Ascorbate peroxidase. A prominent membrane protein in oilseed glyoxysomes

The glyoxysomes of growing oilseed seedlings produce H2O2, a reactive oxygen species, during the beta-oxidation of lipids stored in the cotyledons. An expression library of dark-grown cotton (Gossypium hirsutm L.) cotyledons was screened with antibodies that recognized a 31-kD glyoxysomal membrane polypeptide. A full-length cDNA clone (1258 bp) was isolated that encodes a 32-kD subunit of ascorbate peroxidase (APX) with a single, putative membrane-spanning region near the C-terminal end of the polypeptide. Internal amino acid sequence analysis of the cotton 31-kD polypeptide verified that this clone encoded this protein. This enzyme, designated gmAPX, was immunocytochemically and enzymatically localized to the glyoxysomal membrane in cotton cotyledons. The activity of monodehydroascorbate reductase, a protein that reduces monodehydroascorbate to ascorbate with NADH, also was detected in these membranes. The co-localization of gmAPX and monodehydroascorbate reductase within the glyoxysomal membrane likely reflects an essential pathway for scavenging reactive oxygen species and also provides a mechanism to regenerate NAD+ for the continued operation of the glyoxylate cycle and beta-oxidation of fatty acids. Immunological cross-reactivity of 30- to 32-kD proteins in glyoxysomal membranes of cucumber, sunflower, castor bean, and cotton indicate that gmAPX is common among oilseed species.

Peroxisomal membrane ascorbate peroxidase is sorted to a membranous network that resembles a subdomain of the endoplasmic reticulum

The peroxisomal isoform of ascorbate peroxidase (APX) is a novel membrane isoform that functions in the regeneration of NAD(+) and protection against toxic reactive oxygen species. The intracellular localization and sorting of peroxisomal APX were examined both in vivo and in vitro. Epitope-tagged peroxisomal APX, which was expressed transiently in tobacco BY-2 cells, localized to a reticular/circular network that resembled endoplasmic reticulum (ER; 3,3'-dihexyloxacarbocyanine iodide-stained membranes) and to peroxisomes. The reticular network did not colocalize with other organelle marker proteins, including three ER reticuloplasmins. However, in vitro, peroxisomal APX inserted post-translationally into the ER but not into other purified organelle membranes (including peroxisomal membranes). Insertion into the ER depended on the presence of molecular chaperones and ATP. These results suggest that regions of the ER serve as a possible intermediate in the sorting pathway of peroxisomal APX. Insight into this hypothesis was obtained from in vivo experiments with brefeldin A (BFA), a toxin that blocks vesicle-mediated protein export from ER. A transiently expressed chloramphenicol acetyltransferase-peroxisomal APX (CAT-pAPX) fusion protein accumulated only in the reticular/circular network in BFA-treated cells; after subsequent removal of BFA from these cells, the CAT-pAPX was distributed to preexisting peroxisomes. Thus, plant peroxisomal APX, a representative enzymatic peroxisomal membrane protein, is sorted to peroxisomes through an indirect pathway involving a preperoxisomal compartment with characteristics of a distinct subdomain of the ER, possibly a peroxisomal ER subdomain.

Identification of the peroxisomal targeting signal for cottonseed catalase

Catalase is a ubiquitous peroxisomal matrix enzyme, yet the molecular targeting signal(s) for sorting it in plant cells has not been defined. The most common peroxisome targeting signal (PTS) is a C-terminal tripeptide composed of a conserved SKL motif (type 1 PTS). The PTS for cottonseed catalase (Ccat) was elucidated in this study from immunofluorescence microscopic analyses of tobacco BY-2 suspension cells serving as an in vivo import system. To distinguish biolistically introduced Ccat from endogenous tobacco catalase, Ccat was hemagglutinin (HA)epitope-tagged at its N-terminus. Bombardment with HA-Ccat resulted in the import of Ccat into glyoxysomes, the specialized type of peroxisome in BY-2 cells. The C-terminal tripeptide of Ccat, PSI, is necessary for import. Evidence for this were mislocalizations to the cytosol of PSI-truncated Ccat and AGV-substituted (for PSI) Ccat. PSI-COOH, however, was not sufficient to re-route chloramphenicol acetyltransferase (CAT) from the cytosol to glyoxysomes, whereas the Ccat tetrapeptide RPSI-COOH was sufficient. Surprisingly, substitution of K (common at the fourth position in other plant catalases) for the R (CAT-KPSI) decreased import efficiency. However, substitution of K did not affect import, when additional upstream residues in Ccat were included (e.g. CAT-NVKPSI). Other evidence for the importance of upstream residues comprised abolishment of Ccat import due to substitutions with non-conserved residues (e.g. -AGVNVRPSI for -SRLNVRPSI). These data indicate that Ccat is sorted to plant peroxisomes by a degenerate type 1 PTS (PSI-COOH) whose residues are functionally dependent on a strict context of adjacent C-terminal amino acid residues.

