glycoproteins.