The cargo receptor p24A facilitates calcium sensing receptor maturation and stabilization in the early secretory pathway

The calcium sensing receptor (CaSR) is a Family 3/C G protein-coupled receptor with slow and partial targeting to the plasma membrane in both native and heterologous cells. We identified cargo receptor family member p24A in yeast two-hybrid screens with the CaSR carboxyl terminus. Interactions were confirmed by immunoprecipitation of either p24A or CaSR in transiently transfected HEK293 cells. Only the immaturely glycosylated form of CaSR interacts with p24A. Dissociation likely occurs in the endoplasmic reticulum–Golgi intermediate compartment (ERGIC) or cis-Golgi, since only the uncleaved form of a CaSR mutant sensitive to the trans-Golgi enzyme furin was co-immunoprecipitated with p24A. p24A and p24A(ΔGOLD) significantly increased total and plasma membrane CaSR protein but p24A(FF/AA) did not. The CaSR carboxyl terminus distal to T868 is required for differential sensitivity to p24A and its mutants. Interaction with p24A therefore increases CaSR stability in the ER and enhances plasma membrane targeting. Neither wt Sar1p or the T39N mutant increased CaSR maturation or abundance while the H79G mutant increased abundance but prevented maturation of CaSR. These results suggest that p24A is the limiting factor in CaSR trafficking in the early secretory pathway, and that cycling between the ER and ERGIC protects CaSR from degradation.