Oilseed isocitrate lyases lacking their essential type 1 peroxisomal targeting signal are piggybacked to glyoxysomes

Isocitrate lyase (IL) is an essential enzyme in the glyoxylate cycle, which is a pathway involved in the mobilization of stored lipids during postgerminative growth of oil-rich seedlings. We determined experimentally the necessary and sufficient peroxisome targeting signals (PTSs) for cottonseed, oilseed rape, and castor bean ILs in a well-characterized in vivo import system, namely, suspension-cultured tobacco (Bright Yellow) BY-2 cells. Results were obtained by comparing immunofluorescence localizations of wild-type and C-terminal-truncated proteins transiently expressed from cDNAs introduced by microprojectile bombardment. The tripeptides ARM-COOH (on cottonseed and castor bean ILs) and SRM-COOH (on oilseed rape IL) were necessary for targeting and actual import of these ILs into glyoxysomes, and ARM-COOH was sufficient for redirecting chloramphenicol acetyltransferase (CAT) from the cytosol into the glyoxysomes. Surprisingly, IL and CAT subunits without these tripeptides were still acquired by glyoxysomes, but only when wild-type IL or CAT-SKL subunits, respectively, were simultaneously expressed in the cells. These results reveal that targeting signal-depleted subunits are being piggybacked as multimers to glyoxysomes by association with subunits possessing a PTS1. Targeted multimers are then translocated through membrane pores or channels to the matrix as oligomers or as subunits before reoligomerization in the matrix.

Acquisition of membrane lipids by differentiating glyoxysomes: role of lipid bodies

Glyoxysomes in cotyledons of cotton (Gossypium hirsutum, L.) seedlings enlarge dramatically within 48 h after seed imbibition (Kunce, C.M., R.N. Trelease, and D.C. Doman. 1984. Planta (Berl.). 161:156-164) to effect mobilization of stored cotton-seed oil. We discovered that the membranes of enlarging glyoxysomes at all stages examined contained a large percentage (36-62% by weight) of nonpolar lipid, nearly all of which were triacylglycerols (TAGs) and TAG metabolites. Free fatty acids comprised the largest percentage of these nonpolar lipids. Six uncommon (and as yet unidentified) fatty acids constituted the majority (51%) of both the free fatty acids and the fatty acids in TAGs of glyoxysome membranes; the same six uncommon fatty acids were less than 7% of the acyl constituents in TAGs extracted from cotton-seed storage lipid bodies. TAGs of lipid bodies primarily were composed of palmitic, oleic, and linoleic acids (together 70%). Together, these three major storage fatty acids were less than 10% of both the free fatty acids and fatty acids in TAGs of glyoxysome membranes. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) constituted a major portion of glyoxysome membrane phospholipids (together 61% by weight). Pulse-chase radiolabeling experiments in vivo clearly demonstrated that 14C-PC and 14C-PE were synthesized from 14C-choline and 14C-ethanolamine, respectively, in ER of cotyledons, and then transported to mitochondria; however, these lipids were not transported to enlarging glyoxysomes. The lack of ER involvement in glyoxysome membrane phospholipid synthesis, and the similarities in lipid compositions between lipid bodies and membranes of glyoxysomes, led us to formulate and test a new hypothesis whereby lipid bodies serve as the dynamic source of nonpolar lipids and phospholipids for membrane expansion of enlarging glyoxysomes. In a cell-free system, 3H-triolein (TO) and 3H-PC were indeed transferred from lipid bodies to glyoxysomes. 3H-PC, but not 3H-TO, also was transferred to mitochondria in vitro. The amount of lipid transferred increased linearly with respect to time and amount of acceptor organelle protein, and transfer occurred only when lipid body membrane proteins were associated with the donor lipid bodies. 3H-TO was transferred to and incorporated into glyoxysome membranes, and then hydrolyzed to free fatty acids. 3H-PC was transferred to and incorporated into glyoxysome and mitochondria membranes without subsequent hydrolysis. Our data are inconsistent with the hypothesis that ER contributes membrane lipids to glyoxysomes during postgerminative seedling growth.(ABSTRACT TRUNCATED AT 400 WORDS)

