The association of metalloendopeptidase EC 3.4.24.15 at the extracellular surface of the AtT-20 cell plasma membrane

Mapping sequence differences between thimet oligopeptidase and neurolysin implicates key residues in substrate recognition

The highly homologous endopeptidases thimet oligopeptidase and neurolysin are both restricted to short peptide substrates and share many of the same cleavage sites on bioactive and synthetic peptides. They sometimes target different sites on the same peptide, however, and defining the determinants of differential recognition will help us to understand how both enzymes specifically target a wide variety of cleavage site sequences. We have mapped the positions of the 224 surface residues that differ in sequence between the two enzymes onto the surface of the neurolysin crystal structure. Although the deep active site channel accounts for about one quarter of the total surface area, only 11% of the residue differences map to this region. Four isolated sequence changes (R470/E469, R491/M490, N496/H495, and T499/R498; neurolysin residues given first) are well positioned to affect recognition of substrate peptides, and differences in cleavage site specificity can be largely rationalized on the basis of these changes. We also mapped the positions of three cysteine residues believed to be responsible for multimerization of thimet oligopeptidase, a process that inactivates the enzyme. These residues are clustered on the outside of one channel wall, where multimerization via disulfide formation is unlikely to block the substrate-binding site. Finally, we mapped the regulatory phosphorylation site in thimet oligopeptidase to a location on the outside of the molecule well away from the active site, which indicates this modification has an indirect effect on activity.

Comparative fine structural distribution of endopeptidase 24.15 (EC3.4.24.15) and 24.16 (EC3.4.24.16) in rat brain

Endopeptidase 24.15 (EP24.15) and 24.16 (EP24.16) are closely related metalloendopeptidases implicated in the metabolism of several neuropeptides and widely expressed in mammalian brain. To gain insight into the functional role of these two enzymes in the central nervous system, we examined their cellular and subcellular distribution in rat brain by using electron microscopic immunogold labeling. In all areas examined, EP24.15 and EP24.16 immunoreactivity were observed in selective subpopulations of neuronal and glial cells. Subcellular localization of EP24.15 in neurons revealed that this enzyme was predominantly concentrated in the nucleus, whereas EP24.16 was almost exclusively cytoplasmic. The amount of EP24.15 found in the nucleus was inversely correlated with that found in the cytoplasm, suggesting that the enzyme could be mobilized from one compartment to the other. Within the cytoplasm, EP24.15 and EP24.16 immunoreactivity showed comparable distributional patterns. Both enzymes were detected throughout perikarya and dendrites, as well as within axons and axon terminals. In all neuronal compartments, EP24.15 and EP24.16 showed a major association with membranes of neurosecretory elements, including Golgi cisternae, tubulovesicular organelles, synaptic vesicles, and endosomes. However, whereas EP24.15 always faced the cytoplasmic face of the membranes, EP24.16 was observed on both cytoplasmic and luminal sides, suggesting that the latter was more likely to contribute to the processing of peptides or to the degradation of internalized ligands. Taken together, the present results suggest that EP24.15 could play a major role in the hydrolysis of intranuclear substrates, whereas EP24.16 would be predominantly involved in the processing and inactivation of signaling peptides.

Major histocompatibility complex class I-presented antigenic peptides are degraded in cytosolic extracts primarily by thimet oligopeptidase

