The Mr 115 kd fiber cell-specific protein is a component of the lens cytoskeleton

Effects of truncations in the N- and C-terminal domains of filensin on filament formation with phakinin in cell-free conditions and cultured cells

Filensin and phakinin are lens fiber cell-specific proteins that constitute the beaded filaments (BFs) that are critical for maintaining lens transparency. In the Shumiya cataract rat, filensin 94 kDa undergoes N- and C-terminal proteolytic processing to give a transient 50 kDa fragment and a final 38 kDa fragment, just before opacification. To characterize the effects of this processing on filensin function, recombinant proteins representing the two filensin fragments, termed Fil(30-416) and Fil(30-369), respectively, were examined. Fil(30-416) lacks the N-terminal 29 amino acids and the C-terminal 248 amino acids. Fil(30-369) lacks the N-terminal 29 residues and the C-terminal 295 residues. In cell-free assembly characterized by electron microscopy, filensin and Fil(30-416) co-polymerized with phakinin and formed rugged, entangled filaments, whereas Fil(30-369) formed only aggregates. In cultured SW-13 and MCF-7 cells expressing fluorescent fusion proteins, filensin and Fil(30-416) co-polymerized with phakinin and formed cytoplasmic sinuous filaments with different widths, while Fil(30-369) gave aggregates. Therefore, while truncation of the N-terminal 29 amino acids did not affect filament formation, truncation of the C-terminal 295 but not the 248 residues resulted in failure of filament formation. These results indicate that the tail B region (residues 370-416) of rat filensin is essential for filament formation with phakinin. Truncation of the tail B region by proteolytic processing in the cataract rat lens might interfere with BF formation and thereby contribute to opacification.

Insights into the biochemical and biophysical mechanisms mediating the longevity of the transparent optics of the eye lens

In the human eye, a transparent cornea and lens combine to form the "refracton" to focus images on the retina. This requires the refracton to have a high refractive index "n," mediated largely by extracellular collagen fibrils in the corneal stroma and the highly concentrated crystallin proteins in the cytoplasm of the lens fiber cells. Transparency is a result of short-range order in the spatial arrangement of corneal collagen fibrils and lens crystallins, generated in part by post-translational modifications (PTMs). However, while corneal collagen is remodeled continuously and replaced, lens crystallins are very long-lived and are not replaced and so accumulate PTMs over a lifetime. Eventually, a tipping point is reached when protein aggregation results in increased light scatter, inevitably leading to the iconic protein condensation-based disease, age-related cataract (ARC). Cataracts account for 50% of vision impairment worldwide, affecting far more people than other well-known protein aggregation-based diseases. However, because accumulation of crystallin PTMs begins before birth and long before ARC presents, we postulate that the lens protein PTMs contribute to a "cataractogenic load" that not only increases with age but also has protective effects on optical function by stabilizing lens crystallins until a tipping point is reached. In this review, we highlight decades of experimental findings that support the potential for PTMs to be protective during normal development. We hypothesize that ARC is preventable by protecting the biochemical and biophysical properties of lens proteins needed to maintain transparency, refraction, and optical function.

Deletion of beaded filament proteins or the C-terminal end of Aquaporin 0 causes analogous abnormal distortion aberrations in mouse lens

Lens-specific beaded filament (BF) proteins CP49 and filensin interact with the C-terminus of the water channel protein Aquaporin 0 (AQP0). Previously we have reported that a C-terminally end-deleted AQP0-expressing transgenic mouse model AQP0^ΔC/ΔC developed abnormal optical aberrations in the lens. This investigation was undertaken to find out whether the total loss of the BF structural proteins alter the optical properties of the lens and cause optical aberrations similar to those in AQP0^ΔC/ΔC lenses; also, to map the changes in the optical quality as a function of age in the single or double BF protein knockouts as well as to assess whether there is any significant change in the water channel function of AQP0 in these knockouts. A double knockout mouse (2xKO) model for CP49 and filensin was developed by crossing CP49-KO and filensin-KO mice. Wild type, CP49-KO, filensin-KO, and 2xKO lenses at different ages, and AQP0^ΔC/ΔC lenses at postnatal day-17 were imaged through the optical axis and compared for optical quality and focusing property. All three knockout models showed loss of transparency, and development of abnormal optical distortion aberration similar to that in AQP0^ΔC/ΔC. Copper grid focusing by the lenses at 6, 9 and 12 months of age showed an increase in aberrations as age advanced. With progression in age, the grid images produced by the lenses of all KO models showed a transition from a positive barrel distortion aberration to a pincushion distortion aberration with the formation of three distinct aberration zones similar to those produced by AQP0^ΔC/ΔC lenses. Water permeability of fiber cell membrane vesicles prepared from CP49-KO, filensin-KO and 2xKO models, measured using the osmotic shrinking method, remained similar to that of the wild type without any statistically significant alteration (P > 0.05). Western blotting and quantification revealed the expression of comparable quantities of AQP0 in all three BF protein KOs. Our study reveals that loss of single or both beaded filament proteins significantly affect lens refractive index gradient, transparency and focusing ability in an age-dependent manner and the interaction of BF proteins with AQP0 is critical for the proper functioning of the lens. The presence of BF proteins is necessary to prevent abnormal optical aberrations and maintain homeostasis in the aging lens.

Single cell transcriptomics of the developing zebrafish lens and identification of putative controllers of lens development

The vertebrate lens is a valuable model system for investigating the gene expression changes that coordinate tissue differentiation due to its inclusion of two spatially separated cell types, the outer epithelial cells and the deeper denucleated fiber cells that they support. Zebrafish are a useful model system for studying lens development given the organ's rapid development in the first several days of life in an accessible, transparent embryo. While we have strong foundational knowledge of the diverse lens crystallin proteins and the basic gene regulatory networks controlling lens development, no study has detailed gene expression in a vertebrate lens at single cell resolution. Here we report an atlas of lens gene expression in zebrafish embryos and larvae at single cell resolution through five days of development, identifying a number of novel putative regulators of lens development. Our data address open questions about the temperospatial expression of α-crystallins during lens development that will support future studies of their function and provide the first detailed view of β- and γ-crystallin expression in and outside the lens. We describe divergent expression in transcription factor genes that occur as paralog pairs in the zebrafish. Finally, we examine the expression dynamics of cytoskeletal, membrane associated, RNA-binding, and transcription factor genes, identifying a number of novel patterns. Overall these data provide a foundation for identifying and characterizing lens developmental regulatory mechanisms and revealing targets for future functional studies with potential therapeutic impact.

