Release of glycosylphosphatidylinositol-anchored carboxypeptidase M by phosphatidylinositol-specific phospholipase C upregulates enzyme synthesis

Carboxypeptidase M is a positive allosteric modulator of the kinin B1 receptor

Ligand binding to extracellular domains of G protein-coupled receptors can result in novel and nuanced allosteric effects on receptor signaling. We previously showed that the protein-protein interaction of carboxypeptidase M (CPM) and kinin B1 receptor (B1R) enhances B1R signaling in two ways; 1) kinin binding to CPM causes a conformational activation of the B1R, and 2) CPM-generated des-Arg-kinin agonist is efficiently delivered to the B1R. Here, we show CPM is also a positive allosteric modulator of B1R signaling to its agonist, des-Arg(10)-kallidin (DAKD). In HEK cells stably transfected with B1R, co-expression of CPM enhanced DAKD-stimulated increases in intracellular Ca(2+) or phosphoinositide turnover by a leftward shift of the dose-response curve without changing the maximum. CPM increased B1R affinity for DAKD by ∼5-fold but had no effect on basal B1R-dependent phosphoinositide turnover. Soluble, recombinant CPM bound to HEK cells expressing B1Rs without stimulating receptor signaling. CPM positive allosteric action was independent of enzyme activity but depended on interaction of its C-terminal domain with the B1R extracellular loop 2. Disruption of the CPM/B1R interaction or knockdown of CPM in cytokine-treated primary human endothelial cells inhibited the allosteric enhancement of CPM on B1R DAKD binding or ERK1/2 activation. CPM also enhanced the DAKD-induced B1R conformational change as detected by increased intramolecular fluorescence or bioluminescence resonance energy transfer. Thus, CPM binding to extracellular loop 2 of the B1R results in positive allosteric modulation of B1R signaling, and disruption of this interaction could provide a novel therapeutic approach to reduce pathological B1R signaling.

Interaction of angiotensin-converting enzyme (ACE) with membrane-bound carboxypeptidase M (CPM) - a new function of ACE

Angiotensin-converting enzyme (ACE) demonstrates, besides its typical dipeptidyl-carboxypeptidase activity, several unusual functions. Here, we demonstrate with molecular, biochemical, and cellular techniques that the somatic wild-type murine ACE (mACE), stably transfected in Chinese Hamster Ovary (CHO) or Madin-Darby Canine Kidney (MDCK) cells, interacts with endogenous membranal co-localized carboxypeptidase M (CPM). CPM belongs to the group of glycosylphosphatidylinositol (GPI)-anchored proteins. Here we report that ACE, completely independent of its known dipeptidase activities, has GPI-targeted properties. Our results indicate that the spatial proximity between mACE and the endogenous CPM enables an ACE-evoked release of CPM. These results are discussed with respect to the recently proposed GPI-ase activity and function of sperm-bound ACE.

Kinin- and angiotensin-converting enzyme (ACE) inhibitor-mediated nitric oxide production in endothelial cells

Carboxypeptidase cleavage of the C-terminal Arg of kinins generates specific agonists of the B1 receptor. Activation of B1 receptors produces nitric oxide via eNOS in bovine endothelial cells and iNOS in cytokine-stimulated human endothelial cells. Angiotensin-converting enzyme (ACE) inhibitors are direct agonists of B1 receptors in endothelial cells, although they release NO via a different signaling pathway than peptide ligands in bovine cells. This brief review discusses carboxypeptidase M as a required processing enzyme for generating B1 agonists, how ACE inhibitors and peptide ligands stimulate NO production and the evidence for, as well as some consequences of, the direct activation of B1 receptors by ACE inhibitors.

Effect of mutation of two critical glutamic acid residues on the activity and stability of human carboxypeptidase M and characterization of its signal for glycosylphosphatidylinositol anchoring

Human carboxypeptidase (CP) M was expressed in baculovirus-infected insect cells in a glycosylphosphatidylinositol-anchored form, whereas a truncated form, lacking the putative signal sequence for glycosylphosphatidylinositol anchoring, was secreted at high levels into the medium. Both forms had lower molecular masses (50 kDa) than native placental CPM (62 kDa), indicating minimal glycosylation. The predicted glycosylphosphatidylinositol-anchor attachment site was investigated by mutation of Ser(406) to Ala, Thr or Pro and expression in HEK-293 and COS-7 cells. The wild-type and S406A and S406T mutants were expressed on the plasma membrane in glycosylphosphatidylinositol-anchored form, but the S406P mutant was not and was retained in a perinuclear location. The roles of Glu(260) and Glu(264) in CPM were investigated by site-directed mutagenesis. Mutation of Glu(260) to Gln had minimal effects on kinetic parameters, but decreased heat stability, whereas mutation to Ala reduced the k(cat)/ K(m) by 104-fold and further decreased stability. In contrast, mutation of Glu(264) to Gln resulted in a 10000-fold decrease in activity, but the enzyme still bound to p-aminobenzoylarginine-Sepharose and was resistant to trypsin treatment, indicating that the protein was folded properly. These results show that Glu(264) is the critical catalytic glutamic acid and that Glu(260) probably stabilizes the conformation of the active site.

Carboxypeptidase M, a glycosylphosphatidylinositol-anchored protein, is localized on both the apical and basolateral domains of polarized Madin-Darby canine kidney cells

Carboxypeptidase M, a glycosylphosphatidylinositol-anchored membrane glycoprotein, is highly expressed in Madin-Darby canine kidney (MDCK) cells, where it was previously shown that the glycosylphosphatidylinositol anchor and N-linked carbohydrate are apical targeting signals. Here, we show that carboxypeptidase M has an unusual, non-polarized distribution, with up to 44% on the basolateral domain of polarized MDCK cells grown on semipermeable inserts. Alkaline phosphatase, as well as five other glycosylphosphatidylinositol-anchored proteins, and transmembrane gamma-glutamyl transpeptidase exhibited the expected apical localization. Basolateral carboxypeptidase M was readily released by exogenous phosphatidylinositol-specific phospholipase C, showing it is glycosylphosphatidylinositol-anchored, whereas apical carboxypeptidase M was more resistant to release. In contrast, the spontaneous release of carboxypeptidase M into the medium was much higher on the apical than the basolateral domain. In pulse-chase studies, newly synthesized carboxypeptidase M arrived in equal amounts within 30 min on both domains, indicating direct sorting. After 4-8 h of chase, the steady-state distribution was attained, possibly due to transcytosis from the basolateral to the apical domain. These data suggest the presence of a unique basolateral targeting signal in carboxypeptidase M that competes with its apical targeting signals, resulting in a non-polarized distribution in MDCK cells.