OLIGOMERIZATION SUPPORTS TRANSPORT OF A MUTANT SECRETORY PROTEIN OUT OF THE ENDOPLASMIC-RETICULUM

To examine the transport of the single subunits of the glycoprotein co mplex gp80 (clusterin, apolipoprotein J), a marker protein for apical exocytosis and main secretory product of Madin-Darby canine kidney (MD CK) cells, several mutant cDNAs were constructed and expressed in the fibroblastic baby hamster kidney (BHK-21) cell line. In the absence of the second submit the mutant proteins formed disulfide-linked homodim eric complexes. In the homodimeric form lys-gp45, a lysozyme-tagged mu tant representing the C-terminal submit, acquired competence for trans port to the cell surface. Biogenesis and transport of this hybrid prot ein were also examined in stably transfected MDCK cells. In these cell s which express both the endogenous gene and the mutant cDNA lys-gp45 was linked via disulfide bonds to the gp80 complex. These heterooligom eric complexes were secreted predominantly at the apical cell surface. Thus, oligomerization regardless of whether it resulted in homodimeri c or heterooligomeric complexes conveyed transport competence to the m utant protein.