Immunolocalization of MP70 in lens fiber 16-17-nm intercellular junctions

MP26, a protein of intercellular junctions in the bovine lens: electrophoretic and chromatographic characterization

We have characterized the membrane protein of apparent molecular weight 26 kD from bovine lenses (MP26 or MIP) with respect to six different electrophoretic and chromatographic procedures. These include one- and two-dimensional gel electrophoretic procedures, as well as SDS-hydroxylapatite chromatography. The two-dimensional gels include isoelectric focusing with both conventional ampholytes and buffer focusing methods. With buffer focusing, the membranes are solubilized without the use of SDS and the isoelectric focusing is performed in the absence of SDS. As specific probes for MP26, a monoclonal antibody and an anti-MP26 rabbit serum were used, the latter prepared against electrophoretically purified MP26. These separation techniques were adapted to MP26 in order to permit a more detailed characterization of this protein and to search for any heterogeneity in this size range, specifically other junctional proteins or protein fragments. We have found evidence for charge heterogeneity in MP26, but no evidence for multiple membrane proteins of Mr 26,000 in urea-treated membranes. The charge heterogeneity appears to be related to a phosphorylation of MP26. The results reported here aid the interpretation of a variety of data, especially findings on the reconstitution of MP26 in artificial membranes and results from work with polyclonal MP26 antibodies. These investigations are all designed to evaluate the proposed role of MP26 as a protein of cell-to-cell channels in the lens fiber cell.

Freeze-fracture cytochemistry: a simplified guide and update on developments

A wide variety of methods by which cytochemistry and freeze-fracture can be successfully combined have recently become available. All these techniques are designed to provide information on the chemical nature of structural components revealed by freeze-fracture, but differ in how this is achieved, in precisely what type of information is obtained, and in which types of specimen can be studied. Colloidal gold labelling is the most widely used cytochemical technique in freeze-fracture cytochemistry, and for many of the methods it is indispensable. In principle, there are four points in which the cytochemical labelling step may be integrated into the standard freeze-fracture procedure: (i) before the specimen has been frozen, (ii) after it has been fractured and thawed, (iii) after platinum shadowing or (iv) after completion of the full replication sequence. Retention of the gold label so that it can be viewed with replicas can be achieved by depositing platinum and/or carbon upon the labelled surface, thereby partially entrapping the marker particles within the replica, or by retaining, attached to the replica, fragments of fractured membrane (or other cellular components) that would normally have been lost during the replica cleaning step. Another approach to visualizing the label is to use sections, either with portions of a replica included face-on, or for examining the fracture path through the sample (without replica). Recent developments have centered on the use of replicas to stabilize half-membrane leaflets; not only may these and associated attached components be retained for labelling just before mounting, but they provide a means for manipulating the specimen--specifically, turning it over during processing--so that additional structural information can be obtained. This article aims to explain how modern freeze-fracture cytochemistry works, and how the various techniques differ in what they can tell us about membranes and other cellular structures. With the effectiveness of many of the techniques now demonstrated, freeze-fracture cytochemistry is firmly established, alongside a range of related labelling techniques, for increasing application in cell and membrane biology in the 1990s.

Amino acid sequence of in vivo phosphorylation sites in the main intrinsic protein (MIP) of lens membranes

The main intrinsic membrane protein of the lens fiber cell, MIP, has been previously shown to be phosphorylated in preparations of lens fragments. Phosphorylation occurred on serine residues near the cytoplasmic C-terminus of the molecule. Since MIP is thought to function as a channel protein in lens plasma membranes, possibly as a cell-to-cell channel protein, phosphorylation could regulate the assembly or gating of these channels. We sought to identify the specific serines which are phosphorylated in order to help identify the kinases involved in regulating MIP function. To this end we purified a peptide fragment from native membranes that had not been subjected to any exogenous kinases or kinase activators. Any phosphorylation detected in these fragments must be due to cellular phosphorylation and thus is termed in vivo phosphorylation. Purified membranes were also phosphorylated with cAMP-dependent protein kinase to determine the mobility of phosphorylated and unphosphorylated MIP-derived peptides on different HPLC columns and to determine possible cAMP-dependent protein kinase phosphorylation sites. Lens membranes, which contain 50-60% of the protein as MIP, were digested with lysylendopeptidase C. Peptides were released from the C-terminal region of MIP and a major product of 21-22 kDa remained membrane-associated. Separation of the lysylendopeptidase-C-released peptides on C8 reversed-phase HPLC demonstrated that one of these fragments, corresponding to residues 239-259 in MIP, was partially phosphorylated. The phosphorylated and nonphosphorylated forms of this peptide were separated on QAE HPLC. In vivo phosphorylation sites were found at residues 243 and 245 through phosphoserine modification via ethanethiol and sequence analysis. Phosphorylation was never detected on serine 240. The phosphorylation level of serine 243 could be increased by incubation of membranes with cAMP-dependent protein kinase under standard assay conditions. Other kinases that phosphorylate serines found near acidic amino acids must be responsible for the in vivo phosphorylation demonstrated at serine 245.