Nearly all peptides generated by proteasomes during protein degradation are digested rapidly to amino acids, but a few proteasomal products escape this fate and are presented to the immune system on cell surface major histocompatibility complex class I molecules. To test whether these antigenic peptides may be inherently resistant to cytosolic peptidases, six different antigenic peptides were incubated with HeLa cell extracts. All six were degraded rapidly by a process involving o-phenanthroline-sensitive metallopeptidases. One antigenic peptide, FAPGNYPAL, was rapidly destroyed in the extracts by a bestatin-sensitive exopeptidase, apparently by the puromycin-sensitive aminopeptidase. The disappearance of the other five was reduced 30-90% by a specific inhibitor of the cytosolic endopeptidase, thimet oligopeptidase (TOP) (EC ), whose physiological function(s) have been unclear and controversial. All these peptides were sensitive to pure recombinant TOP. Furthermore, upon fractionation of the extracts, the major peptidase peak that degraded the ovalbumin-derived epitope, SIINFEKL, co-purified with TOP. In the extracts, TOP also catalyzed rapid degradation of N-extended variants of SIINFEKL and of other antigenic peptides, which in vivo can serve as precursors of these major histocompatibility complex-presented epitopes. This enzyme (unlike cell proteins that promote production of antigenic peptides) is not regulated by interferon-gamma. TOP seems to be primarily responsible for the rapid breakdown of antigenic peptides in cytosolic extracts, and our related studies (A. X. Y. Mo, K. Lemerise, W. Zeng, Y. Shen, C. R. Abraham, A. L. Goldberg, and K. L. Rock, submitted for publication) indicate that TOP by destroying such peptides limits antigen presentation in vivo.

The neuropeptide processing enzyme EC 3.4.24.15 is modulated by protein kinase A phosphorylation

The metalloendopeptidase EC (EP24.15) is a neuropeptide-metabolizing enzyme expressed predominantly in brain, pituitary, and testis, and is implicated in several physiological processes and diseases. Multiple putative phosphorylation sites in the primary sequence led us to investigate whether phosphorylation effects the specificity and/or the kinetics of substrate cleavage. Only protein kinase A (PKA) treatment resulted in serine phosphorylation with a stoichiometry of 1.11 +/- 0.12 mol of phosphate/mol of recombinant rat EP24.15. Mutation analysis of each putative PKA site, in vitro phosphorylation, and phosphopeptide mapping indicated serine 644 as the phosphorylation site. Phosphorylation effects on catalytic activity were assessed using physiological (GnRH, GnRH(1-9), bradykinin, and neurotensin) and fluorimetric (MCA-PLGPDL-Dnp and orthoaminobenzoyl-GGFLRRV-Dnp-edn) substrates. The most dramatic change upon PKA phosphorylation was a substrate-specific, 7-fold increase in both K(m) and k(cat) for GnRH. In both rat PC12 and mouse AtT-20 cells, EP24.15 was serine-phosphorylated, and EP24.15 phosphate incorporation was enhanced by forskolin treatment, and attenuated by H89, consistent with PKA-mediated phosphorylation. Cloning of the full-length mouse EP24.15 cDNA revealed 96.7% amino acid identity to the rat sequence, and conservation at serine 644, consistent with its putative functional role. Therefore, PKA phosphorylation is suggested to play a regulatory role in EP24.15 enzyme activity.

N-arginine dibasic convertase (nardilysin) isoforms are soluble dibasic-specific metalloendopeptidases that localize in the cytoplasm and at the cell surface

N-arginine (R) dibasic (NRD) convertase (nardilysin; EC 3.4.24.61), a metalloendopeptidase of the M16 family, specifically cleaves peptide substrates at the N-terminus of arginines in dibasic motifs in vitro. In rat testis, the enzyme localizes within the cytoplasm of spermatids and associates with microtubules of the manchette and axoneme. NRD1 and NRD2 convertases, two NRD convertase isoforms, differ by the absence (isoform 1) or presence (isoform 2) of a 68-amino acid insertion close to the active site. In this study, we overexpressed both isoforms, either by vaccinia virus infection of BSC40 cells or transfection of COS-7 cells. The partially purified enzymes exhibit very similar biochemical and enzymic properties. Microsequencing revealed that NRD convertase is N-terminally processed. Results of immunocytofluorescence, immunoelectron microscopy and subcellular fractionation studies argue in favour of a primary cytosolic localization of both peptidases. Although the putative signal peptide did not direct NRD convertase into microsomes in an in vitro translation assay, biotinylation experiments clearly showed the presence of both isoforms at the cell surface. In conclusion, although most known processing events at pairs of basic residues are achieved by proprotein convertases within the secretory pathway, NRD convertase may fulfil a similar function in the cytoplasm and/or at the cell surface.