Structure, assembly, and secretion of octameric invertase.

Yeast invertase forms a homo-octamer of core glycosylated subunits during assembly in the lumen of the endoplasmic reticulum. This form has been purified from mutant cells (sec18) in which transport of secreted proteins from the endoplasmic reticulum is blocked. No heterologous protein subunits are found in the purified material. Analysis of invertase derived from wild type cells or from mutant cells blocked at subsequent stages in secretion demonstrates that invertase remains a homo-octamer throughout the pathway even though the extent of subunit glycosylation increases. Purified octameric invertase is dissociated into dimer units that reassociate in the presence of polyethylene glycol. Negatively stained preparations show the dissociated enzyme as individual spheres, whereas octameric invertase appears as four associated spheres. Assembly of the octamer in vitro and in vivo is facilitated by the presence of N-linked carbohydrate. Selective release of dimeric glycosylated invertase from intact yeast cells suggests that oligomerization helps retain the enzyme in the periplasmic space.