Rhabdomyosarcoma and Wilms tumors contain a subpopulation of noggin producing, myogenic cells immunoreactive for lens beaded filament proteins

Myo/Nog cells are identified by their expression of the skeletal muscle specific transcription factor MyoD and the bone morphogenetic protein inhibitor noggin, and binding of the G8 monoclonal antibody. Their release of noggin is critical for morphogenesis and skeletal myogenesis. In the adult, Myo/Nog cells are present in normal tissues, wounds and skin tumors. Myo/Nog cells in the lens give rise to myofibroblasts that synthesize skeletal muscle proteins. The purpose of this study was to screen human lens tissue, rhabdomyosarcoma cell lines, and tissue sections from rhabdomyosarcoma, Wilms and tumors lacking features of skeletal muscle for co-localization of antibodies to Myo/Nog cell markers and the lens beaded filament proteins filensin and CP49. Immunofluorescence localization experiments revealed that Myo/Nog cells of the lens bind antibodies to beaded filament proteins. Co-localization of antibodies to G8, noggin, filensin and CP49 was observed in most RC13 and a subpopulation of RD human rhabdomyosarcoma cell lines. Western blotting with beaded filament antibodies revealed bands of similar molecular weights in RC13 and murine lens cells. Human alveolar, embryonal, pleomorphic and spindle cell rhabdomyosarcomas and Wilms tumors contained a subpopulation of cells immunoreactive for G8, noggin, MyoD and beaded filaments. G8 was also co-localized with filensin mRNA. Staining for beaded filament proteins was not detected in G8 positive cells in leiomyosarcomas, squamous and basal cell carcinomas, syringocarciomas and malignant melanomas. Lens beaded filament proteins were thought to be present only in the lens. Myo/Nog-like cells immunoreactive for beaded filaments may be diagnostic of tumors related to the skeletal muscle lineage.

Lens Biology and Biochemistry

The primary function of the lens resides in its transparency and ability to focus light on the retina. These require both that the lens cells contain high concentrations of densely packed lens crystallins to maintain a refractive index constant over distances approximating the wavelength of the light to be transmitted, and a specific arrangement of anterior epithelial cells and arcuate fiber cells lacking organelles in the nucleus to avoid blocking transmission of light. Because cells in the lens nucleus have shed their organelles, lens crystallins have to last for the lifetime of the organism, and are specifically adapted to this function. The lens crystallins comprise two major families: the βγ-crystallins are among the most stable proteins known and the α-crystallins, which have a chaperone-like function. Other proteins and metabolic activities of the lens are primarily organized to protect the crystallins from damage over time and to maintain homeostasis of the lens cells. Membrane protein channels maintain osmotic and ionic balance across the lens, while the lens cytoskeleton provides for the specific shape of the lens cells, especially the fiber cells of the nucleus. Perhaps most importantly, a large part of the metabolic activity in the lens is directed toward maintaining a reduced state, which shelters the lens crystallins and other cellular components from damage from UV light and oxidative stress. Finally, the energy requirements of the lens are met largely by glycolysis and the pentose phosphate pathway, perhaps in response to the avascular nature of the lens. Together, all these systems cooperate to maintain lens transparency over time.

Lens intermediate filaments

The ocular lens assembles two separate intermediate filament systems sequentially with differentiation. Canonical 8-11 nm IFs composed of Vimentin are assembled in lens epithelial cells and younger fiber cells, while the fiber cell-specific beaded filaments are switched on as fiber cell elongation initiates. Some of the key features of both filament systems are reviewed.

Periplakin interactions with lens intermediate and beaded filaments

PURPOSE

The lens assembles two systems of intermediated filaments-vimentin intermediate filament (IF) and highly divergent, lens-specific beaded filament (BF)-sequentially as epithelial cells differentiate into fiber cells. The goal of this study was to identify linker proteins that integrate the different lens IF into the biology of the lens fiber cells.

METHODS

Antibodies to periplakin were used in coimmunoprecipitation studies to identify proteins that complex with BF and IF in detergent extracts of mouse lens. GST-periplakin fusion proteins were used to confirm coimmunoprecipitation

RESULTS

Yeast two-hybrid analysis was used to establish direct linkage between periplakin and BF/IF proteins and to narrow down binding domains. Immunocytochemistry was used to establish spatial and temporal coexpression of periplakin and BF/IF. results. Periplakin is found complexed to BF and IF in the lens. The COOH terminus of periplakin was shown to have a strong affinity for the CP49 rod 2 domain but not its head or rod 1 domains. Low-level affinity was seen between the filensin rod domain and periplakin. Periplakin localization in lens overlapped with BF and IF.

CONCLUSIONS

Despite divergence in primary sequence, predicted secondary structure, and filament structure, CP49 has conserved the capacity to bind a common IF linker protein, periplakin, and shares that binding capacity with the other major lens IF protein, vimentin. This suggests that mutations in periplakin have the potential to emulate the cataract seen in lenses with defective BF proteins.

Resisting the effects of aging: a function for the fiber cell beaded filament

PURPOSE

The beaded filament is a cytoskeletal structure that has been found only in the lens fiber cell. It includes phakosin and filensin, two divergent members of the intermediate filament family of proteins that are also unique to the fiber cell. The authors sought to determine what function the beaded filament fulfills in the lens.

METHODS

Light microscopy and electron microscopy were used to characterize structural changes that occurred in previously generated phakosin and filensin knockout mice. Immunocytochemistry and electron microscopy were used to define the distribution of phakosin, filensin, and beaded filaments.

RESULTS

In phakosin and filensin knockout mice, initial lens development and the early phases of fiber cell differentiation proceed in a manner largely indistinguishable from that of wild type. Fiber cells elongate, undergo organelle elimination, and, in the organelle-free zone, develop the unique paddlelike extensions that characterize cells in this region. Subsequent to those stages, however, fiber cells undergo loss of the differentiated fiber cell phenotype and loss of the long-range stacking that characterizes fiber cells and that has been considered essential for clarity.

CONCLUSIONS

The beaded filament is not required for the generation of the differentiated fiber cell phenotype but is required to maintain that differentiated state and the long range order that characterizes the lens at the tissue level.

Identifying the role of specific motifs in the lens fiber cell specific intermediate filament phakosin

PURPOSE

Phakosin and filensin are lens fiber cell-specific intermediate filament (IF) proteins. Unlike every other cytoplasmic IF protein, they assemble into a beaded filament (BF) rather than an IF. Why the lens fiber cell requires two unique IF proteins and why and how they assemble into a structure other than an IF are unknown. In this report we test specific motifs/domains in phakosin to identify changes that that have adapted phakosin to lens-specific structure and function.

METHODS

Phakosin shows the highest level of sequence identity to K18, whose natural assembly partner is K8. We therefore exchanged conserved keratin motifs between phakosin and K18 to determine whether phakosin's divergent motifs could redirect the assembly of chimeric K18 and K8. Modified proteins were bacterially expressed and purified. Assembly competence was assessed by electron microscopy.

RESULTS

Substitution of the phakosin helix initiation motif (HIM) into K18 does not alter assembly with K8, establishing that the radical divergence in phakosin HIM is not by itself the mechanism by which IF assembly is redirected to BF assembly. Unexpectedly, K18 bearing phakosin HIM resulted in normal IF assembly, despite the presence of an otherwise disease-causing R-C substitution, and two helix-disrupting glycines. This disproves the widely held belief that mutation of the R is catastrophic to IF assembly. Additional data are presented that suggest normal IF assembly is dependent on sequence-specific interactions between the IF head domain and the HIM.

CONCLUSIONS

In the lens fiber cell, two members of the IF family have evolved to produce BFs instead of IFs, a change that presumably adapts the IF to a fiber cell-specific function. The authors establish here that the most striking divergence seen in phakosin is not, as hypothesized, the cause of this altered assembly outcome. The authors further establish that the HIM of IFs is far more tolerant of mutations, such as those that cause some corneal dystrophies and Alexander disease, than previously hypothesized and that normal assembly involves sequence-specific interactions between the head domain and the HIM.