The sorting signals for peroxisomal membrane-bound ascorbate peroxidase are within its C-terminal tail

Peroxisomal ascorbate peroxidase (APX) is a carboxyl tail-anchored, type II (N(cytosol)-C(matrix)) integral membrane protein that functions in the regeneration of NAD(+) in glyoxysomes of germinated oilseeds and protection of peroxisomes in other organisms from toxic H(2)O(2). Recently we showed that cottonseed peroxisomal APX was sorted post-translationally from the cytosol to peroxisomes via a novel reticular/circular membranous network that was interpreted to be a subdomain of the endoplasmic reticulum (ER), named peroxisomal ER (pER). Here we report on the molecular signals responsible for sorting peroxisomal APX. Deletions or site-specific substitutions of certain amino acid residues within the hydrophilic C-terminal-most eight-amino acid residues (includes a positively charged domain found in most peroxisomal integral membrane-destined proteins) abolished sorting of peroxisomal APX to peroxisomes via pER. However, the C-terminal tail was not sufficient for sorting chloramphenicol acetyltransferase to peroxisomes via pER, whereas the peptide plus most of the immediately adjacent 21-amino acid transmembrane domain (TMD) of peroxisomal APX was sufficient for sorting. Replacement of the peroxisomal APX TMD with an artificial TMD (devoid of putative sorting sequences) plus the peroxisomal APX C-terminal tail also sorted chloramphenicol acetyltransferase to peroxisomes via pER, indicating that the peroxisomal APX TMD does not possess essential sorting information. Instead, the TMD appears to confer the proper context required for the conserved positively charged domain to function within peroxisomal APX as an overlapping pER sorting signal and a membrane peroxisome targeting signal type 2.

Purification and biosynthesis of cottonseed (Gossypium hirsutum L.) catalase

As part of our research on peroxisome biogenesis, catalase was purified from cotyledons of dark-grown cotton (Gossypium hirsutum L.) seedlings and monospecific antibodies were raised in rabbits. Purified catalase appeared as three distinct electrophoretic forms in non-denaturing gels and as a single protein band (with a subunit Mr of 57,000) on silver-stained SDS/polyacrylamide gels. Western blots of crude extracts and isolated peroxisomes from cotton revealed one immunoreactive polypeptide with the same Mr (57,000) as the purified enzyme, indicating that catalase did not undergo any detectable change in Mr during purification. Synthesis in vitro, directed by polyadenylated RNA isolated from either maturing seeds or cotyledons of dark-grown cotton seedlings, revealed a predominant immunoreactive translation product with a subunit Mr of 57,000 and an additional minor immunoreactive product with a subunit Mr of 64000. Labelling studies in vivo revealed newly synthesized monomers of both the 64000- and 57,000-Mr proteins present in the cytosol and incorporation of both proteins into the peroxisome without proteolytic processing. Within the peroxisome, the 57,000-Mr catalase was found as an 11S tetramer; whereas the 64,000-Mr protein was found as a relatively long-lived 20S aggregate (native Mr approx. 600,000-800,000). The results strongly indicate that the 64,000-Mr protein (catalase?) is not a precursor to the 57,000-Mr catalase and that cotton catalase is translated on cytosolic ribosomes without a cleavable transit or signal sequence.

How are peroxisomes formed? The role of the endoplasmic reticulum and peroxins

Recent data from studies of peroxisome assembly and the subcellular sorting of peroxisomal matrix and membrane proteins have led to an expansion of the 'growth and division' and 'endoplasmic reticulum-vesiculation' models of peroxisome biogenesis into a more flexible, unified model. Within this context, we discuss the proposed role for the endoplasmic reticulum in the formation of preperoxisomes and the potential for 15 Arabidopsis peroxin homologs to function in the biogenesis of peroxisomes in plant cells.

