Cloning and expression of Aplysia carboxypeptidase D, a candidate prohormone-processing enzyme

A first partial Aplysia californica proteome

Aplysia proteins have not been studied systematically and it was therefore the aim of the study to carry out protein profiling in ganglia from Aplysia californica (AC). AC ganglia were extirpated, proteins extracted and run on 2DE with subsequent in-gel digestion, followed by identification of proteins by nano-LC-ESI-MS/MS on an ion trap. Proteins were identified based upon a public Aplysia EST database. Out of 408 picked spots, 276 spots were identified corresponding to 172 ESTs and 118 individual proteins. The range of sequence coverage was between 14 and 80% and the average amount of peptides used for the identification of proteins was 9 (from 3 to 24). Mean score for protein identification was 516. Comparison of protein levels between cerebral, pleural, pedal and abdominal ganglia revealed a series of significant differences including: signaling, metabolism, cytoskeleton and structural, redox, chaperone, replication/transcription and electron/proton transport proteins. The generation of a protein map complements transcriptional studies carried out in AC ganglia. The findings provide the basis for investigation into post-translational modifications, splice variants and assist in the generation of antibodies against AC proteins. Moreover, differences in protein expression between ganglia may be valuable for the design of future studies in neurobiology of AC.

Characterization of the molecular basis of the Drosophila mutations in carboxypeptidase D. Effect on enzyme activity and expression

Carboxypeptidase D (CPD) functions in the processing of proteins and peptides in the secretory pathway. Drosophila CPD is encoded by the silver gene (svr), which is differentially spliced to produce long transmembrane protein forms with three metallocarboxypeptidase (CP)-like domains and short soluble forms with a single CP domain. Many svr mutants have been reported, but the precise molecular defects have not been previously determined. In the present study, three mutant lines were characterized. svr (PG33) mutants do not survive past the early larval stage. These mutants have a P-element insertion within exon 1B upstream of the initiation ATG, which greatly reduces mRNA levels of all forms of CPD. Both svr (1) and svr (poi) mutants are viable, with a silvery body color and pointed wings. The wing shape is generally similar between these two mutants, although svr (poi) mutants have smaller wings. The svr (1) gene has a three-nucleotide deletion in exon 6, removing a leucine in a region of the protein predicted to function as a folding domain for the second CP-like domain. svr (poi) has a 1072-bp duplication of the gene that introduces a stop codon into the open reading frame, causing the truncation of the protein in the middle of the second CP-like domain. Both deletions eliminate enzyme activity of the second CP-like domain and appear to cause the misfolding of the protein. This greatly reduces the levels of the long forms of CPD protein but do not affect the levels of the short forms. Taken together, these findings suggest that lethal and viable svr alleles differ in which protein forms are affected. Flies that retain the short form are viable, whereas flies that are missing all forms of CPD do not survive past the early larval stages.

Palmitoylation of carboxypeptidase D. Implications for intracellular trafficking

Covalent lipid modifications mediate protein-membrane and protein-protein interactions and are often essential for function. The purposes of this study were to examine the Cys residues of the transmembrane domain of metallocarboxypeptidase D (CPD) that could be a target for palmitoylation and to clarify the function of this modification. CPD is an integral membrane protein that cycles between the trans Golgi network and the plasma membrane. We constructed AtT-20 cells stably expressing various constructs carrying a reporter protein (albumin) fused to a transmembrane domain and the CPD cytoplasmic tail. Some of the constructs contained the three Cys residues present in the CPD transmembrane region, while other constructs contained Ala in place of the Cys. Constructs carrying Cys residues were palmitoylated, while those constructs lacking the Cys residues were not. Because palmitoylation of several proteins affects their association with cholesterol and sphingolipid-rich membrane domains or caveolae, we tested endogenous CPD and several of the reporter constructs for resistance to extraction with Triton X-100. A construct containing the Cys residues of the CPD transmembrane domain was soluble in Triton X-100 as was endogenous palmitoylated CPD, indicating that palmitoylation does not target CPD to detergent-resistant membrane rafts. Interestingly, constructs of CPD that lack palmitoylation sites have an increased half-life, a slightly more diffuse steady-state localization, and a slower rate of exit from the Golgi as compared with constructs containing palmitoylation sites. Thus, the covalent attachment of palmitic acid to the Cys residues of CPD has a functional significance in the trafficking of the protein.