Insights into the beaded filament of the eye lens

Filensin (BFSP1) and CP49 (BFSP2) represent two members of the IF protein superfamily that are thus far exclusively expressed in the eye lens. Mutations in both proteins cause lens cataract and careful consideration of the detail of these cataract phenotypes alerts us to several interesting features concerning the function of filensin (BFSP1) and CP49 (BFSP2) in the lens. With the first filensin (BFSP1) mutation now having been reported to cause a recessive cataract phenotype, there is the suggestion that the mutation could predispose heterozygote carriers to the early onset of age-related nuclear cataract. In the case of CP49 (BFSP2), there are now three unrelated families who have been identified with a common E233 Delta mutation. Very interestingly this is linked to myopia in one family. Despite the apparent phenotypic differences of the filensin (BFSP1) and CP49 (BFSP2) mutations, the data are still consistent with the beaded filament proteins being essential for lens function and specifically contributing to the optical properties of the lens. The fact that none of the mutations thus far reported affect either the conserved LNDR or TYRKLLEGE motifs that flank the central rod domain supports the view that this pair of IF proteins have unusual structural features and a distinctive assembly mechanism. The multiple sequence divergences suggest these proteins have been adapted to the specific functional requirements of lens fibre cells, a function that can be traced from squid to man.

Autosomal recessive juvenile onset cataract associated with mutation in BFSP1

A genome wide scan in a consanguineous family of Indian origin with autosomal recessive developmental cataracts was performed by two-point linkage analysis with 382 microsatellite markers. It showed linkage to markers on chromosome 20q, between D20S852 and D20S912, with a maximum lod score of 5.4 obtained with D20S860. This region encompasses the beaded filament structural protein 1 (BFSP1) gene. Direct sequencing revealed a 3343 bp deletion including exon 6 (c.736-1384_c.957-66 del) predicted to result in a shift of the open reading frame. This mutation was absent in 50 control individuals from south India. This is the first report of a mutation in the BFSP1 gene associated with human inherited cataracts. This further increases the genetic heterogeneity of inherited cataracts and provides clues as to the importance of BFSP1 in the cell biology of intermediate filaments and their role in the eye lens.

Characterization of Covalent Multimers of Crystallins in Aging Human Lenses

The purpose of this study was to characterize covalent multimers with molecular mass of >90 kDa in the water-insoluble (WI) proteins of aging human lenses. The experimental approach was to first separate the multimers (molecular mass >90 kDa) as individual spots by two-dimensional gel electrophoresis and next analyze compositions of each multimers by matrix-assisted laser desorption ionization-time of flight and electrospray ionization-tandem mass spectrometric (ES-MS/MS) methods. The WI proteins from lenses of 25- and 41-year-old subjects showed distinct 5- and 16-multimer spots on two-dimensional gels, respectively, but the spots from 52- and 72-year-old lenses were non-descript and diffused. ES-MS/MS analyses showed two types of covalent multimers in 25- and 41-year-old lenses, i.e. the first type composed of fragments of eight different crystallins (i.e. alphaA, alphaB, betaA3, betaA4, betaB1, betaB2, gammaS, and gammaD), and the second type of alpha-, beta-, and gamma-crystallins (possibly fragments) and two beaded filament proteins (phakinin and filensin). The most commonly identified species in the complexes of 41-year-old lenses were: alphaA-fragment (C-terminally truncated, residues 1-157), alphaB-fragment (residues 83-90), betaB1-crystallin (residues 60-71), betaA3 (residues 33-44), betaA4 (residues 106-117), filensin (residues 78-90), and phakinin (residues 77-89). Three post-translational modifications (i.e. oxidation of Met and Trp, conversion of Ser to dehydroalanine, and formylation of His) were observed in alphaA-crystallin fragment, and the first two modifications could cross-link proteins. Together, the results suggested that covalent multimers appeared early in life (i.e. 25 years of age) and increased in number with aging, and the two beaded filament proteins form covalent complexes with crystallin fragments in vivo.

Knockout of the intermediate filament protein CP49 destabilises the lens fibre cell cytoskeleton and decreases lens optical quality, but does not induce cataract

In this report, the phenotype associated with the first targeted knockout of the lens specific intermediate filament gene CP49 is described. Several surprising observations have been made. The first was that no cataract was observed despite the fact that the beaded filaments of the lens fibre cells had been disrupted. Light scatter and the lens optical properties had, however, deteriorated in the CP49 knockout lenses compared to litter mate controls. These changes were accompanied by dramatic changes in plasma membrane organisation of the fibre cells as revealed by detailed morphological examinations and providing the second surprising result. The CP49 knockout mouse is therefore an important model to study the functional link between lens transparency, the cytoskeleton and plasma membrane organisation.

Lens membrane fraction associated intermediate filaments of different aged rats

PURPOSE

To describe the intermediate filament proteins vimentin, filensin and phakinin associated with different fractions isolated from neonatal, 10 day old and 20 day old rat lenses.

METHODS

Fractions were isolated by differential and density gradient centrifugation of lens homogenates from neonatal, 10 day old and 20 day old rats. Aliquots of the 8 M urea soluble proteins of each fraction were separated by SDS PAGE, transferred to PVDF membranes, the membranes were probed with antibodies to vimentin, filensin or phakinin, and analyzed by computer.

RESULTS

Over the 20 day growth period, the water soluble fraction increased and the most abundant membrane fraction was characterized by a significant increase in its urea insoluble protein and a significant decrease in its urea soluble protein. There were no significant quantitative changes in any of the other fractions. The concentration of each intermediate filament protein was greatest in the cytoskeletal fraction and over the 20 day period, the amount of vimentin associated with this fraction dramatically decreased, and the amounts of filensin and phakinin dramatically increased. Among the membrane fractions, the greatest concentration of each intermediate filament protein was found in the non sedimenting membrane fraction (NSMF) which was the least abundant fraction recovered. Filensin and phakinin associated with the other three major membrane fractions increased over the 20 day growth period, but the level of vimentin did not significantly change.

CONCLUSIONS

The NSMF may represent a domain of the lens plasma membrane particularly important in interaction between plasma membrane and cytoskeleton and as the membrane-cytoskeleton protein architecture of rat lens changes over the first 20 days of life, the changes are readily detected in the different membrane fractions.

Autosomal-dominant congenital cataract associated with a deletion mutation in the human beaded filament protein gene BFSP2

Congenital cataracts are a common major abnormality of the eye that frequently cause blindness in infants. At least one-third of all cases are familial; autosomal-dominant congenital cataract appears to be the most-common familial form in the Western world. Elsewhere, in family ADCC-3, we mapped an autosomal-dominant cataract gene to chromosome 3q21-q22, near the gene that encodes a lens-specific beaded filament protein gene, BFSP2. By sequencing the coding regions of BFSP2, we found that a deletion mutation, DeltaE233, is associated with cataracts in this family. This is the first report of an inherited cataract that is caused by a mutation in a cytoskeletal protein.