Diverse amino acid residues function within the type 1 peroxisomal targeting signal. Implications for the role of accessory residues upstream of the type 1 peroxisomal targeting signal

The purpose of this study was to determine whether the plant type 1 peroxisomal targeting signal (PTS1) utilizes amino acid residues that do not strictly adhere to the serine-lysine-leucine (SKL) motif (small-basic-hydrophobic residues). Selected residues were appended to the C terminus of chloramphenicol acetyltransferase (CAT) and were tested for their ability to target CAT fusion proteins to glyoxysomes in tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 suspension-cultured cells. CAT was redirected from the cytosol into glyoxysomes by a wide range of residues, i.e. A/C/G/S/T-H/K/ L/N/R-I/L/M/Y. Although L and N at the -2 position (-SLL, -ANL) do not conform to the SKL motif, both functioned, but in a temporally less-efficient manner. Other SKL divergent residues, however, did not target CAT to glyoxysomes, i.e. F or P at the -3 position (-FKL, -PKL), S or T at the -2 position (-SSI, STL), or D at the -1 position (-SKD). The targeting inefficiency of CAT-ANL could be ameliorated when K was included at the -4 position (-KANL). In summary, the plant PTS1 mostly conforms to the SKL motif. For those PTS1s that possess nonconforming residue(s), other residues upstream of the PTS1 appear to function as accessory sequences that enhance the temporal efficiency of peroxisomal targeting.

Two temporally synthesized charge subunits interact to form the five isoforms of cottonseed (Gossypium hirsutum) catalase

Five charge isoforms of tetrameric catalase were isolated from cotyledons of germinated cotton (Gossypium hirsutum L.) seedlings. Denaturing isoelectric focusing of the individual isoforms in polyacrylamide gels indicated that isoforms A (most anodic) and E (most cathodic) consisted of one subunit of different charge, whereas isoforms B, C and D each consisted of a mixture of these two subunits. Thus the five isoforms apparently were formed through combinations of two subunits in different ratios. Labelling cotyledons in vivo with [35S]methionine at three daily intervals in the dark, and translation in vivo of polyadenylated RNA isolated from cotyledons at the same ages, revealed synthesis of two different subunits. One of the subunits was synthesized in cotyledons at all ages studied (days 1-3), whereas the other subunit was detected only at days 2 and 3. This differential expression of two catalase subunits helped explain previous results from this laboratory showing that the two anodic forms (A and B) found in maturing seeds were supplemented with three cathodic forms (C-E) after the seeds germinated. These subunit data also helped clarify our new findings that proteins of isoforms A, B and C (most active isoforms) accumulated in cotyledons of plants kept in the dark for 3 days, then gradually disappeared during the next several days, whereas isoforms D and E (least active isoforms) remained in the cells. This shift in isoform pattern occurred whether seedlings were kept in the dark or exposed to continuous light after day 3, although exposure to light enhanced this process. These sequential molecular events were responsible for the characteristic developmental changes (rise and fall) in total catalase activity. We believe that the isoform changeover is physiologically related to the changeover in glyoxysome to leaf-type-peroxisome metabolism.

Biogenesis of catalase in glyoxysomes and leaf-type peroxisomes of sunflower cotyledons

Eight charge isoforms of catalase (EC 1.11.1.6.) appeared in the peroxisomes of sunflower cotyledons during growth after germination (2.5 days of dark, continuous light thereafter). In the light, when glyoxysomes were transformed to leaf-type peroxisomes, the five more-basic forms (CAT 1 through CAT 5) became more prominent, while amounts of the three more-acidic forms (CAT 6 through CAT 8) decreased considerably. The isoforms CAT 1 through CAT 5 were hybrids of 55- and 59-kDa subunits, whereas CAT 6 through CAT 8 contained 55-kDa subunits exclusively. The catalase translation products changed during the transition of glyoxysomes to leaf-type peroxisomes. Polyadenylated RNA from 2-day-old cotyledons directed synthesis of 56-kDa subunits, whereas 59-kDa subunits predominated after in vitro translation of RNA from 4-day-old cotyledons. Both translation products were processed to lower molecular weight forms in vivo. The 56-kDa translation products were precursors for 55-kDa subunits in glyoxysomes. It could not be decided however, whether the 59-kDa precursors were processed to 56-kDa or 55-kDa subunits, because both subunits of lower molecular weight were present in leaf-type peroxisomes. Some of the 59-kDa precursors escaped proteolytic processing and formed hybrid isoforms (CAT 1 through CAT 5) with mature 55-kDa subunits. This type of isoform formation, i.e., condensation of mature and unprocessed subunits, has not yet been described for other plant catalases. In summary, the results showed that the postgerminative changes in the number and abundance of catalase isoforms resulted from changes in translation (transcription) of catalase precursors and assembly of proteolytically processed and unprocessed subunits into tetramers within peroxisomes acquiring leaf peroxisomal function.

