Two carboxylesterases bind C-reactive protein within the endoplasmic reticulum and regulate its secretion during the acute phase response.

We have previously reported that C-reactive protein (CRP) is normally synthesized by rabbit hepatocytes at relatively low rates and is retained in the endoplasmic reticulum (ER), apparently by specific interaction with a 60-kDa lumenal ER protein. During the acute phase response to tissue injury, a marked increase in CRP synthesis is associated with a decrease in the CRP binding capacity of the 60-kDa protein, with accompanying rapid secretion of CRP. In the present studies, we purified two 60-kDa ER lumenal glycoproteins (referred to as gp60a and gp60b) capable of binding CRP. gp60b, though present at only 5% the level of gp60a, was found to account for 80% of the total CRP binding capacity. Amino-terminal amino acid sequence analysis and biochemical characterization identified gp60a and gp60b as two microsomal carboxylesterases previously reported by others to contain COOH-terminal ER retention signals (HIEL and HTEL). The CRP binding activities of gp60a and gp60b were found to be independent of their esterase activities. In animals undergoing the acute phase response, the levels of gp60a and gp60b were diminished by about 50%, but the CRP binding capacities were reduced by 4-6-fold for gp60a and 25-30-fold for gp60b. These findings indicate that CRP is normally retained within the ER via interaction with gp60a and gp60b, while during the acute phase response a decrease in the CRP binding affinity of these proteins, particularly gp60b, results in efficient secretion of CRP.