The eye lens cytoskeleton

During lens cell differentiation there are a number of very characteristic morphological changes that occur. These include a 50- to 100-fold increase in cell length as the equatorial lens epithelial cells differentiate into fibre cells and the loss of the cellular organelles such as mitochondria, nuclei, Golgi apparatus and endoplasmic reticulum. Coincident with these changes are dramatic alterations in the organisation of the lens fibre cell cytoskeleton and in particular the lens-specific intermediate filament network comprising CP49 and filensin. Cell shape and cell polarisation as well as tissue integrity are all processes that depend upon the cytoskeleton and are therefore important to the lens. The unique aspects of the lenticular cytoskeleton are the subject of this review.

Primary sequence, secondary structure, gene structure, and assembly properties suggests that the lens-specific cytoskeletal protein filensin represents a novel class of intermediate filament protein

The ocular lens fiber cell assembles a novel cytoskeletal element, the Beaded Filament, from CP49 and filensin, two proteins expressed only in the differentiated lens fiber cell. We report the primary sequence, secondary structural analysis, gene structure and Yeast Two Hybrid interaction data for human filensin, and develop a consensus model of filensin from the human and previously reported bovine and chicken filensin sequences. This consensus model, combined with gene structure and Yeast Two Hybrid studies establish that filensin is a member of the Intermediate Filament family of proteins. Specifically, filensin exhibits (1) divergence at amino acid sequence motifs otherwise highly conserved among intermediate filament proteins, (2) a loss of 29 amino acids from the central rod domain which is unique among cytoplasmic intermediate filament proteins, (3) an absence of sequence identity with any existing class of intermediate filament protein, (4) a gene structure unique among intermediate filament family, (5) an inability to dimerize with representatives of Type I, II, and III intermediate filament proteins. Thus, at each level of analysis, we find that filensin is similar to the consensus model of intermediate filament proteins, supporting our conclusion that filensin's relatedness to the IF family is not the consequence of convergent evolution. However, filensin also shows unique or extreme distinctions from the consensus intermediate filament protein at each level of analysis, indicating that filensin constitutes a novel class of IF protein. Some of filensin's unique features are incompatible with current models of IF assembly. Analysis of filensin gene structure suggests that the 29 amino acid reduction in the central rod domain was not the result of a single splice site mutation, the mechanism suggested for the transition between nuclear lamins and cytoplasmic intermediate filament proteins.

Intermediate filament cytoskeletal proteins associated with bovine lens native membrane fractions

PURPOSE

To examine the intermediate filament cytoskeletal proteins associated with native membrane fractions isolated from bovine lenses.

METHODS

Decapsulated bovine lenses were divided into cortex and nucleus. The lens regions were homogenized and separated into water-soluble and water-insoluble fractions by centrifugation. Sedimenting membrane fractions were isolated from the water-insoluble fraction by discontinuous sucrose-density-gradient centrifugation and the non-sedimenting membrane fractions were isolated from the Kbr high-density water-soluble fractions by flotation, during overnight centrifugation. The intermediate filament peptides of the membrane fractions were examined by Western blot analysis, using monoclonal antibodies to filensin, cytoskeletal protein 49 (CP49) and vimentin.

RESULTS

Filensin immunoreactive peptides were found in all membrane fractions of both cortex and nucleus. The parent 115 kDa filensin was found almost exclusively in the urea-soluble protein of cortical membrane fractions, and was the predominant filensin immunoreactive peptide only in the urea-soluble protein of the cortical sedimenting membrane fraction isolated from the 25%/45% sucrose density interface. The predominant filensin immunoreactive peptide of all other samples migrated with a M(r) of 53 kDa. CP49 immunoreactive peptides were found almost exclusively in the urea-soluble protein of all membrane fractions from both the cortex and nucleus. The cortical non-sedimenting membrane fraction and the nuclear membrane fraction of the 25%/45% sucrose density interface were notably deficient in CP49. Vimentin immunoreactive peptides were found in both urea-soluble and urea-insoluble proteins of membrane fractions from the cortex only. Vimentin was particularly enriched in the cortical non-sedimenting membrane fraction. The urea-insoluble filensin immunoreactive peptides were only partially removed by alkali extraction, indicating a very avid association with the membrane. Two dimensional electrophoresis revealed that the urea-soluble protein of the major cortical membrane fraction contained two different filensin-derived 53 kDa fragments.

CONCLUSIONS

The non-sedimenting membrane fraction, which may reflect a distinct domain of the lens plasma membrane, possesses a membrane-associated cytoskeletal composition different from that of the major sedimenting membrane fractions.

Actin filament bundles in cortical fiber cells of the rat lens

The distribution and organization of actin filament bundles were studied in cortical fiber cells of rat lenses at various ages (3 days to 2.5 months old), using thin-section electron microscopy, immunocytochemistry and immunoblotting. Electron microscopy showed that actin bundles were regularly found along cortical fiber cell membranes of the lens at all ages studied. The actin bundles were commonly arranged in three distinct units, one bundle in each fiber cell, located at the intersections where three hexagonal fiber cells meet as seen in cross sections. These actin bundles were approximately 150 nm in diameter and were composed of 7-nm small filaments. They were aligned parallel to the long axis of fiber cells as judged by both cross and longitudinal sections. The outside border of each bundle was always surrounded by a zone of 10-nm intermediate filaments which have the same orientation as that of the actin bundles. In longitudinal sections, elongated actin bundles were always parallel to the cell membranes. A number of individual actin bundles sometimes were found to form a chain with periodic short intervals. In addition, actin bundles were frequently associated with adherens junctions near the intersections and other regions of fiber cell membranes. By immunoelectron microscopy, we demonstrated that these filament bundles indeed contained actins. By rhodamine-phalloidin labelling, we found that labeled actin bundles appeared as large, distinct dots at the corners of hexagonal fiber cells in all ages studied. In addition, non-bundle F-actins were labeled preferentially along the cell membranes of the short sides of hexagonal fiber cells. This resulted in a unique zigzag pattern of actin labeling commonly seen in the cortical fiber cells of a mature rat lens. Finally, we showed that alpha-actinin was associated with the actin bundles in the fiber cells by immunofluorescent double labeling and immunoblotting. It is suggested that this unique arrangement of actin bundles in fiber cells may provide a stabilizing structure for forming a sharp angle at each corner of fiber cells, thereby the hexagonal shape of the cells can be maintained.

The mouse filensin gene: structure and evolutionary relation to other intermediate filament genes

Filensin and phakinin are two lens-specific members of the intermediate filament (IF) superfamily of proteins. They coassemble to form a beaded submembraneous filamentous network, the beaded filaments (BFs). The low sequence homology and differences in assembly compared to other IF proteins do not allow their classification in any of the five IF subgroups. The organization of the phakinin gene exon/intron boundaries provides evidence that this partner may be sharing a common origin with type I cytokeratin genes. Here we report the molecular cloning, sequence and characterization of the mouse filensin gene. The filensin gene consists of 8 exons and 7 introns, with 6 introns interrupting its rod domain in a highly conserved manner characteristic of type III IF genes, like vimentin, desmin, or peripherin. Of the two tail domain exons the one adjacent to the rod domain, compares to exon 7 of the non-neuronal cytoplasmic IF gene of helix aspersa and to the lamin region bridging the end of the rod domain to the nuclear localization signal. Altogether, these observations indicate that the lens beaded filaments form an independent class of IF.

