Biomedical subjects

C Enenkel

Publications and source records attributed to C Enenkel.

9 recordsLinked to original sources

PubMed1999

GFP-labelling of 26S proteasomes in living yeast: insight into proteasomal functions at the nuclear envelope/rough ER.

26S proteasomes are multisubunit protease complexes that play the central role in the ubiquitin-dependent protein degradation pathway. The proteolytically active core is formed by the 20S proteasome. Regulatory subunits, principally the 19S cap complex, confer the specificity towards ubiquitinated substrates and an ATP-dependence on proteolysis. Green fluorescence protein (GFP)-tagged versions of either an alpha-subunit of the 20S core or an ATPase subunit of the 19S cap complex were functionally incorporated into the protease complex, thus allowing to monitor the subcellular distribution of 26S proteasomes in living yeast. Our localization studies suggest that proteasomal proteolysis mainly occurs at the nuclear envelope (NE)/rough ER. Implications of proteasomal functions at the NE/rough ER are discussed in the context of published work on ER degradation and with regard to possible targeting mechanisms.

Adenosine Triphosphatases↗

PubMed1998

Subcellular distribution of proteasomes implicates a major location of protein degradation in the nuclear envelope-ER network in yeast.

26S proteasomes are the key enzyme complexes responsible for selective turnover of short-lived and misfolded proteins. Based on the assumption that they are dispersed over the nucleoplasm and cytoplasm in all eukaryotic cells, we wanted to determine the subcellular distribution of 26S proteasomes in living yeast cells. For this purpose, we generated yeast strains that express functional green fluorescent protein (GFP) fusions of proteasomal subunits. An alpha subunit of the proteolytically active 20S core complex of the 26S proteasome, Pre6/YOL038w, as well as an ATPase-type subunit of the regulatory 19S cap complex, Cim5/YOL145w, were tagged with GFP. Both chimeras were shown to be incorporated completely into active 26S proteasomes. Microscopic analysis revealed that GFP-labelled 20S as well as 19S subunits are accumulated mainly in the nuclear envelope (NE)-endoplasmic reticulum (ER) network in yeast. These findings were supported by the co-localization and co-enrichment of 26S proteasomes with NE-ER marker proteins. A major location of proteasomal peptide cleavage activity was visualized in the NE-ER network, indicating that proteasomal degradation takes place mainly in this subcellular compartment in yeast.

Adenosine Triphosphatases↗

PubMed1998

Mutations in the yeast proteasome beta-type subunit Pre3 uncover position-dependent effects on proteasomal peptidase activity and in vivo function.

Proteasomes are highly complex proteases responsible for selective protein degradation in the eukaryotic cell. 26 S proteasomes consist of two regulatory 19 S cap complexes and the 20 S proteasome, which acts as the proteolytic core module. We isolated six mutants of the yeast Saccharomyces cerevisiae containing mutations in the 20 S proteasome beta-type subunit Pre3. Three mutations (pre3-2, pre3-3, and pre3-5) which reside at the active site cleft of the Pre3 subunit solely caused reduction of the proteasomal peptidylglutamyl peptide-hydrolyzing activity but did not lead to detectable defects in protein degradation nor to any other phenotype. However, the pre3-2 mutation strengthened phenotypes induced by other 20 S proteasomal mutations, indicating that the peptidylglutamyl peptide-hydrolyzing activity has to fulfill some rescue functions. The other three mutations (pre3-1, pre3-4, and pre3-6) are located at diverse sites of the Pre3 protein and caused multiple defects in proteasomal peptide cleaving activities. pre3-1 and pre3-6 mutants exhibited significant defects in proteasomal protein degradation; they accumulated ubiquitinated proteins and stabilized defined substrate proteins as, e.g. fructose-1,6-bisphosphatase. In addition, pre3-1 and pre3-6 mutant cells exhibited pleiotropic phenotypes as temperature sensitivity and cell cycle-related effects.

Amino Acid Sequence↗

PubMed1996

Expression in yeast of binding regions of karyopherins alpha and beta inhibits nuclear import and cell growth.

Using truncated forms of recombinant yeast karyopherins alpha and beta in in vitro binding assays, we mapped the regions of karyopherin alpha that bind to karyopherin beta and the regions of karyopherin beta that interact with karyopherin alpha and with Ran-GTP. Karyopherin alpha's binding region for karyopherin beta was localized to its N-terminal domain, which contains several clusters of basic residues, whereas karyopherin beta's binding region for karyopherin alpha was localized to an internal region containing two clusters of acidic residues. Karyopherin beta's binding region for Ran-GTP overlaps with that for karyopherin alpha and comprises at least one of the two acidic clusters required for karyopherin alpha binding in addition to further downstream determinants not required for karyopherin alpha binding. Overexpression in yeast of fragments containing either karyopherin beta's binding region for alpha and Ran-GTP or karyopherin alpha's binding region for beta resulted in sequestration of most of the cytosolic karyopherin alpha or karyopherin beta, respectively, in complexes containing the truncated proteins. As these binding region-containing fragments lack other domains required for function of the corresponding protein, the overexpression of either fragment also inhibited in vivo nuclear import of a model reporter protein as well as cell growth.

Animals↗

PubMed1995

Identification of a yeast karyopherin heterodimer that targets import substrate to mammalian nuclear pore complexes.