Microbodies (Glyoxysomes and Peroxisomes) in Cucumber Cotyledons: Correlative Biochemical and Ultrastructural Study in Light- and Dark-grown Seedlings

The changes in activities of glyoxysomal and peroxisomal enzymes have been correlated with the fine structure of microbodies in cotyledons of the cucumber (Cucumis sativus L.) during the transition from fat degradation to photosynthesis in light-grown plants, and in plants grown in the dark and then exposed to light. During early periods of development in the light (days 2 through 4), the microbodies (glyoxysomes) are interspersed among lipid bodies and contain relatively high activities of glyoxylate cycle enzymes involved in lipid degradation. Thereafter, these activities decrease rapidly as the cotyledons expand and become photosynthetic, and the activity of glycolate oxidase rises to a peak (day 7); concomitantly the microbodies (peroxisomes) become preferentially associated with chloroplasts.In seedlings grown in the dark for 10 days, the reserve lipid and the glyoxylate cycle enzyme activities persist for a longer time than in the light; correlated with this, there is a continued association of the microbodies with the lipid bodies. When these dark-grown seedlings are then exposed to 51 hours of the light-dark cycle, peroxisomal marker enzymes increase rapidly in activity, and the microbodies become appressed to chloroplasts. We conclude that the characteristic association observed between glyoxysomes and lipid bodies reflects their mutual involvement in net gluconeogenesis through the conversion of fatty acids to carbohydrate, while the close spatial relationship observed between peroxisomes and chloroplasts at later stages of development reflects their mutual involvement in glycolate metabolism.Although glyoxysomal enzyme activities are dropping rapidly while peroxisomal enzyme activities are increasing rapidly during the transition period in the light, the electron microscopic evidence does not indicate that glyoxysomes are being degraded or peroxisomes are being formed. Since in the dark-grown seedlings the activities of peroxisomal enzymes remain low and do not increase as they do in the light, an opportunity is afforded to compare quantitatively any changes in numbers of microbodies per cell with the changes in activities of glyoxysomal enzymes. It is found that the magnitude of the decrease in numbers of microbodies is considerably less than that of the decrease in glyoxysomal enzyme activities between days 4 and 10. When the cotyledons are exposed to light, peroxisomal enzyme activities increase greatly, but again there is no ultrastructural evidence for the synthesis of a new population of microbodies to accommodate this increase. These results allow us to conclude that the developmental transition from glyoxysomal to peroxisomal function almost certainly does not involve the actual replacement of one population of microbodies by another. Rather, the transition probably occurs within existing particles, either by a sequential functioning of two different kinds of microbodies or by a change in enzyme complement within a single population. Our findings with both light- and dark-grown cotyledons favor the latter possibility. The cytoplasmic invaginations into microbodies seen during greening of both light-grown cotyledons and etiolated cotyledons exposed to light may be morphological manifestations of the mechanism by which the microbodies lose or gain enzymes.

Mutational analyses of a type 2 peroxisomal targeting signal that is capable of directing oligomeric protein import into tobacco BY-2 glyoxysomes

In this study of the type 2 peroxisomal targeting signal (PTS2) pathway, we examined the apparent discontinuity and conservation of residues within the PTS2 nonapeptide and demonstrated that this topogenic signal is capable of directing heteromultimeric protein import in plant cells. Based on cumulative data showing that at least 26 unique, putative PTS2 nonapeptides occur within 12 diverse peroxisomal-destined proteins, the current (-R/K-L/V/I-X5-H/Q-L/A-) as well as the original (-R-L-X5-H/Q-L-) PTS2 motif appear to be oversimplified. To assess the functionality of residues within the motif, rat liver thiolase (rthio) and various chimeric chloramphenicol acetyltransferase (CAT) proteins were expressed transiently in suspension-cultured tobacco (Nicotiana tabaccum L.) cv Bright Yellow cells (BY-2), and their subcellular location was determined by immunofluoresence microscopy. Hemagglutinin (HA)-epitope-tagged-CAT subunits, lacking a PTS2 (CAT-HA), were 'piggybacked' into glyoxysomes by PTS2-bearing CAT subunits (rthio-CAT), whereas signal-depleted CAT-HA subunits that were modified to prevent oligomerization did not import into glyoxysomes. These results provided direct evidence that signal-depleted subunits imported into peroxisomes were targeted to the organelle as oligomers (heteromers) by a PTS2. Mutational analysis of residues within PTS2 nonapeptides revealed that a number of amino acid substitutions were capable of maintaining targeting function. Furthermore, functionality of residues within the PTS2 nonapeptide did not appear to require a context-specific environment conferred by adjacent residues. These results collectively suggest that the functional PTS2 is not solely defined as a sequence-specific motif, i.e. -R/K-X6-H/Q-A/L/F-, but defined also by its structural motif that is dependent upon the physiochemical properties of residues within the nonapeptide.