The beaded filament of the eye lens: an unexpected key to intermediate filament structure and function

In 1959, an unusual filamentous polymer, now called the beaded filament, was described in the lens of the eye. The constituent proteins, assembly properties and functions of the beaded filament have been elusive. The recent publication of the sequences for two major lens filament proteins (CP49 and filensin) and the reconstitution in vitro of structures closely resembling beaded filaments, suggests that the beaded filament is related structurally to intermediate filaments (IFs). The association of the lenticular chaperones, the alpha-crystallins, with the filament contributes to the characteristic beaded morphology, as well as giving important clues to the function of this unusual filament in the lens. These recent results have several implications for IF function and assembly.

Gene structure and cDNA sequence identify the beaded filament protein CP49 as a highly divergent type I intermediate filament protein

The fiber cell of the vertebrate ocular lens assembles a cytoskeletal structure, the beaded filament, which contains two proteins unique to the fiber cell: CP49 (phakinin) and CP115/CP95 (filensin). We report here the complete primary sequence and gene structure for human CP49. These data show that CP49 is a member of the intermediate filament family, but highly unusual in several regards. 1) CP49 primary sequence does not permit unambiguous assignment to any existing class of intermediate filament protein, but exhibits a gene structure that is identical to the Type I cytokeratins. 2) CP49 essentially lacks one of the three major domains that characterize all intermediate filament proteins, the carboxyl-terminal tail domain. 3) CP49 shows substitutions at 3 of 4 residues in the otherwise highly conserved intermediate filament protein motif LNDR. Notably, this divergence includes an Arg to Cys substitution that has only been observed in the mutant human cytokeratin K14, a mutation shown to cause the skin blistering seen in the genetic disorder Dowling-Meara epidermolysis bullosa simplex.

The predicted structure of chick lens CP49 and a variant thereof, CP49ins, the first vertebrate cytoplasmic intermediate filament protein with a lamin-like insertion in helix 1B

The full length cDNA sequence for the lens-specific intermediate filament protein, CP49, from chicken is presented. The sequence contains features typical of the other intermediate filament proteins, including two major alpha-helical regions, helix I and II and appropriate linker regions. CP49 lacks a C-terminal non-alpha-helical domain and is only the second intermediate filament protein to be described missing this feature. Comparison to the bovine CP49 shows significant homology in all domains except the N-terminal non-alpha-helical domain. Besides bovine CP49, the other protein most homologous to chicken CP49 in the database was keratin 18, a type I keratin. A variant of CP49 is also described, called CP49ins. Of the 61 positive clones identified in the library, two encoded CP49ins, one of these being a full-length clone. The sequence differed to CP49 by the insertion of 49 amino acids in helix IB. This is the first chordate cytoplasmic intermediate filament protein sequence to be identified with an archetypal lamin-like insertion in this helical subdomain and represents a key discovery in tracing the evolutionary pathway of intermediate filament protein family.

Vimentin and CP49/filensin form distinct networks in the lens which are independently modulated during lens fibre cell differentiation

The cells of the eye lens contain the type III intermediate filament protein vimentin, as well as two other intermediate filament proteins, CP49 and filensin. These two proteins appear to be unique to the differentiated lens fibre cell. Immunoblotting and confocal microscopy were used to describe changes which occur in these three intermediate filament proteins and the networks they form during fibre cell differentiation and maturation. The vimentin network was present in both epithelial cells and some fibre cells. Fibre cells were vimentin positive up to a specific point 2–3 mm in from the lens capsule where the vimentin signal was drastically reduced. The CP49/filensin network was not present in the undifferentiated epithelial cells but emerged in the differentiating fibre cells. This latter network exhibited a principally plasma membrane localization in younger fibre cells but became more cytoplasmic in older fibre cells. This change also occurred at a distinct point in fibre cell differentiation, much earlier than the observed loss of the vimentin network. The subcellular changes in the distributions of these cytoskeletal networks were correlated to the loss of the fibre cell nucleus, another feature of fibre cell differentiation. No correlation was found to changes in the vimentin network but nuclear loss did coincide with changes in the CP49/filensin network. Concomitant with nuclear pyknosis, there were also changes in the nuclear lamina as well as infringement of the nuclear compartment by CP49, as shown by confocal microscopy. This study demonstrates vimentin and the CP49/filensin network to be independent in the lens but both networks undergo dramatic changes in subcellular distribution during the differentiation/maturation of the fibre cell. Only changes in the CP49/filensin network can be correlated to nuclear loss. Thus in the lens, unlike mammalian erythropoiesis which is also characterized by nuclear loss, the vimentin network does not appear linked to nuclear retention.

In vitro studies on the assembly properties of the lens proteins CP49, CP115: coassembly with alpha-crystallin but not with vimentin

A rapid one-step purification procedure for CP49, an intermediate filament protein found in the lens, is described using reverse-phase HPLC. This protein is one of the major intermediate filament proteins of the lens fibre cells and is found in both the water insoluble fraction (WIF) and the water soluble fraction (WSF) of the lens. In order to better understand the physiological role of CP49 in lens transparency we have purified CP49 from both compartments and compared the in vitro assembly characteristics of both by electron microscopy and sedimentation assays. Our studies showed that CP49, when mixed with another lens intermediate filament protein, CP115, forms 10 nm intermediate filaments. Vimentin, another intermediate filament protein found in the lens, was unable to coassemble with CP115, thus demonstrating the specificity of the interaction of CP49 with CP115. CP49 isolated from either the WIF or the WSF formed 10-nm filaments with CP115 and indicated that CP49 from both these lens cell compartments had similar in vitro assembly characteristics. This also suggested that the post-translational modifications observed for CP49 from the different compartments was of little apparent consequence to filament formation. The inability to reconstitute beaded filaments from CP49 and CP115 suggested that other lens proteins may be needed in the reconstitution assay before these lens specific cytoskeletal elements could be repolymerised from their purified protein components. CP49 and CP115 were therefore assembled in the presence of alpha-crystallins and a beaded filament structure was observed as has been seen with type III intermediate filament proteins assembled with alpha-crystallins.

Chromosomal locations of the genes for the beaded filament proteins CP 115 and CP 47

We have used the polymerase chain reaction (PCR) to amplify CP 115 and CP 47 encoding sequences from human lens cDNA samples. DNA sequence and northern blot analysis were used to confirm human origin. From the determined cDNA sequences, human-specific oligonucleotides were synthesized and assessed for the ability to amplify human genomic DNA. After empirically selecting a primer pair for each gene able to amplify human genomic DNA, and optimizing PCR conditions for human specificity, we used the PCR to screen a panel of mouse/human somatic cell hybrid DNA samples. Amplification of CP 115 or CP 47 sequences in each of the somatic cell hybrid samples was correlated with the presence/absence of human genomic DNA sequences encoding the respective gene sequences. From our results, we conclude that the gene for human CP 115 resides on chromosome 20 and the gene for human CP 47 on chromosome 3. Further mapping using somatic cell lines carrying derivatives of human chromosome 3 localize the gene for CP 47 to 3q21-25. We propose LIFL-H (Lens Intermediate Filament Like-Heavy) for CP 115 and LIFL-L (Lens Intermediate Filament Like-Light) for CP 47 as the gene symbols for these loci.

