Post-translational assembly of lens alpha-crystallin in the reticulocyte lysate and in Xenopus laevis oocytes

Subunit Exchange of Polydisperse Proteins

The small heat shock protein, alpha-crystallin, plays a key role in maintaining lens transparency by chaperoning structurally compromised proteins. This is of particular importance in the human lens, where proteins are exposed to post-translational modifications over the life-time of an individual. Here, we examine the structural and functional consequences of one particular modification of alphaA-crystallin involving the truncation of 5 C-terminal residues (alphaA(1-168)). Using novel mass spectrometry approaches and established biophysical techniques, we show that alphaA(1-168) forms oligomeric assemblies with a lower average molecular mass than wild-type alphaA-crystallin (alphaA(WT)). Also apparent from the mass spectra of both alphaA(WT) and alphaA(1-168) assemblies is the predominance of oligomers containing even numbers of subunits; interestingly, this preference is more marked for alphaA(1-168). To examine the rate of exchange of subunits between assemblies, we mixed alphaB crystallin with either alphaA(WT) or alphaA(1-168) and monitored in a real-time mass spectrometry experiment the formation of heteroligomers. The results show that there is a significant decrease in the rate of exchange when alphaA(1-168) is involved. These reduced exchange kinetics, however, have no effect upon chaperone efficiency, which is found to be closely similar for both alphaA(WT) and alphaA(1-168). Overall, therefore, our results allow us to conclude that, in contrast to mechanisms established for analogous proteins from plants, yeast, and bacteria, the rate of subunit exchange is not the critical parameter in determining efficient chaperone behavior for mammalian alphaA-crystallin.

An immunocytochemical technique for analysis of regulation of genes encoding early differentiation marker antigens in an oocyte translation system

Monoclonal antibodies are useful probes for analyzing cells at the molecular level at various developmental stages. Although identification of the genes encoding tissue- and stage-specific antigens could be informative for further molecular analysis, gene cloning is usually a time-consuming step, particularly when a monoclonal antibody is the only probe available. We describe here an immunocytochemical method for preliminary and immediate analysis of the regulation of antigen-coding genes. mRNAs purified from stage 27 and 38 Xenopus tadpoles were fractionated by size and injected into newt oocytes, from which frozen sections were prepared for immunostaining with tissue-specific monoclonal antibodies. Both of the antigens we tested, which are early markers for differentiating epidermal cells of Xenopus tadpoles, were detected in mRNA injected oocytes, but not in control oocytes. Immunostaining for each of the antigens showed that their relative levels in stage 27 and 38 tadpole tissue were reflected in those oocytes injected with mRNA purified from tadpoles of the respective stages. We suggest that this oocyte translation system combined with immunostaining provides for rapid analysis of changes in levels of antigen coding mRNAs throughout development.

Acid-induced dissociation of alpha A- and alpha B-crystallin homopolymers

Homopolymers were constructed from the alpha A and alpha B polypeptides isolated from the lens protein alpha-crystallin. As the pH is lowered from 7.0 to 3.4, these homopolymers dissociate to smaller species with molecular masses ranging from 80 to 250 kDa for the alpha A and around 140 kDa for the alpha B dissociation products. The pKa for this dissociation was 3.8 +/- 0.2 for alpha A and 4.1 +/- 0.1 for alpha B homopolymers. Further decreases in pH, to 2.5, resulted in the presence of only denatured alpha B polypeptides, whereas the alpha A dissociation products remained intact. Fractionation of the acid dissociation products from the alpha A homopolymer at pH 2.5 yielded stable species with molecular masses of 220 +/- 30, 160 +/- 20, and 90 +/- 10 kDa. The majority of the population at acid pH consisted of the 160 kDa species. Conformational analysis of these species revealed that most of the secondary structure of the original alpha A homopolymer was retained but that the tertiary structure was perturbed. Fluorescence quenching and energy transfer measurements suggested that the molecule had undergone acid expansion, with the greatest perturbation observed in the smallest particles. The results from this work suggest that alpha A homopolymers are heterogeneous populations of aggregates of a "monomeric" molecule with a molecular mass of 160 kDa. This "monomeric" molecule may be formed from the association of two tetrameric units.