Cytochemical localization of malate synthase in glyoxysomes

Cytochemical staining techniques for microbodies (peroxisomes) are limited at present to the enzymes catalase and alpha-hydroxy acid oxidase, and neither technique can distinguish glyoxysomes from other microbodies. Described here is a procedure using ferricyanide for the cytochemical demonstration by light and electron microscopy of malate synthase activity in glyoxysomes of cotyledons from fat-storing cucumber and sunflower seedlings. Malate synthase, a key enzyme of the glyoxylate cycle, catalyzes the condensation of acetyl CoA with glyoxylate to form malate and release free coenzyme A. Localization of the enzyme activity is based on the reduction by free CoA of ferricyanide to ferrocyanide, and the visualization of the latter as an insoluble, electron-opaque deposit of copper ferrocyanide (Hatchett's brown). The conditions and optimal concentrations for the cytochemical reaction mixture were determined in preliminary studies using a colorimetric assay developed to measure disappearance of ferricyanide at 420 nm. Ultrastructural observation of treated tissue reveals electron-opaque material deposited uniformly throughout the matrix portion of the glyoxysomes, with little background deposition elsewhere in the cell. The reaction product is easily visualized in plastic sections by phase microscopy without poststaining. Although the method has been applied thus far only to cotyledons of fat-storing seedlings, it is anticipated that the technique will be useful in localizing and studying glyoxylate cycle activity in a variety of tissues from both plants and animals.

A correlative ultrastructural and enzymatic study of cotyledonary microbodies following germination of fat-storing seeds

Sunflower, cucumber, and tomato cotyledons, which contain microbodies in both the early lipid-degrading and the later photosynthetic stages of post-germinative growth, were processed for electron microscopy according to conventional procedures and examined 1, 4 and 7 days after germination. Homogenates of sunflower cotyledons were assayed for enzymes characteristic of glyoxysomes and leaf peroxisomes (both of which are defined morphologically as microbodies) at stages corresponding to the fixations for electron microscopy. The particulate nature of these enzymes was demonstrated by differential and equilibrium density centrifugation, making it possible to relate them to the microbodies seen in situ.One day after germination, the microbodies are present as small organelles among large numbers of protein and lipid storage bodies; the cell homogenate contains catalase but no detectable isocitrate lyase (characteristic of glyoxysomes) or glycolic acid oxidase (characteristic of leaf peroxisomes). 4 days after germination, numerous microbodies (glyoxysomes) are in extensive and frequent contact with lipid bodies. The microbodies often have cytoplasmic invaginations. At this stage the cells are rapidly converting lipids to carbohydrates, and the homogenate has high isocitrate lyase activity. 7 days after germination, microbodies (peroxisomes) are appressed to chloroplasts and frequently squeezed between them in the green photosynthetic cells. The homogenate at this stage has substantial glycolic acid oxidase activity but a reduced level of isocitrate lyase. It is yet to be determined whether the peroxisomes present at day 7 are derived from preexisting glyoxysomes or arise as a separate population of organelles.

Characterization of a cDNA encoding cottonseed catalase

A 1.7 kb cDNA clone was isolated from our lambda gt11 library constructed from poly(A) RNA of 24-h-old cotyledons. The cDNA encodes a full-length catalase peptide (492 amino acid residues). The calculated molecular mass is 56,800, similar to that determined for purified enzyme (57,000 SDS-PAGE). Among higher plant catalases, this cotton catalase shows the highest amino acid sequence identity (85%) to the subunit of homotetrameric maize CAT 1, a developmental counterpart to the homotetrameric CAT A isoform of cotton seeds. Comparison of sequences from cotton, sweet potato, maize CAT 1, and yeast with bovine catalase revealed that the amino acid residues and regions that are involved in catalytic activity and/or required to maintain basic catalase structure, are highly conserved. The C-terminus region, which has the lowest nucleotide sequence identity between plant and mammalian catalases, does not terminate with a tripeptide, S-K/R/H-L, a putative targeting signal for peroxisomal proteins.