The beaded intermediate filaments and their potential functions in eye lens

The elongated fiber cells of the eye lens contain a unique cytoskeletal system, the beaded chain filaments (BFs). The BFs had been morphologically identified more than two decades ago, but the precise identity of their subunit molecules remained unknown. Recently, use of recombinant DNA approaches, refined morphological and immunochemical studies and experiments with mutant mice have allowed the molecular dissection of these structures and provided clues about their potential functions. The BFs represent a highly specialized network of intermediate filaments (IFs) juxtaposed to the plasma membrane. They are obligate heteropolymers composed of two lens-specific polypeptides, filensin and phakinin. In this review we discuss the properties, molecular interactions and in situ arrangement of these two proteins, and comment on their potential roles during lens development.

The 47-kD lens-specific protein phakinin is a tailless intermediate filament protein and an assembly partner of filensin

In previous studies we have characterized a lens-specific intermediate filament (IF) protein, termed filensin. Filensin does not self-assemble into regular IFs but is known to associate with another 47-kD lens-specific protein which has been suggested to represent its assembly partner. To address this possibility, we cloned and sequenced the cDNA coding for the bovine 47-kD protein which we have termed phakinin (from the greek phi alpha kappa omicron sigma = phakos = lens). The predicted sequence comprises 406 amino acids and shows significant similarity (31.3% identity over 358 residues) to type I cytokeratins. Phakinin possesses a 95-residue, non-helical domain (head) and a 311 amino acid long alpha-helical domain punctuated with heptad repeats (rod). Similar to cytokeratin 19, phakinin lacks a COOH-terminal tail domain and it therefore represents the second known example of a naturally tailless IF protein. Confocal microscopy on frozen lens sections reveals that phakinin colocalizes with filensin and is distributed along the periphery of the lens fiber cells. Quantitative immunoblotting with whole lens fiber cell preparations and fractions of washed lens membranes suggest that the natural stoichiometry of phakinin to filensin is approximately 3:1. Under in vitro conditions, phakinin self-assembles into metastable filamentous structures which tend to aggregate into thick bundles. However, mixing of phakinin and filensin at an optimal ratio of 3:1 yields stable 10-nm filaments which have a smooth surface and are ultrastructurally indistinguishable from "mainstream" IFs. Immunolabeling with specific antibodies shows that these filaments represent phakinin/filensin heteropolymers. Despite its homology to the cytokeratins, phakinin does not coassemble with acidic (type I), or basic (type II) cytokeratins. From these data we conclude that filensin and phakinin are obligate heteropolymers which constitute a new membrane-associated, lens-specific filament system related to, but distinct from the known classes of IFs.

Primary structure and lens-specific expression of genes for an intermediate filament protein and a beta-tubulin in cephalopods

Intermediate filament (IF) protein and tubulin cDNAs of cephalopod eye lenses were cloned and sequenced. The rod regions of the deduced IF proteins of the squid and octopus were more similar (68% identical) than were head (33% identical) and tail (40% identical) regions. The rod sequences were closer to squid neuronal IF protein (39% identical) than to any other known IF protein. There was only 31% identity between the rod regions, 21-30% identity between the head regions and 23-32% identity between the tail regions of the present IF proteins of cephalopods and other invertebrates. The rod regions of the cephalopod IF proteins contained the 6 heptads characteristic of nuclear lamins, consistent with an evolutionary relationship between IF proteins and lamins. The present octopus alpha-tubulin was 93% and beta-tubulin was 87% identical to the corresponding tubulins of insects and vertebrates. SDS-PAGE and peptide sequencing indicated that the order of abundance of the cephalopod lens cytoskeletal proteins was IF proteins, actin and tubulins. Northern blot hybridization revealed a 4 kb mRNA for the octopus IF protein and 2.9 and 7.3 kb mRNAs for the squid IF protein; the alpha-tubulin mRNA was about 1.8 kb in the octopus and squid, and the beta-tubulin mRNA was about 2.8 kb in the octopus. The alpha-tubulin mRNA was present in all tissues examined; by contrast, the present beta-tubulin and IF protein mRNAs appeared specialized for lens expression.

Evidence that the chick lens cytoskeletal protein CP 49 belongs to the family of intermediate filament proteins

A partial cDNA sequence for chick lens beaded-filament protein CP 49 showed the greatest similarity to the sequence of acidic cytokeratins, especially human cytokeratin 18. The predicted amino acid sequence of chick CP 49 corresponded to the entire coil 1a region of the rod domain of human cytokeratin 18, spacer 1, coil 1b, spacer 2 and about half of coil 2. For this sequence of 242 amino acids, there was an overall 38% identity and 76.8% similarity between the chick CP 49 and human cytokeratin 18. This is further evidence that CP 49 belongs to the family of intermediate filament proteins.

Bovine filensin possesses primary and secondary structure similarity to intermediate filament proteins

The cDNA coding for calf filensin, a membrane-associated protein of the lens fiber cells, has been cloned and sequenced. The predicted 755-amino acid-long open reading frame shows primary and secondary structure similarity to intermediate filament (IF) proteins. Filensin can be divided into an NH2-terminal domain (head) of 38 amino acids, a middle domain (rod) of 279 amino acids, and a COOH-terminal domain (tail) of 438 amino acids. The head domain contains a di-arginine/aromatic amino acid motif which is also found in the head domains of various intermediate filament proteins and includes a potential protein kinase A phosphorylation site. By multiple alignment to all known IF protein sequences, the filensin rod, which is the shortest among IF proteins, can be subdivided into three subdomains (coils 1a, 1b, and 2). A 29 amino acid truncation in the coil 2 region accounts for the smaller size of this domain. The filensin tail contains 6 1/2 tandem repeats which match analogous motifs of mammalian neurofilament M and H proteins. We suggest that filensin is a novel IF protein which does not conform to any of the previously described classes. Purified filensin fails to form regular filaments in vitro (Merdes, A., M. Brunkener, H. Horstmann, and S. D. Georgatos. 1991. J. Cell Biol. 115:397-410), probably due to the missing segment in the coil 2 region. Participation of filensin in a filamentous network in vivo may be facilitated by an assembly partner.

cDNA analysis of the 49 kDa lens fiber cell cytoskeletal protein: a new, lens-specific member of the intermediate filament family?