On the structure of alpha-crystallin: construction of hybrid molecules and homopolymers

The alpha A2 and alpha B2 subunits of bovine alpha-crystallin were purified by chromatofocussing in urea and assembled into homopolymers. Light-scattering measurements indicated their molecular masses were 360 and 420 kDa. The alpha A2 and alpha B2 polypeptides were also used to construct a series of hybrid molecules with alpha A/alpha B ratios ranging from 7:1 to 1:7. Sedimentation velocity analyses, isoelectric focussing under non-deaggregating conditions, circular dichroism spectroscopy and immunochemical analysis indicated that all of the subunits had copolymerized to alpha-crystallin-like aggregates with complete regeneration of the native structure. The polymers could be distinguished on the basis of their differing affinities for the antiserum. This was directly related to the proportion of alpha A2 subunits in each polymer. It was concluded that the alpha A2 and alpha B2 subunits are structurally equivalent and occupy equivalent site in the alpha-crystallin aggregates. It was also concluded that a micellar-like quaternary structure was consistent with most previous observations on the protein.

Use of Xenopus oocytes to study the expression of cloned genes and translation of mRNA

One of the most active areas of research in the field of molecular biology is the examination of the mechanisms associated with the regulation of gene expression. Our understanding of the events in eukaryotic transcription has been aided by the ability to test the expression of various genomic DNA constructs after their microinjection into the germinal vesicle of Xenopus oocytes. This in vivo transcription system has permitted the analysis of the involvement of cis-acting DNA sequences and the examination of the effect of co-injected trans-acting factors on gene expression. Furthermore, the Xenopus oocyte has been employed widely as an in vivo translation system. Not only is the Xenopus oocyte system a sensitive assay for the translation of rare mRNAs but it also has the ability to post-translationally modify and compartmentalize numerous types of proteins. Finally, the Xenopus oocyte has proven useful in the procedures associated with the cloning and screening of cDNAs.

A possible structure for alpha-crystallin

alpha-Crystallin, the major protein of the mammalian eye lens, is found in vivo as a multimeric aggregate composed of two closely related subunits whose molar ratio is widely variable from species to species. Attempts to determine the arrangement of the subunits within the aggregate, or even to determine the size of the aggregate and the number of subunits composing it, have not resulted in general agreement. Because of the variability in alpha-crystallin particle size, the apparent dependence of this parameter on certain environmental factors (e.g. temperature), the absence of a specific requirement for either alpha-crystallin isoform in aggregation, and the sharp division in the amino acid sequence between a strong hydrophobic region and a sharply hydrophilic one, it is suggested that the alpha-crystallin aggregate has the properties of a protein micelle. This hypothesis is consistent with what is known of the alpha-crystallin molecule and aggregate, and can be tested experimentally. If this hypothesis is shown to be true, then alpha-crystallin will be the first example of a naturally occurring protein micelle.

The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

The function of N alpha-acetylation of the eye-lens crystallins

The putative protective role of the N alpha-acetyl group of proteins has been investigated. Synthetic, non-acetylated N-terminal tetrapeptides of the alpha A2- and gamma II-crystallin chains are good substrates for leucine aminopeptidase, while the acetylated ones are completely resistant. In the native, non-acetylated, gamma-crystallin the N terminus is not degraded by leucine aminopeptidase. Newly synthesized alpha A2-crystallin, in which the normally occurring N-terminal acetylation has been prevented during cell-free translation, is virtually resistant against degradation by leucine aminopeptidase. Only at extreme enzyme-substrate ratios the N-terminal methionine is removed. Although the N alpha-acetyl group by its very nature protects against this exopeptidase, we conclude that the group is not essential for this purpose in the native crystallins.