Immunohistochemical localization of carboxypeptidases D, E, and Z in pituitary adenomas and normal human pituitary

Carboxypeptidases may play important role(s) in prohormone processing in normal and neoplastic adenohypophyseal cells of the pituitary. We have recently demonstrated carboxypeptidase E (CPE) and carboxypeptidase Z (CPZ) in the majority of adenohypophyseal cells with carboxypeptidase D (CPD) immunoreactivity largely confined to adrenocorticotrophs. This study evaluated the expression patterns of CPE, CPD, and CPZ immunoreactivity in 48 pituitary adenomas. Our immunohistochemistry demonstrated extensive intracytoplasmic immunoreactivity for CPE, CPD, and CPZ in adrenocorticotrophic hormone (ACTH)-producing adrenocorticotroph cells, prolactin-producing lactotroph cells, and growth hormone (GH)-producing somatotroph cell adenomas, all of which require carboxypeptide processing of prohormones to produce active endocrine hormones. In contrast to the restricted expression in the normal adenohypophysis, CPD appeared to be widespread in the majority of adenomas, suggesting that CPD levels are increased in adenomas. In luteinizing hormone/follicle-stimulating hormone (LH/FSH)-producing gonadotroph adenomas, which do not require carboxypeptidases to produce gonadotropins, only CPZ immunostaining was demonstrated. In null-cell adenomas, CPE immunoreactivity was detected in the majority of tumors, but CPD and CPZ were identified only in a minority of cases. CPE in these cells may process other peptides critical for pituitary cell function, such as chromogranin A or B. These findings suggest that CPs participate in the functioning of pituitary adenomas.

Purification and characterization of arginine carboxypeptidase produced by Porphyromonas gingivalis

Arginine carboxypeptidase was isolated from the cytoplasm of Porphyromonas gingivalis 381 and purified by DEAE-Sephacel column chromatography, followed by high-performance liquid chromatography on DEAE-5PW and TSK G2000SW(XL). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme revealed the presence of three major bands at 42, 33, and 32 kDa with identical N-terminal sequences. By Western blotting analysis and immunoelectron microscopy, the arginine carboxypeptidase was found to be widely distributed in the cytoplasm and on the surface of the outer membrane. The open reading frame corresponding to the N-terminal amino acids of the arginine carboxypeptidase was detected by a search of the sequence of the P. gingivalis W83 genome. This sequence showed homology with mammalian carboxypeptidases (M, N, and E/H) and included a zinc-binding region signature, suggesting that the enzyme is a member of the zinc carboxypeptidase family. The purified enzyme was inhibited by EGTA, o-phenanthroline, DL-2-mercaptomethyl-3-guanidinoethylthiopropanoic acid, and some metal ions, such as Cu(2+), Zn(2+), and Cd(2+). On the other hand, Co(2+) activated the enzyme. The enzyme released arginine and/or lysine from biologically active peptides containing these amino acids at the C terminus but did not cleave substrates when proline was present at the penultimate position. These results indicate that the arginine carboxypeptidase produced by P. gingivalis is an exo type of metallocarboxypeptidase. This enzyme may function to release arginine in collaboration with an arginine aminopeptidase, e.g., Arg-gingipain, to obtain specific amino acids from host tissues during the growth of P. gingivalis.

The crystal structure of the inhibitor-complexed carboxypeptidase D domain II and the modeling of regulatory carboxypeptidases

The three-dimensional crystal structure of duck carboxypeptidase D domain II has been solved in a complex with the peptidomimetic inhibitor, guanidinoethylmercaptosuccinic acid, occupying the specificity pocket. This structure allows a clear definition of the substrate binding sites and the substrate funnel-like access. The structure of domain II is the only one available from the regulatory carboxypeptidase family and can be used as a general template for its members. Here, it has been used to model the structures of domains I and III from the former protein and of human carboxypeptidase E. The models obtained show that the overall topology is similar in all cases, the main differences being local and because of insertions in non-regular loops. In both carboxypeptidase D domain I and carboxypeptidase E slightly different shapes of the access to the active site are predicted, implying some kind of structural selection of protein or peptide substrates. Furthermore, emplacement of the inhibitor structure in the active site of the constructed models showed that the inhibitor fits very well in all of them and that the relevant interactions observed with domain II are conserved in domain I and carboxypeptidase E but not in the non-active domain III because of the absence of catalytically indispensable residues in the latter protein. However, in domain III some of the residues potentially involved in substrate binding are well preserved, together with others of unknown roles, which also are highly conserved among all carboxypeptidases. These observations, taken together with others, suggest that domain III might play a role in the binding and presentation of proteins or peptide substrates, such as the pre-S domain of the large envelope protein of duck hepatitis B virus.