Two proteins, with molecular weights of 49 (CP49) and 115 kDa (CP115) as judged by SDS PAGE, have been shown by immunocytochemistry to be components of the beaded filament, a cytoskeletal structure thus far demonstrated only in the lens fiber cell. We have used antibodies reactive with CP49 to screen a mouse lens cDNA expression library. An immunoreactive clone with an approximately 1.0 kb insert was identified and purified. DNA sequence analysis shows the presence of an open reading frame that extends from the upstream cloning site for 660 bases. Contained within this reading frame are 2 peptide sequences nearly identical to 2 peptide sequences obtained from purified bovine CP49. Northern analysis revealed that the mRNA for the CP49 is not detectable in mouse brain, muscle, lung, liver, or heart. A search of the Genbank database showed that the partial cDNA sequence for the murine CP49 is unique, but that this partial sequence shows a strong similarity to multiple members of the intermediate filament family, with greatest similarity to type I acidic cytokeratins. The data presented here suggests that the CP49 is related to, and possibly represents a new member of the intermediate filament family. These data, in concert with previously published work, suggests 1) the CP49 and CP115, which have been localized to the beaded filament, are related to the family of IF proteins, and 2) these two proteins comprise a cytoskeletal structure which is structurally distinct from classical 8-11 nm intermediate filaments, thus possibly comprising a structurally novel form of intermediate filament.

Membrane-binding properties of filensin, a cytoskeletal protein of the lens fiber cells

Filensin is a 100/110 kDa membrane-associated protein found in lens fiber cells. Previous studies have shown that this protein polymerizes in vitro and binds strongly to vimentin and to another 47 kDa lens membrane protein. Using cosedimentation assays, flotation assays and immunoelectron microscopy, we have examined the properties of purified filensin and measured its binding to lens membranes. Filensin behaves as a ureaextractable, hydrophilic protein which does not partition with Triton X-114 and is not affected by 1 M hydroxylamine at alkaline pH, an agent known to release fatty-acylated proteins from the membrane. Immunoblotting of urea-extracted lens membranes with two different affinity-purified antibodies reveals that, unlike intact filensin, a COOH-terminal filensin degradation product (51 kDa) remains tightly associated with the membranes. Purified filensin binds directly to urea-stripped lens membranes, but not to protein-free vesicles reconstituted from total lens lipids. The binding of filensin is not significantly influenced by the purified 47 kDa protein. Interestingly, the filensin-binding capacity of urea-extracted membranes is increased at least two-fold after trypsin treatment, which removes entirely the 51 kDa peptide from the membranes and presumably unmasks additional filensin-acceptor sites. Consistent with this, filensin binds to trypsinized and non-trypsinized membranes with similar affinities (2 × 10(−7) and 4 × 10(−7) M, respectively). Treatment of the membranes with thrombin, which also eliminates the 51 kDa peptide, does not increase their binding capacity, apparently because filensin-acceptor sites are also destroyed during proteolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

The 53kDa polypeptide component of the bovine fibre cell cytoskeleton is derived from the 115kDa beaded filament protein: evidence for a fibre cell specific intermediate filament protein

The 115kDa protein found enriched in the PMCC (plasma membrane-cytoskeleton complex) fraction of the cortex in bovine lens fibre cells is proteolytically processed to a stable 53kDa product. The 115 kDa protein and the 53kDa polypeptide have been purified by a combination of ion exchange and hydroxyapatite chromatography. Tryptic peptide mapping using reverse phase HPLC and subsequent peptide sequencing confirmed that the 53kDa polypeptide is derived from the 115kDa protein. The 53kDa fragment is also a component of the PMCC as well as being a major component of the urea soluble fraction of lens plasma membranes which have been extracted with buffers containing 1M KC1. The 53kDa polypeptide has escaped identification as a breakdown product of the 115kDa protein because it is not recognised by a commonly used monoclonal antibody, R2D2, specific for the bovine 115kDa protein. This result suggests that proteolysis is important in determining the function(s) of the 115kDa protein, and that part of this function is satisfied by the 53kDa protein core. Both the purified 115kDa protein and the 53kDa polypeptide were unable to form either beaded or intermediate filaments on their own but they were able to form short 10nm rods indicative of an intermediate stage in intermediate filament assembly. Comparison ot the assembly properties of the 53 and 115kDa proteins indicate that there are sequences in the 115kDa protein which inhibit in vitro assembly. This is similar to the situation with neurofilament proteins. We suggest that the 115kDa protein is a lens-specific intermediate filament protein.

Susceptibility of the bovine lens 115kDa beaded filament protein to degradation by calcium and calpain

The 115kDa cytoskeletal beaded filament protein of bovine lens fibres is degraded during opacification induced by increased internal calcium. The monoclonal antibody R2D2 to this protein has been used in whole lenses and native homogenates to follow the process of degradation and the production of break-down products. In the opaque outer cortex of whole bovine lenses with an internal Ca2+ of 2.0mM, both the 115kDa parent protein and the main degradation product (57kDa) were reduced in amount by almost 60%. No additional products were detected by the antibody. When native homogenates were incubated overnight with 10mM Ca2+ the protein could no longer be detected in SDS gels, but faintly reactive bands were detected by the antibody. Since these changes were dependent on the presence of increased calcium they were compared with changes induced by incubating freshly isolated cytoskeletal proteins with Ca2+ and the Ca(2+)-activated protease, calpain. The 115kDa protein was shown to be susceptible to degradation by calpain, with the formation of a number of breakdown products. These results indicate that degradation of the beaded filament protein can be brought about by the activation of calpain. Since the enzyme is present in lens cortex it is likely to have a role in the protein degradation observed during Ca(2+)-induced opacification, and may also be involved in the changes occurring as the lens fibres mature.

Reconstitution of the filamentous backbone of lens beaded-chain filaments from a purified 49kD polypeptide

The beaded-chain filaments unique to the fiber cells of the crystalline lens are composed of a linear array of spheroidal particles which appear to be connected by a filamentous backbone. In order to determine the existence of the putative backbone and to characterize its constituents, one of the major proteins associated with beaded-chains in the chicken lens was investigated. 49kD was isolated in an enriched fraction derived from the 8M urea extract of the lens cell water-insoluble residue. The polypeptide (which exists in several charge isoforms, the major at pI 5.2) was purified sequentially by gel filtration on Sephacryl S-200, hydrophobic interaction chromatography on phenyl-Sepharose, and anionic exchange chromatography on Mono Q, all under denaturing conditions. Immunoblot analyses established that 49kD was immunologically distinct from vimentin, actin, and tubulin/MAPs (representing the three classes of cytoplasmic filaments), as well as from the crystallins. Amino acid analyses demonstrated compositional differences for 49kD compared with lens actin and vimentin, and one- and two-dimensional peptide mapping of 49kD and vimentin revealed no homology. Electron microscopy demonstrated that short, contorted filaments were produced upon removal of purified 49kD from urea to low-salt buffers. In the presence of physiological salt concentrations 49kD assembled into extensive 4-6nm diameter, straight filaments similar to the backbone seen in native beaded-chain filaments, but morphologically distinct from the other cytoplasmic filament classes.