State of differentiation of bovine epithelial lens cells in vitro. Modulation of the synthesis and of the polymerization of specific proteins (crystallins) and non-specific proteins in relation to cell divisions

Maintenance of the state of differentiation in serially cultured bovine epithelial lens cells has been investigated. The radioactive labelled soluble proteins were studied by gel filtration and gel electrophoresis. 1. In the lens epithelium on its capsule, preferential synthesis of alpha B2 vs alpha A2 crystallin subunits and synthesis of beta-crystallins (mainly beta Bp) were observed. 2. Epithelial lens cells cultured on plastic Petri dishes for up to 35 divisions still synthesized alpha B2 and beta Bp, but no longer alpha A2. Conversely, the same cells injected into nude mice synthesized alpha B and alpha A, but no beta-crystallin could be detected. 3. The ratio of non-crystallin proteins to crystallin polypeptides increased drastically with the number of cell divisions. Among these proteins, both Mr 45 000 and Mr 57 000 proteins are probably constituents of the water-soluble cytoskeletal proteins, respectively actin and vimentin. A Mr 17 000 polypeptide was observed and its relationship with a metabolic product of alpha-crystallin is proposed. 4. The polymerization process of crystallin polypeptides in these cells was studied and compared with crystallin aggregates found in the lens. Newly synthesized alpha crystallins were readily involved in high molecular aggregates. This process does not seem to require alpha A, since only alpha B was detected. Interestingly, non-crystallin-soluble proteins form the bulk of proteins found in high molecular weight (HMW) polymers. The time course of crystallin aggregate formation, in long-term culture cells, seems to be different for alpha- vs beta-polypeptides. These results allowed us to conclude that bovine epithelial lens cells in vitro, although they do not undergo terminal differentiation into fibers, are not dedifferentiated, since they still express specific features of the epithelium in situ.

Translation of mRNA for Limulus polyphemus haemocyanin polypeptides in vitro: studies on subunit heterogeneity

The haemocyanin of Limulus polyphemus is composed of a number (possibly 10-15) of polypeptides and is believed to be synthesised in cells called cyanoblasts. In vitro translation in the rabbit reticulocyte haemolysate system and in Xenopus oocytes, of mRNA isolated from cyanoblast-containing tissue, allowed the detection of several haemocyanin polypeptides amongst the products of translation. At least seven polypeptides with molecular weights in the range 68 000-71 000 were identified by an immunological method followed by electrophoretic characterisation on two-dimensional polyacrylamide gels. Comparison of the polypeptide patterns of authentic haemocyanin, reticulocyte lysate translation products and Xenopus oocyte translation products led to the conclusion that the polypeptides are unlikely to undergo significant post-translational modification or to possess cleavable signal sequences. It is proposed that release of haemocyanin into the haemolymph in vivo may involve bursting of the cyanoblasts.

Structural characterization of the proteins encoded by adenovirus early region 2A

Proteins encoded by adenovirus type 2 and type 5 early region 2A isolated from infected HeLa cells were compared to translation products of E2A-specific messenger RNA in a reticulocyte cell-free system and in Xenopus oocytes. The main cell-free translation product is a 72,000 Mr polypeptide which in HeLa cells as well as in Xenopus oocytes is converted into a 75,000 Mr phosphoprotein capable of binding to single-stranded DNA. Some minor proteins are proteolytic cleavage products of the major protein. In the cell-free system, three E2A polypeptides, 32,000, 37,000 and 44,000 Mr, are translated from minor polyadenylated mRNA species that can be separated from the major mRNA. Synthesis of all E2A polypeptides in vitro is inhibited by cap-analogs. The 44,000 Mr protein is also synthesized in Xenopus oocytes. Tryptic peptide maps of [35S]methionine-labeled E2A proteins were constructed using high pressure liquid chromatography and the position of the methionyl residues within each peptide was determined by amino acid sequencing procedures. This information and the DNA sequence of the adenovirus 5 E2A gene published by Kruijer et al. (1981) were used to align the peptides and to construct a map of the E2A proteins. Our data demonstrate that the major 75,000 Mr protein is coded for by a leftward reading frame of 529 amino acid residues located between 62 and 66 map units. The data also map six sites as targets for proteolytic enzymes. The minor E2A translation products have the same carboxy terminus as the major protein. The initiation codons of the 44,000, 37,000 and 32,000 Mr polypeptides probably correspond to amino acids 170, 243 or 244 and 290 of the major protein. Some functional properties of the major E2A protein are shared by the minor proteins and thus could be mapped. Major sites of phosphorylation, the region involved in binding to single-stranded DNA and the antigenic regions recognized by immune sera are located between amino acid residues 50 to 120, 170 to 470 and 170 to 240, respectively.