Neuropeptide amidation: cloning of a bifunctional alpha-amidating enzyme from Aplysia

One of the most common mechanisms of posttranslational modifications to generate biologically active (neuro)peptides is the process of peptide alpha-amidation. The only enzyme known to catalyze this important modification is peptidylglycine alpha-amidating monooxygenase (PAM): a (bifunctional) zymogen, giving rise to a monooxygenase (PHM) and a lyase (PAL). The highly peptidergic central nervous system and endocrine system of the marine mollusk Aplysia has homologs of various mammalian peptide processing enzymes, including furin, Afurin2, prohormone convertase 1 (PC1), PC2, carboxypeptidase E (CPE) and CPD. Previously, it has been shown that the abdominal ganglion of Aplysia, which contains approximately 800 peptidergic bag cell neurons, contains the highest specific alpha-amidating activity. We have identified and cloned multiple overlapping central nervous system and bag cell cDNAs that encode a predicted 748-residue protein that is a member of the PAM family. The protein sequence contains the contiguous sequence of the catalytic domains of PHM and PAL, clearly demonstrating the existence of bifunctional Aplysia PAM, the first invertebrate PAM zymogen with an organization similar to that in vertebrates. None of the characterized clones encoded the so-called exon A domain between the PHM and PAL domains. Furthermore, in a specific search by reverse transcription-polymerase chain reaction of RNA from multiple tissues we could only detect exon A-less transcripts. PAM expression was detected in the central nervous system, and in several endocrine and exocrine organs. Aplysia PAM is a candidate prohormone processing enzyme that plays an important role in the processing of Aplysia prohormones in the secretory pathway.

Characterization of the enzymatic properties of the first and second domains of metallocarboxypeptidase D

Carboxypeptidase D (CPD) contains three domains with homology to other metallocarboxypeptidases. To further characterize the various domains, we constructed a series of point mutants with a critical active site Glu of duck CPD converted to Gln. The proteins were expressed in the baculovirus system, purified to homogeneity, and characterized. Point mutations within both the first and second domains eliminated enzyme activity, indicating that the third domain is inactive toward dansyl-Phe-Ala-Arg. CPD removed only the C-terminal Lys or Arg from peptides, with the first domain more efficient toward Arg and the second domain more efficient toward Lys. Peptides containing Pro in the penultimate position were poorly cleaved by either domain. Cleavage of a peptide with Ala in the penultimate position was most efficient, with the relative order Ala >/= Met > Ser, Phe > Tyr > Trp > Thr >/= Gln, Asp, Leu, Gly >> Pro for CPD with both domains active. There were only minor differences between the first and the second domains regarding the influence of the penultimate amino acid. The first domain was optimally active at pH 6.3-7.5, whereas the second domain was optimally active at pH 5. 0-6.5. Thus, the first and second carboxypeptidase domains have complementary enzyme activities. Furthermore, the finding that CPD with both domains active shows a broad activity to a wide range of substrates is consistent with a role for this enzyme in the processing of many proteins that transit the secretory pathway.

Localization of metallocarboxypeptidase D in AtT-20 cells. Potential role in prohormone processing

Carboxypeptidase D (CPD) is a recently discovered metallocarboxypeptidase that is predominantly located in the trans-Golgi network (TGN), and also cycles between the cell surface and the TGN. In the present study, the intracellular distribution of CPD was examined in AtT-20 cells, a mouse anterior pituitary-derived corticotroph. CPD-containing compartments were isolated using antibodies to the CPD cytosolic tail. The immunopurified vesicles contained TGN proteins (TGN38, furin, syntaxin 6) but not lysosomal or plasma membrane proteins. The CPD-containing vesicles also contained neuropeptide-processing enzymes and adrenocorticotropic hormone, a product of proopiomelanocortin proteolysis. Electron microscopic analysis revealed that CPD is present within the TGN and immature secretory granules but is virtually absent from mature granules, suggesting that CPD is actively removed from the regulated pathway during the process of granule maturation. A second major finding of the present study is that a soluble truncated form of CPD is secreted mainly via the constitutive pathway in AtT-20 cells, indicating that the lumenal domain does not contain signals for the sorting of CPD to mature secretory granules. Taken together, these data are consistent with the proposal that CPD participates in the processing of proteins within the TGN and immature secretory vesicles.