Targeting protein tyrosine phosphatase σ after myocardial infarction restores cardiac sympathetic innervation and prevents arrhythmias.

Millions of people suffer a myocardial infarction (MI) every year, and those who survive have increased risk of arrhythmias and sudden cardiac death. Recent clinical studies have identified sympathetic denervation as a predictor of increased arrhythmia susceptibility. Chondroitin sulfate proteoglycans present in the cardiac scar after MI prevent sympathetic reinnervation by binding the neuronal protein tyrosine phosphatase receptor σ (PTPσ). Here we show that the absence of PTPσ, or pharmacologic modulation of PTPσ by the novel intracellular sigma peptide (ISP) beginning 3 days after injury, restores sympathetic innervation to the scar and markedly reduces arrhythmia susceptibility. Using optical mapping we observe increased dispersion of action potential duration, supersensitivity to ß-adrenergic receptor stimulation and Ca(2+) mishandling following MI. Sympathetic reinnervation prevents these changes and renders hearts remarkably resistant to induced arrhythmias.

Pichia pastoris Pex14p, a phosphorylated peroxisomal membrane protein, is part of a PTS-receptor docking complex and interacts with many peroxins.

The peroxisomal protein import machinery plays a central role in the assembly of this organelle in all eukaryotes. Genes encoding components of this machinery, termed peroxins or Pex proteins, have been isolated and characterized in several yeast species and in mammals, including humans. Here we report on one of these components, Pex14p, from the methylotrophic yeast Pichia pastoris. Work in other organisms has shown that Pex14p is located on the cytoplasmic surface of the peroxisomal membrane and binds peroxisomal targeting signal (PTS) receptors carrying proteins bound for the peroxisomal matrix, results that have led to the hypothesis that Pex14p is a receptor-docking protein. P. pastoris Pex14p (PpPex14p) behaves like an integral membrane protein, with its C-terminus exposed on the cytosolic side of the peroxisomal membrane. PpPex14p complexes with many peroxins, including Pex3p (Snyder et al., 1999b), Pex5p, Pex7p, Pex13p, Pex17p, itself, and a previously unreported peroxin, Pex8p. A portion of Pex14p is phosphorylated, but both phosphorylated and unphosphorylated forms of Pex14p interact with several peroxins. The interactions between Pex14p and other peroxins provide clues regarding the function of Pex14p in peroxisomal protein import.

New selectable marker/auxotrophic host strain combinations for molecular genetic manipulation of Pichia pastoris.

We describe the isolation and characterization of three new biosynthetic genes-ARG4, ADE1, and URA3-from the methylotrophic yeast Pichia pastoris. The predicted products of the genes share significant sequence similarity to their Saccharomyces cerevisiae counterparts, namely argininosuccinate lyase, PR-aminoimidazolesuccinocarboxamide synthase, and orotidine-5'-phosphate decarboxylase, respectively. Along with the previously described HIS4 gene, each gene was incorporated as the yeast selectable marker into a set of shuttle vectors designed to express foreign genes in P. pastoris. In addition, we have constructed a series of host strains containing all possible combinations of ade1, arg4, his4, and ura3 auxotrophies to be used with these new vectors.

Recombinant protein expression in Pichia pastoris.

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

Heterologous protein expression in the methylotrophic yeast Pichia pastoris.

During the past 15 years, the methylotrophic yeast Pichia pastoris has developed into a highly successful system for the production of a variety of heterologous proteins. The increasing popularity of this particular expression system can be attributed to several factors, most importantly: (1) the simplicity of techniques needed for the molecular genetic manipulation of P. pastoris and their similarity to those of Saccharomyces cerevisiae, one of the most well-characterized experimental systems in modern biology; (2) the ability of P. pastoris to produce foreign proteins at high levels, either intracellularly or extracellularly; (3) the capability of performing many eukaryotic post-translational modifications, such as glycosylation, disulfide bond formation and proteolytic processing; and (4) the availability of the expression system as a commercially available kit. In this paper, we review the P. pastoris expression system: how it was developed, how it works, and what proteins have been produced. We also describe new promoters and auxotrophic marker/host strain combinations which extend the usefulness of the system.

A Pichia pastoris VPS15 homologue is required in selective peroxisome autophagy.

Methylotrophic yeasts contain large peroxisomes during growth on methanol. Upon exposure to excess glucose or ethanol these organelles are selectively degraded by autophagy. Here we describe the cloning of a Pichia pastoris gene (PpVPS15) involved in peroxisome degradation, which is homologous to Saccharomyces cerevisiae VPS15. In methanol-grown cells of a P. pastoris VPS15 deletion strain, the levels of peroxisomal marker enzymes remained high after addition of excess glucose or ethanol. Electron microscopic studies revealed that the organelles were not taken up by vacuoles, suggesting that PpVPS15 is required at an early stage in peroxisome degradation.

