Nucleotide binding and dimerization at the chloroplast pre-protein import receptor, atToc33, are not essential in vivo but do increase import efficiency.

The atToc33 protein is one of several pre-protein import receptors in the outer envelope of Arabidopsis chloroplasts. It is a GTPase with motifs characteristic of such proteins, and its loss in the plastid protein import 1 (ppi1) mutant interferes with the import of photosynthesis-related pre-proteins, causing a chlorotic phenotype in mutant plants. To assess the significance of GTPase cycling by atToc33, we generated several atToc33 point mutants with predicted effects on GTP binding (K49R, S50N and S50N/S51N), GTP hydrolysis (G45R, G45V, Q68A and N101A), both binding and hydrolysis (G45R/K49N/S50R), and dimerization or the functional interaction between dimeric partners (R125A, R130A and R130K). First, a selection of these mutants was assessed in vitro, or in yeast, to confirm that the mutations have the desired effects: in relation to nucleotide binding and dimerization, the mutants behaved as expected. Then, activities of selected mutants were tested in vivo, by assessing for complementation of ppi1 in transgenic plants. Remarkably, all tested mutants mediated high levels of complementation: complemented plants were similar to the wild type in growth rate, chlorophyll accumulation, photosynthetic performance, and chloroplast ultrastructure. Protein import into mutant chloroplasts was also complemented to >50% of the wild-type level. Overall, the data indicate that neither nucleotide binding nor dimerization at atToc33 is essential for chloroplast import (in plants that continue to express the other TOC receptors in native form), although both processes do increase import efficiency. Absence of atToc33 GTPase activity might somehow be compensated for by that of the Toc159 receptors. However, overexpression of atToc33 (or its close relative, atToc34) in Toc159-deficient plants did not mediate complementation, indicating that the receptors do not share functional redundancy in the conventional sense.

In vivo assessment of the significance of phosphorylation of the Arabidopsis chloroplast protein import receptor, atToc33.

atToc33 is a transit peptide receptor of the chloroplast outer envelope membrane, and possesses GTPase activity. In vitro, its transit peptide- and GTP-binding properties are abrogated by its phosphorylation at serine 181, which was proposed to represent an important regulatory mechanism. We mutated S181 to alanine (to prevent phosphorylation), and to aspartate and glutamate (to mimic the effects of phosphoserine), and expressed all three proteins in ppi1 (atToc33 knockout) plants using the native promoter. The mutants complemented ppi1 with equal efficiency in respect of all criteria tested, including protein import efficiency and light stress tolerance. The data suggest that atToc33 phosphorylation may not play an important role in vivo.

Tryptophan tryptophylquinone cofactor biogenesis in the aromatic amine dehydrogenase of Alcaligenes faecalis. Cofactor assembly and catalytic properties of recombinant enzyme expressed in Paracoccus denitrificans.

The heterologous expression of tryptophan trytophylquinone (TTQ)-dependent aromatic amine dehydrogenase (AADH) has been achieved in Paracoccus denitrificans. The aauBEDA genes and orf-2 from the aromatic amine utilization (aau) gene cluster of Alcaligenes faecalis were placed under the regulatory control of the mauF promoter from P. denitrificans and introduced into P. denitrificans using a broad-host-range vector. The physical, spectroscopic and kinetic properties of the recombinant AADH were indistinguishable from those of the native enzyme isolated from A. faecalis. TTQ biogenesis in recombinant AADH is functional despite the lack of analogues in the cloned aau gene cluster for mauF, mauG, mauL, mauM and mauN that are found in the methylamine utilization (mau) gene cluster of a number of methylotrophic organisms. Steady-state reaction profiles for recombinant AADH as a function of substrate concentration differed between 'fast' (tryptamine) and 'slow' (benzylamine) substrates, owing to a lack of inhibition by benzylamine at high substrate concentrations. A deflated and temperature-dependent kinetic isotope effect indicated that C-H/C-D bond breakage is only partially rate-limiting in steady-state reactions with benzylamine. Stopped-flow studies of the reductive half-reaction of recombinant AADH with benzylamine demonstrated that the KIE is elevated over the value observed in steady-state turnover and is independent of temperature, consistent with (a) previously reported studies with native AADH and (b) breakage of the substrate C-H bond by quantum mechanical tunnelling. The limiting rate constant (k(lim)) for TTQ reduction is controlled by a single ionization with pK(a) value of 6.0, with maximum activity realized in the alkaline region. Two kinetically influential ionizations were identified in plots of k(lim)/K(d) of pK(a) values 7.1 and 9.3, again with the maximum value realized in the alkaline region. The potential origin of these kinetically influential ionizations is discussed.

Unusual nucleotide-binding properties of the chloroplast protein import receptor, atToc33.

Arabidopsis Toc33 (atToc33) is a GTP-binding protein of the chloroplast outer envelope membrane. We studied its nucleotide-binding properties in vitro, and found that it binds GTP, GDP and XTP, with similar efficiencies, but not ATP. We further demonstrated that atToc33 has intrinsic GTPase activity. Mutations within the putative G4 motif of the atToc33 nucleotide-binding domain (D217N, D219N and E220Q) had no effect on nucleotide specificity or GTPase activity. Similarly, a mutation in the newly assigned G5 motif (E208Q) did not affect nucleotide specificity or GTPase activity. Furthermore, the D217N and D219N mutations did not affect atToc33 functionality in vivo. The data demonstrate that atToc33 belongs to a novel class of GTPases with unusual nucleotide-binding properties.