Targeting of import substrate to nuclear pore complexes of permeabilized vertebrate cells was previously shown to require a protein complex composed of two subunits, termed karyopherin. Yeast contain a homologue of karyopherin alpha named Srp1p, which was initially identified as a genetic suppressor of mutations in a subunit of RNA polymerase I. To determine whether yeast contain a karyopherin complex that includes Srp1p as the karyopherin alpha homologue, we genetically replaced Srp1p with a Srp1-Protein A chimera. Cytosol from this strain contained a complex, composed of the chimera and a protein of 95 kDa, that was purified using affinity chromatography on IgG Sepharose. Microsequence analysis showed that the 95-kDa protein was identical with a yeast protein encoded by gene L8300.15 on chromosome XII. Sequence comparison revealed that the L8300.15 gene product is the closest structural homologue of vertebrate karyopherin beta. The yeast alpha and beta karyopherin subunits were expressed in Escherichia coli and were purified. When combined, they formed a heterodimeric complex and were active in targeting import substrate to nuclear envelopes of mammalian cells. We propose that all karyopherins function as alpha/beta heterodimers.

Amino Acid Sequence↗

PubMed1994

PRE3, highly homologous to the human major histocompatibility complex-linked LMP2 (RING12) gene, codes for a yeast proteasome subunit necessary for the peptidylglutamyl-peptide hydrolyzing activity.

20S proteasomes are multifunctional proteinase complexes ubiquitous in eucaryotes. We have cloned the yeast PRE3 gene by complementation of the pre3-2 mutation, which leads to a defect in the peptidylglutamyl-peptide hydrolyzing activity of the 20S proteasome. The PRE3 gene, a beta-type member of the proteasomal gene family, is essential for cellular life and codes for a 193-amino acid proteasomal subunit with a predicted molecular mass of 21.2 kDa. The Pre3 protein shows striking homology to the human proteasome subunits Hs delta and Lmp2 (Ring12). Lmp2 is encoded in the major histocompatibility complex class II region implicating proteasomes in antigen processing.

Amino Acid Sequence↗

PubMed1993

BLH1 codes for a yeast thiol aminopeptidase, the equivalent of mammalian bleomycin hydrolase.

We have cloned the BLH1 gene of the yeast Saccharomyces cerevisiae coding for a peptidase with significant homology to rabbit bleomycin hydrolase. Bleomycin is a glycopeptide antibiotic used for the treatment of human cancers. The antitumor activity of the drug is limited by its metabolic inactivation caused by bleomycin hydrolase, a member of the cysteine protease family. The open reading frame of BLH1 consists of 1,449 base pairs encoding a 55.4-kDa protein consistent with the molecular mass found in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The consensus sequence within the active site region of cysteine proteases is conserved in the yeast enzyme. Biochemical studies on the purified enzyme confirm its classification as a thiol protease. The nonvacuolar enzyme has a molecular mass of 220 kDa, suggesting a homotetrameric structure. It exhibits an aminopeptidase activity with broad substrate specificity. Biochemical and genetic linkage data give evidence that the BLH1 and the LAP3 (Trumbly, R. J., and Bradley, G. (1983) J. Bacteriol. 156, 36-48) encoded aminopeptidases are identical. Deletion of the BLH1 gene is not lethal under normal growth conditions. However, blh1 mutants show hypersensitivity to bleomycin, indicating that bleomycin hydrolase is able to inactivate bleomycin in vivo and to protect cells from bleomycin-induced toxicity.

Amino Acid Sequence↗

PubMed1993

The PRE4 gene codes for a subunit of the yeast proteasome necessary for peptidylglutamyl-peptide-hydrolyzing activity. Mutations link the proteasome to stress- and ubiquitin-dependent proteolysis.

Proteinase yscE, the yeast proteasome, is a member of the nonlysosomal, high molecular mass (approximately 700 kDa) multifunctional proteinase complexes that are highly conserved from yeast to man. We have isolated mutants defective in one of the three proteolytic activities of the enzyme complex, i.e. in cleavage of peptide bonds after acidic amino acids. Using one of these mutants (pre4-1), we cloned the PRE4 gene and uncovered an open reading frame with 266 amino acids coding for a predicted protein of 29.4 kDa. The protein proved to be a subunit of proteinase yscE. The Pre4 amino acid sequence shows strong homology to the beta-subunit of the Xenopus laevis proteasome. Chromosomal deletion of the PRE4 gene is lethal. The pre4-1 mutant allele was cloned and sequenced. The mutant protein is shortened by 15 amino acids at the carboxyl terminus. Mutations (pre1-1, pre2-2) in the chymotrypsin-like activity of proteinase yscE uncovered the enzyme to be involved in ubiquitin-linked and stress-dependent proteolytic pathways. In contrast to these mutants, pre4-1 mutants did not exhibit any apparent stress-dependent phenotypes. However, pre1-1 pre4-1 double mutants showed enhanced canavanine sensitivity and increased accumulation of ubiquitin protein conjugates, as compared with pre1-1 single mutants.

Amino Acid Sequence↗

PubMedDate not supplied

20S proteasome biogenesis.

26S proteasomes are multi-subunit protease complexes responsible for the turnover of short-lived proteins. Proteasomal degradation starts with the autocatalytic maturation of the 20S core particle. Here, we summarize different models of proteasome assembly. 20S proteasomes are assembled as precursor complexes containing alpha and unprocessed beta subunits. The propeptides of the beta subunits are thought to prevent premature conversion of the precursor complexes into matured particles and are needed for efficient beta subunit incorporation. The complex biogenesis is tightly regulated which requires additional components such as the maturation factor Ump1/POMP, an ubiquitous protein in eukaryotic cells. Ump1/POMP is associated with precursor intermediates and degraded upon final maturation. Mammalian proteasomes are localized all over the cell, while yeast proteasomes mainly localize to the nuclear envelope/endoplasmic reticulum (ER) membrane network. The major localization of yeast proteasomes may point to the subcellular place of proteasome biogenesis.

Amino Acid Sequence↗