Pex17p is required for import of both peroxisome membrane and lumenal proteins and interacts with Pex19p and the peroxisome targeting signal-receptor docking complex in Pichia pastoris.

Pichia pastoris PEX17 was cloned by complementation of a peroxisome-deficient strain obtained from a novel screen for mutants disrupted in the localization of a peroxisomal membrane protein (PMP) reporter. PEX17 encodes a 267-amino-acid protein with low identity (18%) to the previously characterized Saccharomyces cerevisiae Pex17p. Like ScPex17p, PpPex17p contains a putative transmembrane domain near the amino terminus and two carboxyl-terminal coiled-coil regions. PpPex17p behaves as an integral PMP with a cytosolic carboxyl-terminal domain. pex17Delta mutants accumulate peroxisomal matrix proteins and certain integral PMPs in the cytosol, suggesting a critical role for Pex17p in their localization. Peroxisome remnants were observed in the pex17Delta mutant by morphological and biochemical means, suggesting that Pex17p is not absolutely required for remnant formation. Yeast two-hybrid analysis demonstrated that the carboxyl terminus of Pex19p was required for interaction with Pex17p lacking the carboxyl-terminal coiled-coil domains. Biochemical evidence confirmed the interaction between Pex19p and Pex17p. Additionally, Pex17p cross-linked to components of the peroxisome targeting signal-receptor docking complex, which unexpectedly contained Pex3p. Our evidence suggests the existence of distinct subcomplexes that contain separable pools of Pex3p, Pex19p, Pex17p, Pex14p, and the peroxisome targeting signal receptors. These distinct pools may serve different purposes for the import of matrix proteins or PMPs.

Applications of yeast in biotechnology: protein production and genetic analysis.

Improvements in yeast expression systems, coupled with the development of yeast surface display and refinements in two-hybrid methodology, are expanding the role of yeasts in the process of understanding and engineering eukaryotic proteins.

Hansenula polymorpha Pex1p and Pex6p are peroxisome-associated AAA proteins that functionally and physically interact.

We have cloned the Hansenula polymorpha PEX1 and PEX6 genes by functional complementation of the corresponding peroxisome-deficient (pex) mutants. The gene products, HpPex1p and HpPex6p, are ATPases which both belong to the AAA protein family. Cells deleted for either gene (Deltapex1 or Deltapex6) were characterized by the presence of small peroxisomal remnants which contained peroxisomal membrane proteins and minor amounts of matrix proteins. The bulk of the matrix proteins, however, resided in the cytosol. In cell fractionation studies HpPex1p and HpPex6p co-sedimented with the peroxisomal membrane protein HpPex3p in both wild-type cells and in Deltapex4, Deltapex8 or Deltapex14 cells. Both proteins are loosely membrane-bound and face the cytosol. Furthermore, HpPex1p and HpPex6p physically and functionally interact in vivo. Overexpression of PEX6 resulted in defects in peroxisomal matrix protein import. By contrast, overexpression of PEX1 was not detrimental to the cells. Interestingly, co-overproduction of HpPex1p rescued the protein import defect caused by HpPex6p overproduction. Overproduced HpPex1p and HpPex6p remained predominantly membrane-bound, but only partially co-localized with the peroxisomal membrane protein HpPex3p. Our data indicate that HpPex1p and HpPex6p function in a protein complex associated with the peroxisomal membrane and that overproduced, mislocalized HpPex6p prevents HpPex1p from reaching its site of activity.

Positive selection of novel peroxisome biogenesis-defective mutants of the yeast Pichia pastoris.

We have developed two novel schemes for the direct selection of peroxisome-biogenesis-defective (pex) mutants of the methylotrophic yeast Pichia pastoris. Both schemes take advantage of our observation that methanol-induced pex mutants contain little or no alcohol oxidase (AOX) activity. AOX is a peroxisomal matrix enzyme that catalyzes the first step in the methanol-utilization pathway. One scheme utilizes allyl alcohol, a compound that is not toxic to cells but is oxidized by AOX to acrolein, a compound that is toxic. Exposure of mutagenized populations of AOX-induced cells to allyl alcohol selectively kills AOX-containing cells. However, pex mutants without AOX are able to grow. The second scheme utilizes a P. pastoris strain that is defective in formaldehyde dehydrogenase (FLD), a methanol pathway enzyme required to metabolize formaldehyde, the product of AOX. AOX-induced cells of fld1 strains are sensitive to methanol because of the accumulation of formaldehyde. However, fld1 pex mutants, with little active AOX, do not efficiently oxidize methanol to formaldehyde and therefore are not sensitive to methanol. Using these selections, new pex mutant alleles in previously identified PEX genes have been isolated along with mutants in three previously unidentified PEX groups.