Development and application of an in vivo plant peroxisome import system

The purposes of this study are to develop an in vivo cell system that is suitable for the immunofluorescent detection of transiently expressed proteins targeted to plant peroxisomes and to determine whether a C-terminal serine-lysine-leucine (SKL) tripeptide, a consensus-targeting signal for mammalian peroxisomes, also targets proteins to plant peroxisomes. Protoplasts from mesophyll cells and from suspension-cultured cells initially were examined for their potential as an in vivo import system. Several were found suitable, but based on a combination of criteria, suspension-cultured tobacco (Nicotiana tabacum L. cv Bright Yellow 2) cells (TBY-2) were chosen. The tobacco cell extracts had catalase activity, and two polypeptides of approximately 55 and 57 kD specifically were detected on immunoblots with anti-cottonseed catalase immunoglobulins G as the probe. Indirect immunofluorescence microscopy with these immunoglobulins G revealed a punctate labeling pattern indicative of endogenous catalase localization within putative TBY-2 peroxisomes. The cells did not have to be completely converted to protoplasts for optimal microscopy; treatment with 0.1% (w/v) pectolyase for 2 h was sufficient. Microprojectile bombardment proved superior for transient transformation of the TBY-2 cells with plasmids encoding beta-glucuronidase, or chloramphenicol acetyltransferase (CAT), or CAT with an added C-terminal tripeptide (CAT-SKL). C-terminal SKL is a consensus, type 1, peroxisome targeting signal. Double indirect immunofluorescent labeling showed that CAT-SKL co-localized with endogenous catalase. Non-punctate, diffuse localization of CAT without SKL provided direct evidence that the C-terminal SKL tripeptide was necessary and sufficient for targeting of CAT to plant peroxisomes. These data demonstrate the effectiveness of this peroxisome targeting signal for plant cells.

Characterization of a cDNA clone encoding the complete amino acid sequence of cotton isocitrate lyase

A cDNA clone encoding the glyoxysomal enzyme isocitrate lyase (ICL) (EC 4.1.3.1) was isolated from a library prepared from cotton (Gossypium hirsutum L.) cotyledon poly(A)+ RNA. The clone is 1893 basepairs (bp) in length and contains a 1728 bp open reading frame encoding a polypeptide of 576 residues (Mr = 64,741). The deduced amino acid sequence of cotton ICL is 85.2%, 90.3% and 41.1% identical to ICL from rapeseed, castor bean and E. coli, respectively. Cotton ICL has a C-terminal tripeptide of A-R-M which is a putative trafficking signal for peroxisome (glyoxysome) proteins.

Subcellular localization of glyoxylate cycle enzymes in Ascaris suum larvae

Evidence is presented on the particulate nature of glyoxylate cycle enzymes in metazoa with the use of 15-day old larvae of the nematode Ascaris suum. Homogenization procedures were developed to disrupt the resistant nematode cuticle. Malate synthase and isocitrate lyase, key enzymes of the glyoxylate cycle, consistently sedimented with mitochondrial enzymes in differential pellets while catalase, a major peroxisomal enzyme, was always soluble. Isopycnic sucrose gradient centrifugation of the differential pellet yielded two protein peaks: one at 1.18 g/cm3 (characteristic for mitochondria), and another at 1.23 g/cm3 (common for glyoxysomes and peroxisomes). Electron microscopy of these fractions revealed that the lighter peak consisted primarily of mitochondria, while the heavier band contained proteinaceous bodies termed "dense granules" morphologically resembling microbodies. SIgnificantly, both malate synthase and isocitrate lyase cosedimented with the mitochondrial marker enzymes in the lighter peak (1.18 g/cm3) and not with the dense granules. Further purification of mitochondria, accomplished by separating dense granules with a step gradient before isopycnic centrifugation, substantiated the evidence that microbodies (glyoxysomes) do not occur in these nematode larvae. Rough-surfaced membranes were alternatively considered as the subcellular site, but the evidence tends to favor localization of the glyoxylate bypass enzymes in the mitochondria.

