Cloning and expression of protein tyrosine phosphatase-like protein derived from a rat pheochromocytoma cell line

Expression and function of IA-2 family proteins, unique neuroendocrine-specific protein-tyrosine phosphatases

IA-2 (also known as islet cell antigen ICA-512) and IA-2 beta (also known as phogrin, phosphatase homologue in granules of insulinoma) are major autoantigens in insulin-dependent diabetes mellitus (IDDM). Autoantibodies against both proteins are expressed years before clinical onset, and they become predictive markers for high-risk subjects. However, the role of these genes in the IDDM pathogenesis has been reported fairly negative by recent studies. IA-2 and IA-2 beta are type I transmembrane proteins that possess one inactive protein-tyrosine phosphatase (PTP) domain in the cytoplasmic region, and act as one of the constituents of regulated secretory pathways in various neuroendocrine cell types including pancreatic beta-cells. Existence of IA-2 homologues in different species suggests a fundamental role in neuroendocrine function. Studies of knockout animals have shown their involvement in maintaining hormone content, however, their specific steps in the secretory pathway IA-2 functions as well as their molecular mechanisms in the hormone content regulation are still unknown. More recent studies have suggested a novel function showing that they contribute to pancreatic beta-cell growth. This review attempts to show the possible biological functions of IA-2 family, focusing on their expression and localization in the neuroendocrine cells.

Evolution of the multifunctional protein tyrosine phosphatase family

The protein tyrosine phosphatase (PTP) family plays a central role in signal transduction pathways by controlling the phosphorylation state of serine, threonine, and tyrosine residues. PTPs can be divided into dual specificity phosphatases and the classical PTPs, which can comprise of one or two phosphatase domains. We studied amino acid substitutions at functional sites in the phosphatase domain and identified putative noncatalytic phosphatase domains in all subclasses of the PTP family. The presence of inactive phosphatase domains in all subclasses indicates that they were invented multiple times in evolution. Depending on the domain composition, loss of catalytic activity can result in different consequences for the function of the protein. Inactive single-domain phosphatases can still specifically bind substrate and protect it from dephosphorylation by other phosphatases. The inactive domains of tandem phosphatases can be further subdivided. The first class is more conserved, still able to bind phosphorylated tyrosine residues and might recruit multiphosphorylated substrates for the adjacent active domain. The second has accumulated several variable amino acid substitutions in the catalytic center, indicating a complete loss of tyrosine-binding capabilities. To study the impact of substitutions in the catalytic center to the evolution of the whole domain, we examined the evolutionary rates for each individual site and compared them between the classes. This analysis revealed a release of evolutionary constraint for multiple sites surrounding the catalytic center only in the second class, emphasizing its difference in function compared with the first class. Furthermore, we found a region of higher conservation common to both domain classes, suggesting a new regulatory center. We discuss the influence of evolutionary forces on the development of the phosphatase domain, which has led to additional functions, such as the specific protection of phosphorylated tyrosine residues, substrate recruitment, and regulation of the catalytic activity of adjacent domains.

Post-translational modifications of ICA512, a receptor tyrosine phosphatase-like protein of secretory granules

The autoantigen of type I diabetes ICA512 is a receptor tyrosine phosphatase-like protein enriched in the secretory granule membranes of neurons and peptide secreting endocrine cells. While the function of ICA512 remains unknown, it is thought to link regulated neuropeptide and peptide hormone secretion with signal transduction pathways involving tyrosine phosphorylation/dephosphorylation. To characterize further its biochemical properties, we conducted studies in the bovine pituitary, an abundant source of native ICA512, as well as in fibroblasts transfected with various human ICA512 cDNA constructs. Based on these studies we have established that the signal peptide of ICA512 encompasses residues 1-34 and that the ectodomain of ICA512 undergoes multiple post-translation modifications, including N-glycosylation. Newly synthesized ICA512 appears first as a pro-protein of 110 kDa that is then converted by post-translational modifications into a 130-kDa species. Cleavage of pro-ICA512 at a consensus for furin-like convertases generates a 60-66-kDa ICA512 transmembrane fragment (amino acids 449-979). Such processing ICA512 is not restricted to neuroendocrine cells, as it can also occur in transfected fibroblasts. Finally, the predicted N-terminal fragment of ICA512 resulting from this cleavage (amino acids 35-448) or parts thereof are present in the neurosecretosomes of posterior pituitary, raising the possibility that they may be secreted upon exocytosis of secretory granules.