Primary gene products of bovine beta-crystallin and reassociation behavior of its aggregates

beta-Crystallin from calf lens cortex was fractionated in three different aggregates of increasing size: beta L2- beta L1 and beta H. of which the subunit composition was revealed by 2-dimensional gel electrophoresis. While beta L2 mainly consists of beta Bp (the major polypeptide chain in all three aggregates). beta L1 is characterized by the addition of a neutral and two acidic chains, and beta H contains moreover two basic chains. Translation of calf lens polyribosomes in a reticulocyte cell-free system allowed the identification of six beta-crystallin subunits as primary gene products. The distribution of these newly synthesized polypeptides over the three aggregates was established after gel filtration in the presence of carrier lens proteins. The aggregation behavior of the beta-crystallin chains was studied by dissociation reassociation experiments. The three separate aggregates could be reversibly dissociated. Reassociation of basic, neutral and acidic polypeptides, isolated by ion-exchange chromatography of beta-crystallin, produced a beta H-like aggregate. The neutral and acidic polypeptides reassociated into a beta L1-like aggregate, while the neutral polypeptides gave dimers like beta L2. A beta H-like aggregate could also be obtained by reaggregation of beta L2 with the acidic and basic chains of beta H. On the basis of these results a preliminary model for the formation of beta-crystallin aggregates is discussed.

Lenticular intermediate-sized filaments: biosynthesis and interaction with plasma membrane

Electron microscopical features of the lens fiber plasma membrane-cytoskeleton complex are suggestive of an intimate association between the intermediate-sized filaments (IF) and the lipid bilayer. Biochemical analysis of this complex reveals the occurrence of an appreciable amount of vimentin as a protein subunit of lenticular IF. Additional evidence for association between IF and membranes is provided by the observation that newly synthesized vimentin is associated with plasma membranes added to a reticulocyte lysate programmed with lens polyribosomes. Concomitantly alpha-crystallin polypeptide chains (alpha A2) are also found associated with the plasma membrane together with a hitherto unidentified 47-kilodalton protein. Once associated with the lipid bilayer, the vimentin polypeptide resists urea treatment, suggesting that it has become an integral constituent associated with part of the membrane.

The quaternary structure of bovine alpha-crystallin. Effects of variation in alkaline pH, ionic strength, temperature and calcium ion concentration

The stability of the native quaternary structure of bovine alpha-crystallin was studied, by sedimentation analysis and electron microscopy, as a function of pH (7--11), ionic strength (0.01--0.5), temperature (6--60 degrees C) and calcium ion concentration (0 and 10 mM). Three successive transitions are distinguished at 20 degrees C. Firstly, a slow transconformation step, which is independent of pH, ionic strength or calcium ions. Secondly, an irreversible primary dissociation step, favoured by increasing pH above 8 and/or a lower ionic strength, with formation of 'alkali-modified alpha-crystallin', which is spherically shaped like the native protein but has a smaller average diameter, sedimentation coefficient and molecular weight. Thirdly, with further increase of pH above 9, a rapidly reversible dissociation of alkali-modified alpha-crystallin characterized by a single reaction boundary in sedimentation velocity analysis. In the presence of calcium ions the quaternary structure is stabilized to the extent that no dissociation is observed up to at least pH 10.3. Upon increase of temperature, at pH 7.3, a slow irreversible dissociation and swelling run parallel until a limit is reached around 37 degrees C with formation of 'temperature-modified alpha-crystallin', which is indistinguishable from the native protein by electron microscopy, but has a higher relative viscosity and lower sedimentation coefficient and molecular weight. Calcium ions have little or no effect on this transition. Above 37 degrees C a reversal of this transition or aggregation is indicated. These findings, together with previous structural data on microheterogeneity, reassociation from urea, and aging of alpha-crystallin in vivo, are incorporated into a hypothetical scheme of transitions, based on a three-layer model for the quaternary structure.