39-kDa receptor-associated protein (RAP) facilitates secretion and ligand binding of extracellular region of very-low-density-lipoprotein receptor: implications for a distinct pathway from low-density-lipoprotein receptor

We have expressed the extracellular regions of the low-density-lipoprotein (LDL) receptor and the very-low-density-lipoprotein (VLDL) receptor, along with the full-length forms of the receptors, in insect cells in a baculovir us system. The extracellular region of the LDL receptor has been secreted s uccessfully into the culture medium, and it retained the capacities of bind ing I-125-labelled LDL and beta-VLDL. In contrast, the extracellular region of the VLDL receptor remained intracellular and it did not bind I-125-beta -VLDL. This difference in expression behaviour between the homologous regio ns of the two receptors suggests that the two receptor systems are differen t in receptor-protein maturation or protein targeting. Next we developed th e co-expression system with 39-kDa receptor-associated protein (RAP). This co-expression facilitated the secretion of the extracellular region of the VLDL receptor into the culture medium and the secreted receptor bound I-125 -beta-VLDL. Th, VLDL receptor remaining intracellular that was co-expressed with RAP also showed binding capacity to I-125-beta-VLDL, implying that th e existence of RAP prevented receptor-protein aggregation or improved prote in-folding status of the truncated VLDL receptor. On the other hand, expres sion of the extracellular region of the LDL receptor was not facilitated by RAP co-expression. Thus RAP plays an essential role in maintenance of the active conformation and secretion of the extracellular region of the VLDL r eceptor.