Identification of porin-like polypeptide(s) in the boundary membrane of oilseed glyoxysomes

A 36-kDa polypeptide of unknown function was identified by us in the boundary membrane fraction of cucumber seedling glyoxysomes. Evidence is presented in this study that this 36-kDa polypeptide is a glyoxysomal membrane porin. A sequence of 24 amino acid residues derived from a CNBr-cleaved fragment of the 36-kDa polypeptide revealed 72% to 95% identities with sequences in mitochondrial or non-green plastid porins of several different plant species. Immunological evidence indicated that the 36-kDa (and possibly a 34-kDa polypeptide) was a porin(s). Antiserum raised against a potato tuber mitochondrial porin recognized on immunoblots 34-kDa and 36-kDa polypeptides in detergent-solubilized membrane fractions of cucumber seedling glyoxysomes and mitochondria, and in similar glyoxysomal fractions of cotton, castor bean, and sunflower seedlings. The 36-kDa polypeptide seems to be a constitutive component because it was detected also in membrane protein fractions derived from cucumber leaf-type peroxisomes. Compelling evidence that one or both of these polypeptides were authentic glyoxysomal membrane porins was obtained from electron microscopic immunogold analyses. Antiporin IgGs recognized antigen(s) in outer membranes of glyoxysomes and mitochondria. Taken together, the data indicate that membranes of cucumber (and other oilseed) glyoxysomes, leaf-type peroxisomes, and mitochondria possess similar molecular mass porin polypeptide(s) (34 and 36 kDa) with overlapping immunological and amino acid sequence similarities.

Post-Transcriptional Regulation of Catalase Isozyme Expression in Cotton Seeds

We reported previously that expression of the five tetrameric catalase isozymes during postgerminative growth of cotton seedings was a consequence of interactions between two subunits (SU 1 and SU 2) temporally synthesized from two distinct catalase genes. In this study, we focused on the regulation of the expression of these two catalase subunits during the changeover from glyoxysomal to leaf-type peroxisomal metabolism. The steady-state level of glyoxysomal SU 1 protein (present in 12-hour-old seeds) increased through day 3 and then declined linearly through day 6, whereas SU 2 protein (first detected in 24-hour-old seeds) increased continuously through day 6. The time courses for steady-state levels of the mRNAs encoding these two subunits revealed two clearly separated peaks: the first at day 1 (SU 1) and the other at day 4 (SU 2). Accumulation of these mRNAs preceded the accumulation of their corresponding proteins by at least 24 hours, suggesting temporal, pretranslational regulation of synthesis of both subunits. Results from run-on transcriptional assays with isolated nuclei, however, revealed that transcripts encoding both subunits were synthesized together on days 1 through 5. Hence, it appears that the accumulations of SU 1 and SU 2 mRNAs are controlled primarily at the post-transcriptional level, which has not been reported for catalase or any other eukaryotic peroxisomal enzymes. The accumulation of SU 1 mRNA is not light dependent, whereas the accumulation of SU 2 mRNA, which directs synthesis of the predominant subunit comprising the leaf-type peroxisomal isozyme, occurs only after exposure of seedlings to light.

Stable and transient expression of chimeric peroxisomal membrane proteins induces an independent "zippering" of peroxisomes and an endoplasmic reticulum subdomain

Peroxisomal ascorbate peroxidase (APX) (EC 1.11.1.11) was shown recently to sort through a subdomain of the ER (peroxisomal endoplasmic reticulum; pER), and in certain cases, alter the distribution and/or morphology of peroxisomes and pER when overexpressed transiently in Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) cells. Our goal was to gain insight into the dynamics of peroxisomal membrane protein sorting by characterizing the structure and formation of reorganized peroxisomes and pER. Specifically, we test directly the hypothesis that the observed phenomenon is due to the oligomerization of cytosol-facing, membrane-bound polypeptides. a process referred to as membrane "zippering". Results from differential detergent permeabilization experiments confirmed that peroxisomal APX is a C-terminal "tail-anchored" (Cmatrix-Ncytosol) membrane protein with a majority of the polypeptide facing the cytosol. Transient expression of several APX chimeras whose passenger polypeptides can form dimers or trimers resulted in the progressive formation of "globular" peroxisomes and circular pER membranes. Stable expression of the trimer-capable fusion protein yielded suspension cultures that reproducibly maintained a high degree of peroxisomal globules but relatively few detectable pER membranes. Electron micrographs revealed that the globules consisted of numerous individual peroxisomes, seemingly in direct contact with other peroxisomes and/or mitochondria. These peroxisomal clusters or aggregates were not observed in cells transiently expressing monomeric versions of APX. These findings indicate that the progressive, independent "zippering" of peroxisomes and pER is due to the post-sorting oligomerization of monomeric, cytosol-facing polypeptides that are integrally inserted into the membranes of "like" organelles. The dynamics of this process are discussed, especially with respect to the involvement of the microtubule cytoskeleton.