A novel putative protein-tyrosine phosphatase contains a BRO1-like domain and suppresses Ha-ras-mediated transformation

To investigate a potential role of protein-tyrosine phosphatases (PTPases) in myocardial growth and signaling, a degenerate primer-based reverse transcription-polymerase chain reaction approach was used to isolate cDNAs for proteins that contain a PTPase catalytic domain. Among the 16 cDNA clones isolated by reverse transcription-polymerase chain reaction from total neonatal rat cardiomyocyte RNA, one, designated PTP-TD14, was unique. Subsequent isolation and sequencing of a full-length PTP-TD14 cDNA confirmed that it encodes a novel 164-kDa protein, p164(PTP-TD14). The C-terminal region contains the PTP-like domain, whereas the N-terminal region shows no homology to any known mammalian protein. However, this region is homologous to a yeast protein, BRO1, that is involved in the mitogen-activated protein kinase signaling pathway. Like BRO1, p164(PTP-TD14) contains a proline-rich region with two putative SH3-domain binding sites. By Northern blot analysis, PTP-TD14 is expressed as a 5.3-kilobase pair transcript, not only in neonatal heart but also in many adult rat tissues. When expressed in either COS-7 or NIH-3T3 cells, p164(PTP-TD14) localizes to the cytoplasm in association with vesicle-like structures. Expression of p164(PTP-TD14) in NIH-3T3 cells inhibits Ha-ras-mediated transformation more than 3-fold. This inhibitory activity is localized to the C-terminal PTPase homology domain, since no inhibition of Ha-ras-mediated focus formation was observed with a PTP-TD14 mutant, in which the putative catalytic activity was presumably inactivated by a point mutation. These findings indicate that PTP-TD14 encodes a novel protein that may be critically involved in regulating Ha-ras-dependent cell growth.

Solid-phase synthesis of tyrosyl H-phosphonopeptides and methylphosphonopeptides

Phosphopeptides are a useful tool for the investigation of phosphorylation as a reversible posttranslational modification. There is a growing interest in using mimics of phosphoamino acids involved in phosphorylation in order to study the enzymes concerned in these processes. These mimics should contain a non-hydrolysable or isoelectrically modified phosphate moiety to be used as a specific inhibitor of phosphatases and kinases. We introduce sold-phase synthesis of H- and methylphosphonopeptides as a new class of mimics of phosphotyrosyl peptides. The peptides were synthesized on solid phase using the standard fluorenyl-methyloxycarbonyl (Fmoc) strategy. Tyrosine residues were incorporated as allyl-protected derivatives, which were selectively deprotected on the resin by treatment with Pd(PPh3)4. The peptide resin carrying the side-chain unprotected tyrosine of the model peptide Gly-Gly-Tyr-Ala was phosphonylated with di-tert-butyl-N,N-diethyl-phosphoramidite in the presence of 1H-tetrazole, yielding H-phosphonopeptides after trifluoroacetic acid (TFA) cleavage. Alternatively, phosphonylation of the unprotected tyrosine with O-tert-butyl-N,N-diethyl-P-methylphosphonamidite catalysed by 1H-tetrazole and followed by oxidation led to the methyphosphonopeptides after TFA cleavage. We obtained both the H-phosphonopeptides and the methylphosphonopeptides of the tetrapeptide in high yields and purities above 90%, according to reversed-phase high-performance liquid chromatography (RP-HPLC). To investigate the general applicability of our new methodology, we synthesized phosphonopeptides up to 13 amino acids long, corresponding to recognition sequences of tyrosine kinases. After cleavage and deprotection, all phosphonopeptides were obtained in high yields and purities of about 90%, as shown by mass spectrometry. The only by-product found was the unmodified peptide.

The insulin-dependent diabetes mellitus-associated ICA 105 autoantigen in stiff-man syndrome patients

The pathogenetic role of anti-glutamic acid decarboxylase (GAD) antibodies found in up to 60% of patients with stiff-man syndrome (SMS) is still controversial. GAD, in fact, is also one of the major target antigen of insulin-dependent diabetes mellitus (IDDM), a disease affecting one third of anti-GAD antibody-positive patients with SMS. To better define the role of autoimmunity in SMS we looked for molecular and immunological evidence of an autoimmune recognition of a second IDDM-associated autoantigen, the pancreatic 37/40 kDa IDDM-autoantigen, whose gene called ICA 105 has been recently cloned. By Northern blot analysis we found that tissue distribution of human ICA 105 is restricted to pancreas and brain and within the central nervous system (CNS) its distribution is similar to GAD. We also measured anti-ICA 105 antibodies in 11 SMS patients and 56 control patients with other neurological diseases (OND). Anti-ICA 105 antibodies were found in 4/11 (36%) patients with SMS (a frequency similar to that of anti-GAD-antibodies in our SMS population) but in only 2/56 (3%) patients with OND (P < 0.001). Anti-ICA 105 and anti-GAD antibodies were associated in 3/4 (75%) patients with SMS but in none of the 2 anti-ICA 105 antibody-positive OND patients. Among anti-ICA 105 antibody-positive patients with SMS, only 1 suffered also from IDDM. In contrast, the only 2 anti-ICA 105 antibody-positive with OND had IDDM. Our results indicate that ICA 105 represents another putative neuroendocrine autoantigen in SMS. The presence of circulating anti-GAD and/or anti-ICA 105 antibodies might help the diagnosis of SMS. The absence, however, of antibodies recognising specific CNS autoantigens (e.g. GAD, ICA 105) does not rule out SMS.