A Histidine-rich Linker Region in Peptidylglycine Alpha-Amidating Monooxygenase has the Properties of a pH-sensor [Neurobiology]

March 13th, 2014 by Vishwanatha, K., Back, N., Mains, R. E., Eipper, B. A.

Decreasing luminal pH is thought to play a role in the entry of newly synthesized and endocytosed membrane proteins into secretory granules. The two catalytic domains of PAM, a type I integral membrane protein, catalyze the sequential reactions that convert peptidyl-Gly substrates into amidated products. We explored the hypothesis that a conserved His-rich cluster (His-Gly-His-His) in the linker region connecting its two catalytic domains senses pH and affects PAM trafficking by mutating these His residues to Ala (Ala-Gly-Ala-Ala; H3A). Purified recombinant wildtype and H3A linker peptides were examined using circular dichroism and tryptophan fluorescence; mutation of the His-cluster largely eliminated its pH-sensitivity. An enzymatically active PAM protein with the same mutations (PAM-1/H3A) was expressed in hEK-293 cells and AtT-20 corticotrope tumor cells. Metabolic labeling followed by immunoprecipitation revealed more rapid loss of newly synthesized PAM-1/H3A than PAM-1; although release of newly synthesized monofunctional PHM/H3A was increased, release of soluble bifunctional PAM/H3A, a product of the endocytic pathway was decreased. Surface biotinylation revealed rapid loss of PAM-1/H3A, with no detectable return of the mutant protein to secretory granules. Consistent with its altered endocytic trafficking, little PAM-1/H3A was subjected to regulated intramembrane proteolysis followed by release of a small nuclear-targeted cytosolic fragment. AtT-20 cells expressing PAM-1/H3A adopted the morphology of wildtype AtT-20 cells; secretory products no longer accumulated in the TGN and secretory granule exocytosis was more responsive to secretagogue.