A strong nitrogen source-regulated promoter for controlled expression of foreign genes in the yeast Pichia pastoris.

In methylotrophic yeasts, glutathione-dependent formaldehyde dehydrogenase (FLD) is a key enzyme required for the metabolism of methanol as a carbon source and certain alkylated amines such as methylamine as nitrogen sources. We describe the isolation and characterization of the FLD1 gene from the yeast Pichia pastoris. The gene contains a single short intron with typical yeast-splicing signals near its 5' end, the first intron to be demonstrated in this yeast. The predicted FLD1 product (Fld1p) is a protein of 379 amino acids (approx. 40 kDa) with 71% identity to the FLD protein sequence from the n-alkane-assimilating yeast Candida maltosa and 61-65% identity with dehydrogenase class III enzymes from humans and other higher eukaryotes. Using beta-lactamase as a reporter, we show that the FLD1 promoter (PFLD1) is strongly and independently induced by either methanol as sole carbon source (with ammonium sulfate as nitrogen source) or methylamine as sole nitrogen source (with glucose as carbon source). Furthermore, with either methanol or methylamine induction, levels of beta-lactamase produced under control of PFLD1 are comparable to those obtained with the commonly used alcohol oxidase I gene promoter (PAOX1). Thus, PFLD1 is an attractive alternative to PAOX1 for expression of foreign genes in P. pastoris, allowing the investigator a choice of carbon (methanol) or nitrogen source (methylamine) regulation with the same expression strain.

Deletion of the conserved first 18 N-terminal amino acid residues in rat liver carnitine palmitoyltransferase I abolishes malonyl-CoA sensitivity and binding.

To assess the role of the 130 N-terminal amino acid residues of rat liver carnitine palmitoyltransferase I (L-CPTI) on malonyl-CoA sensitivity and binding, we constructed a series of mutants with deletions of the 18, 35, 52, 73, 83, or 129 most N-terminal amino acid residues. The deletion mutants were expressed in the yeast Pichia pastoris. We determined the effects of these mutations on L-CPTI activity, malonyl-CoA sensitivity, and binding in isolated mitochondria prepared from the yeast strains expressing the wild-type and deletion mutants. The mutant protein that lacked the first 18 N-terminal amino acid residues, Delta18, had activity and kinetic properties similar to wild-type L-CPTI, but it was almost completely insensitive to malonyl-CoA inhibition (I50 = 380 microM versus 2.0 microM). In addition, loss of malonyl-CoA sensitivity in Delta18 was accompanied by a 70-fold decrease in affinity for malonyl CoA (KD = 70 nM versus 1.1 nM) compared to wild-type L-CPTI. Deletion of the first 35, 52, 73, and 83 N-terminal amino acid residues had a similar effect on malonyl-CoA sensitivity as did the 18-residue deletion mutant, and there was a progressive reduction in the affinity for malonyl-CoA binding. By contrast, deletion of the first 129 N-terminal amino acid residues resulted in the synthesis of an inactive protein. To our knowledge, this is the first report to demonstrate a critical role for these perfectly conserved first 18 N-terminal amino acid residues of L-CPTI in malonyl-CoA sensitivity and binding.

The ubiquitin-conjugating enzyme Pex4p of Hansenula polymorpha is required for efficient functioning of the PTS1 import machinery.

We have cloned the Hansenula polymorpha PEX4 gene by functional complementation of a peroxisome-deficient mutant. The PEX4 translation product, Pex4p, is a member of the ubiquitin-conjugating enzyme family. In H.polymorpha, Pex4p is a constitutive, low abundance protein. Both the original mutant and the pex4 deletion strain (Deltapex4) showed a specific defect in import of peroxisomal matrix proteins containing a C-terminal targeting signal (PTS1) and of malate synthase, whose targeting signal is not yet known. Import of the PTS2 protein amine oxidase and the insertion of the peroxisomal membrane proteins Pex3p and Pex14p was not disturbed in Deltapex4 cells. The PTS1 protein import defect in Deltapex4 cells could be suppressed by overproduction of the PTS1 receptor, Pex5p, in a dose-response related manner. In such cells, Pex5p is localized in the cytosol and in peroxisomes. The peroxisome-bound Pex5p specifically accumulated at the inner surface of the peroxisomal membrane and thus differed from Pex5p in wild-type peroxisomes, which is localized throughout the matrix. We hypothesize that in H. polymorpha Pex4p plays an essential role for normal functioning of Pex5p, possibly in mediating recycling of Pex5p from the peroxisome to the cytosol.

Functional studies of yeast-expressed human heart muscle carnitine palmitoyltransferase I.

