Cleavage in the GDPH sequence of the C-terminal cysteine-rich part of the human MUC5AC mucin

Martin E. Lidell , Gunnar C. Hansson (1)

1 Department of Medical Biochemistry

MUC5AC is the main gel-forming mucin expressed by goblet cells of the airways and stomach where it protects the underlying epithelia. We expressed the C-terminal cysteine-rich part of the human MUC5AC mucin in CHO-K1 cells where it formed disulfide-linked dimers in the endoplasmic reticulum. After reducing the disulfide bonds of these dimers, not only the expected monomers were found, but also two smaller fragments indicating that the protein was partially cleaved. The site of cleavage was located to an Asp-Pro bond situated in a GDPH-sequence found in the D4-domain. This sequence is also found in the human MUC2 mucin, previously shown to be cleaved at the same site by a slow, non-enzymatic process triggered by a pH below 6. In contrast to this, the cleavage of MUC5AC started already in the neutral endoplasmic reticulum. However, it continued and was slightly accelerated at a pH below 6.5, a pH found in the later parts of the secretory pathway. The cleavage generated a reactive group in the new C-terminus that could link the protein to a primary amine. No cleavage of MUC5AC has so far been reported. By using an antibody reacting with the C-terminal cleavage fragment, we could verify that the cleavage occurs in wild-type MUC5AC produced by HT-29 cells. The cleavage of MUC5AC and the generation of the reactive new C-terminus could contribute to the adherent and viscous mucus found at chronic lung diseases such as asthma and cystic fibrosis, characterized by mucus hypersecretion and lowered pH of the airways.

Mots clés: Life Sciences

Martin E. Lidell, Gunnar C. Hansson. Cleavage in the GDPH sequence of the C-terminal cysteine-rich part of the human MUC5AC mucin. Biochemical Journal, 2006, 399 (1), pp.121-129. ⟨10.1042/BJ20060443⟩. ⟨hal-00478563⟩