Lys-D48 is required for charge stabilization, rapid flavin reduction, and internal electron transfer in the catalytic cycle of dihydroorotate dehydrogenase B of Lactococcus lactis.

Dihydroorotate dehydrogenase B (DHODB) catalyzes the oxidation of dihydroorotate (DHO) to orotate and is found in the pyrimidine biosynthetic pathway. The Lactococcus lactis enzyme is a dimer of heterodimers containing FMN, FAD, and a 2Fe-2S center. Lys-D48 is found in the catalytic subunit and its side-chain adopts different positions, influenced by ligand binding. Based on crystal structures of DHODB in the presence and absence of orotate, we hypothesized that Lys-D48 has a role in facilitating electron transfer in DHODB, specifically in stabilizing negative charge in the reduced FMN isoalloxazine ring. We show that mutagenesis of Lys-D48 to an alanine, arginine, glutamine, or glutamate residue (mutants K38A, K48R, K48Q, and K48E) impairs catalytic turnover substantially (approximately 50-500-fold reduction in turnover number). Stopped-flow studies demonstrate that loss of catalytic activity is attributed to poor rates of FMN reduction by substrate. Mutation also impairs electron transfer from the 2Fe-2S center to FMN. Addition of methylamine leads to partial rescue of flavin reduction activity. Nicotinamide coenzyme oxidation and reduction at the distal FAD site is unaffected by the mutations. Formation of the spin-interacting state between the FMN semiquinone-reduced 2Fe-2S centers observed in wild-type enzyme is retained in the mutant proteins, consistent with there being little perturbation of the superexchange paths that contribute to the efficiency of electron transfer between these cofactors. Our data suggest a key charge-stabilizing role for Lys-D48 during reduction of FMN by dihydroorotate, or by electron transfer from the 2Fe-2S center, and establish a common mechanism of FMN reduction in the single FMN-containing A-type and the complex multicenter B-type DHOD enzymes.

Translation initiation factors eIF4E and eIFiso4E are required for polysome formation and regulate plant growth in tobacco.

Eukaryotic initiation factor eIF4E plays a pivotal role in translation initiation. As a component of the ternary eIF4F complex, eIF4E interacts with the mRNA cap structure to facilitate recruitment of the 40S ribosomal subunit onto mRNA. Plants contain two distinct cap-binding proteins, eIF4E and eIFiso4E, that assemble into different eIF4F complexes. To study the functional roles of eIF4E and eIFiso4E in tobacco, we isolated two corresponding cDNAs, NteIF4E1 and NteIFiso4E1, and used these to deplete cap-binding protein levels in planta by antisense downregulation. Antibodies raised against recombinant NteIF4E1 detected three distinct cap-binding proteins in tobacco leaf extracts; NteIF4E and two isoforms of NteIFiso4E. The three cap-binding proteins were immuno-detected in all tissues analysed and were coordinately regulated, with peak expression in anthers and pollen. Transgenic tobacco plants showing significant depletion of either NteIF4E or the two NteIFiso4E isoforms displayed normal vegetative development and were fully fertile. Interestingly, NteIFiso4E depletion resulted in a compensatory increase in NteIF4E levels, whereas the down-regulation of NteIF4E did not trigger a reciprocal increase in NteIFiso4E levels. The antisense depletion of both NteIF4E and NteIFiso4E resulted in plants with a semi-dwarf phenotype and an overall reduction in polyribosome loading, demonstrating that both eIF4E and eIFiso4E support translation initiation in planta, which suggests their potential role in the regulation of plant growth.

Functional specialization amongst the Arabidopsis Toc159 family of chloroplast protein import receptors.

The initial stages of preprotein import into chloroplasts are mediated by the receptor GTPase Toc159. In Arabidopsis thaliana, Toc159 is encoded by a small gene family: atTOC159, atTOC132, atTOC120, and atTOC90. Phylogenetic analysis suggested that at least two distinct Toc159 subtypes, characterized by atToc159 and atToc132/atToc120, exist in plants. atTOC159 was strongly expressed in young, photosynthetic tissues, whereas atTOC132 and atTOC120 were expressed at a uniformly low level and so were relatively prominent in nonphotosynthetic tissues. Based on the albino phenotype of its knockout mutant, atToc159 was previously proposed to be a receptor with specificity for photosynthetic preproteins. To elucidate the roles of the other isoforms, we characterized Arabidopsis knockout mutants for each one. None of the single mutants had strong visible phenotypes, but toc132 toc120 double homozygotes appeared similar to toc159, indicating redundancy between atToc132 and atToc120. Transgenic complementation studies confirmed this redundancy but revealed little functional overlap between atToc132/atToc120 and atToc159 or atToc90. Unlike toc159, toc132 toc120 caused structural abnormalities in root plastids. Furthermore, when proteomics and transcriptomics were used to compare toc132 with ppi1 (a receptor mutant that is specifically defective in the expression, import, and accumulation of photosynthetic proteins), major differences were observed, suggesting that atToc132 (and atToc120) has specificity for nonphotosynthetic proteins. When both atToc159 and the major isoform of the other subtype, atToc132, were absent, an embryo-lethal phenotype resulted, demonstrating the essential role of Toc159 in the import mechanism.