Long-chain fatty acids are the primary source of energy production in the heart. Carnitine palmitoyltransferase I (CPT-I) catalyzes the first reaction in the transport of long-chain fatty acids from the cytoplasm to the mitochondrion, a rate-limiting step in beta-oxidation. In this study, we report the functional expression of the human heart/skeletal muscle isoform of CPT-I (M-CPT-I) in the yeast Pichia pastoris. Screening of a human heart cDNA library with cDNA fragments encoding the rat heart M-CPT-I resulted in the isolation of a single full-length human heart M-CPT-I cDNA clone. The clone has an open reading frame of 2316 bp with a 5' untranslated region of 38 bp and a 256-bp 3' untranslated region with the poly(A)+ addition sequence AATAAA. The predicted protein has 772 amino acids and a molecular mass of 88 kDa. Northern blot analysis of mRNAs from different human tissues using the human M-CPT-I cDNA as a probe revealed an abundant transcript of approximately 3.1 kb that was only present in human heart and skeletal muscle tissue. Expression of the human M-CPT-I cDNA in P. pastoris, a yeast with no endogenous CPT activity, produced an 80-kDa protein that was located in the mitochondria. Isolated mitochondria from the M-CPT-I expression strain exhibited a malonyl-coenzyme A (CoA)-sensitive CPT activity that was detergent labile. The I50 for malonyl-CoA inhibition of the yeast-expressed M-CPT-I was 69 nM, and the Kms for carnitine and palmitoyl-CoA were 666 and 42 microM, respectively. The I50 for malonyl-CoA inhibition of the heart enzyme is 30 times lower than that of the yeast-expressed liver CPT-I, and the Km for carnitine is more than 20 times higher than that of the liver CPT-I. This is the first report of the expression of a heart CPT-I in a system devoid of endogenous CPT activity and the functional characterization of a human heart M-CPT-I in the absence of the liver isoform and CPT-II.

Reconstitution of highly expressed human heart muscle carnitine palmitoyltransferase I.

The human heart muscle carnitine palmitoyltransferase I (M-CPTI) gene was expressed at high levels from a strain of the methylotrophic yeast Pichia pastoris containing approximately 24 copies of the expression vector. Levels of M-CPTI were more than ten-fold higher than previously reported by our group with a single-copy strain (Arch. Biochem. Biophys., in press) and were sufficient to perform reconstitution studies on the membrane protein, a key step in purification and structural analysis of the enzyme. Solubilization of yeast mitochondria containing M-CPTI in 5% Triton X-100 abolished M-CPTI activity. The detergent-inactivated M-CPTI was then reconstituted by removal of the detergent in the presence of phospholipids. The reconstituted proteoliposomes exhibited M-CPTI activity of 2.4 nmol palmitoylcarnitine formed/mg protein/min, a recovery of 23% of the activity present in the starting mitochondrial preparation. The malonyl-CoA sensitivity of the reconstituted reactivated M-CPTI was 88%. This is the first demonstration of direct reactivation of malonyl-CoA-sensitive M-CPTI activity from solubilized materials from any organism. Previously, M-CPTI was presumed to be irreversibly inactivated by detergents.

Functional characterization of mitochondrial carnitine palmitoyltransferases I and II expressed in the yeast Pichia pastoris.

The rate-limiting step in beta oxidation is the conversion of long-chain acyl-CoA to acylcarnitine, a reaction catalyzed by the outer mitochondrial membrane enzyme carnitine palmitoyltransferase I (CPTI) and inhibited by malonyl-CoA. The acylcarnitine is then translocated across the inner mitochondrial membrane by the carnitine/acylcarnitine translocase and converted back to acyl-CoA by CPTII. Although CPTII has been examined in detail, studies on CPTI have been hampered by an inability to purify CPTI in an active form from CPTII. In particular, it has not been conclusively demonstrated that CPTI is even catalytically active, or whether sensitivity of CPTI to malonyl-CoA is an intrinsic property of the enzyme or is contained in a separate regulatory subunit that interacts with CPTI. To address these questions, the genes for CPTI and CPTII were separately expressed in Pichia pastoris, a yeast with no endogenous CPT activity. High levels of CPT activity were present in purified mitochondrial preparations from both CPTI- and CPTII-expressing strains. Furthermore, CPTI activity was highly sensitive to inhibition by malonyl-CoA while CPTII was not. Thus, CPT catalytic activity and malonyl-CoA sensitivity are contained within a single CPTI polypeptide in mammalian mitochondrial membranes. We describe the kinetic characteristics for the yeast-expressed CPTs, the first such report for a CPTI enzyme in the absence of CPTII. Yeast-expressed CPTI is inactivated by detergent solubilization. However, removal of the detergent in the presence of phospholipids resulted in the recovery of malonyl-CoA-sensitive CPTI activity, suggesting that CPTI requires a membranous environment. CPTI is thus reversibly inactivated by detergents.