Filensin: a new vimentin-binding, polymerization-competent, and membrane-associated protein of the lens fiber cell

We have studied the molecular properties of a 100-kD protein, termed filensin, which we have isolated from porcine lens membranes. Filensin represents a membrane-associated element, resistant to salt and nonionic detergent treatment, and extractable only by alkali or high concentrations of urea. By indirect immunofluorescence and immunoelectron microscopy, this protein can be localized at the periphery of the lens fiber cells. Immunochemical analysis suggests that filensin originates from a larger 110-kD component which is abundantly expressed in lens but not in other tissues. Purified filensin polymerizes in a salt-dependent fashion and forms irregular fibrils (integral of 10 nm in diameter) when reconstituted into buffers of physiological ionic strength and neutral pH. Radiolabeled filensin binds specifically to lens vimentin under isotonic conditions, as demonstrated by affinity chromatography and ligand-blotting assays. By the latter approach, filensin also reacts with a 47-kD peripheral membrane protein of the lens cells. Purified filensin binds to PI, a synthetic peptide modelled after a segment of the COOH-terminal domain of peripherin (a type III intermediate filament protein highly homologous to vimentin), but not to various other peptides including the NH2-terminal headpiece of vimentin and derivatives of its middle (rod) domain. The filensin-PI binding is inhibited by purified lamin B, which is known to interact in vitro with PI (Djabali, K., M.-M. Portier, F. Gros, G. Blobel, and S. D. Georgatos. 1991. Cell. 64:109-121). Finally, limited proteolysis indicates that the filensin-vimentin interaction involves a 30-kD segment of the filensin molecule. Based on these observations, we postulate that the lens fiber cells express a polymerization-competent protein which is tightly associated with the plasma membrane and has the potential to serve as an anchorage site for vimentin intermediate filaments.

Defect of a fiber cell-specific 94-kDa protein in the lens of inherited microphthalmic mutant mouse Elo

Deficiency in a 94,000-dalton protein in the non-crystallin fraction from the Elo mouse lens was shown. To perform further investigations, we raised an antibody against the 94,000-dalton protein isolated from normal mouse lens. Western blot analysis with the antibody indicated that the protein was only present in the lens and not in the brain, lung, heart, liver, and kidney. In the lens, it was unique to the cortex and nucleus fractions, not being present in the epithelial cells. Furthermore, it was observed in the water-soluble fraction as well as in the urea-soluble fraction. The antibody weakly but clearly reacted with the chick CP97 lens peptide, a fiber cell-specific protein, and anti-CP97 antibody also reacted with the 94,000-dalton protein. From these results, we concluded that the protein corresponds to CP97 cytoskeletal protein in the mouse lens. The protein was deficient in the lenses from Elo mice, but microphthalmic lenses from CTA mice contained a normal level.

Immunologic conservation of the fiber cell beaded filament

Lenses were obtained from the eyes of four different classes of Chordates, including Mammalia (rat, mouse, cow, human), Aves (chicken), Amphibia (tiger salamander), and Osteichthyes (steelhead), as well as from one Mollusca (squid). Buffer soluble, urea soluble and urea insoluble fractions were prepared from each, and probed by western blot analysis for the presence of the lens fiber cell 115 and 49 kD beaded filament proteins. Application of both polyclonal and monoclonal antibodies revealed that an immunologic homologue to the bovine fiber cell 115 kD protein is present in all examples of Chordates tested, and that this homologue possessed properties very similar to those of its bovine counterpart. Both monoclonal and polyclonal antibodies revealed an immunologically cross-reactive homologue in squid as well, but suggested that the squid protein had a native molecular weight of closer to 70-80 kD. A monoclonal antibody to the bovine 49 kD beaded filament protein was successful at identifying an immunologic homologue to this protein in mouse, chicken, and tiger salamander. Ultrastructural analysis of rat, human, and fish lenses showed that a beaded filament was present in these lenses, which was indistinguishable from that seen in the bovine lens. In the squid a filamentous, beaded structure was observed, but it differed from that seen in the bovine lens. We conclude from the data presented that the beaded filament, and its constituent proteins, are well-conserved. This data should facilitate the identification of lens cytoskeletal proteins and structure in a wide range of animal models, and establish that probes for these proteins may be of broad applicability.

Ultrastructural localization of alpha A-crystallin to the bovine lens fiber cell cytoskeleton

Monoclonal antibodies to the bovine alpha A-crystallin were developed and used to probe the relationship between alpha A-crystallin and the bovine lens fiber cell Plasma Membrane-Cytoskeleton Complex (PMCC). Superficial bovine lens cortex was washed by repeated homogenization/centrifugation to remove "soluble protein." The resulting Plasma Membrane-Cytoskeleton Complex was covalently immobilized to inert resin, and extensively buffer washed. SDS PAGE and immunoblot analysis of both the covalently immobilized PMCC and of the sequentially-generated subcellular fractions shows that most of the lens alpha crystallin is "soluble", and readily extracted with physiologic buffers. However, this data also shows that 1) Non-alpha crystallins are progressively and quantitatively extracted from the PMCC with buffer, 2) An irreducible level of non-covalently bound alpha crystallin is achieved which cannot be readily extracted from the PMCC, even with 2 M urea, 1% NP40 or 0.4M KCl. Electron microscope level immunocytochemistry was performed on both the covalently immobilized PMCC, as well as on buffer-extracted thick frozen sections, using monoclonal antibodies to the alpha A-crystallin. The results show a very heavy labelling of both intermediate filaments and beaded filaments, but little or no labelling of fiber cell membranes. The data presented argues that a subfraction of the total alpha A-crystallin is strongly associated with the fiber cell cytoskeleton complex, and constitutes a quantitatively major component of the lens cytoskeleton fraction.

Methods for the circumvention of problems associated with the study of the ocular lens plasma membrane-cytoskeleton complex

Two alternative methods for the study of the lens cytoskeleton are described which serve to overcome some of the difficulties imparted by the unique biology of the lens. The first technique involves rapid freezing, thick sectioning, and selective extraction and/or fixation of the lens section. This approach offers several advantages: 1) enhanced visualization of the cytoskeleton, 2) avoidance of fixation gradients, 3) free access for immunocytochemical probes, 4) retention of tissue-wide spatial relationships, with a sharp increase in the resolution of regional analysis, and 5) the capacity for correlative morphological and biochemical comparisons. The second method involves the covalent immobilization of the plasma membrane-cytoskeleton complex (PMCC) to acrylamide beads. This approach permits: 1) avoidance of fixation in the immunocytochemical analysis of lens cytoskeleton and plasma membranes 2) rapid processing of multiple, small-quantity samples for immunocytochemistry/biochemical analysis 3) cleaner and more rapid analysis of cytoskeletal extraction conditions. Both approaches, while particularly suited to the study of the lens PMCC, may also be of value to the study of the PMCC of other tissues, particularly where preservation/analysis of regional relationships is essential.

Discrimination between the lens fiber cell 115 kd cytoskeletal protein and alpha-actinin

The lens fiber cell cytoskeleton includes a protein with a relative molecular weight, by SDS PAGE, of 115 kD. This protein has been purported to be, or be related to alpha-actinin, a highly-conserved family of actin-binding cytoskeletal proteins common to many tissues across a wide phylogenetic range. In this report we assess the relationship between the 115 kd lens fiber cell protein and alpha-actinin. Assessment of relative molecular weight, immunologic cross-reactivity, and partial sequence analysis suggest that the 115 kD lens fiber cell cytoskeletal protein and alpha-actinin are either unrelated, or, at best, that the lens protein represents an unusually divergent isoform of the alpha-actin family of proteins. Immunochemical analysis of homogenates of bovine heart and red blood cells indicate that these tissues express a protein which is weakly cross-reactive with the lens 115 kD protein, but that this cross-reactive protein is not alpha-actinin.