Molecular cloning and complete nucleotide sequence of the cDNA encoding a bovine lens intrinsic membrane protein (MP19)

Recently, we reported the partial characterization of bovine lens intrinsic membrane proteins having apparent SDS-PAGE derived molecular mass of 19, 21, and 23 kDa, and determined that they contained identical NH2- terminal amino acid sequences for the first 20 amino acids. From this amino acid sequence information, a mixed synthetic oligonucleotide was constructed and used to screen a calf lens lambda gt11 cDNA library in order to isolate and characterize the cDNA coding for this membrane polypeptide(s). Two separate cDNA clones were isolated and sequenced, and were found to have an identical sequence of 883 bases with an open reading frame coding for a polypeptide of 173 amino acids, having a molecular mass of 19,683 Daltons. The first 20 amino acids of the translated sequence were identical to that determined by our laboratory previously, and the last seven amino acids were identical to that recently determined by another laboratory from analysis of the extracted polypeptides, indicating that this cDNA is the authentic molecule coding for MP19.

Properties of channels reconstituted from the major intrinsic protein of lens fiber membranes

Detergent-solubilized plasma membrane protein of either adult bovine or calf lens and high-performance liquid chromatography-purified major intrinsic protein (MIP) of the lens were reconstituted into unilamellar vesicles and planar lipid bilayers. Freeze-fracture studies showed that the density of intramembrane particles in the vesicles was proportional to the protein/lipid ratio. At high ratios, these particles crystallized into tetragonal arrays as does MIP in lens fibers. Channels induced by either purified MIP or detergent-solubilized protein had essentially identical properties. The conductance of multichannel membranes was maximal near 0 mV and decreased to 0.49 +/- 0.08 of the maximum value at voltages greater than 80 mV. The dependence of the conductance on voltage was well fit by a two-state Boltzmann distribution. Voltage steps greater than 30 mV elicited an ohmic current step followed by a slow (seconds) biexponential decrease. The amplitudes and time constants depended on the magnitude but not the sign of the voltage. Steps from 100 mV to voltages less than 30 mV caused the channels to open exponentially with a millisecond time constant. Analysis of latency to first closure after a voltage step gave nearly the same time constants as multichannel kinetics. Single-channel conductance is proportional to salt concentration from 0.1 to 1.0 M in KCl. In 0.1M KCl, the channel had two preferred conductance states with amplitudes of 380 and 160 pS, as well as three additional substates. Multi- and single-channel data suggest that the channel has two kinetically important open states. The channel is slightly anion selective. The properties of the channel do not vary appreciably from pH 7.4 to 5.8 or from pCa 7 to 2. We propose that a channel with these properties could contribute to maintenance of lens transparency and fluid balance.

Expression of the gap junction protein connexin43 in embryonic chick lens: molecular cloning, ultrastructural localization, and post-translational phosphorylation

Lens epithelial cells are physiologically coupled to each other and to the lens fibers by an extensive network of intercellular gap junctions. In the rat, the epithelial-epithelial junctions appear to contain connexin43, a member of the connexin family of gap junction proteins. Limitations on the use of rodent lenses for the study of gap junction formation and regulation led us to examine the expression of connexin43 in embryonic chick lenses. We report here that chick connexin43 is remarkably similar to its rat counterpart in primary amino acid sequence and in several key structural features as deduced by molecular cDNA cloning. The cross-reactivity of an anti-rat connexin43 serum with chick connexin43 permitted definitive immunocytochemical localization of chick connexin43 to lens epithelial gap junctional plaques and examination of the biosynthesis of connexin43 by metabolic radiolabeling and immunoprecipitation. We show that chick lens cells synthesize connexin43 as a single, 42-kD species that is efficiently posttranslationally converted to a 45-kD form. Metabolic labeling of connexin43 with 32P-orthophosphate combined with dephosphorylation experiments reveals that this shift in apparent molecular weight is due solely to phosphorylation. These results indicate that embryonic chick lens is an appropriate system for the study of connexin43 biosynthesis and demonstrate for the first time that connexin43 is a phosphoprotein.

Mammalian lens inter-fiber resistance is modulated by calcium and calmodulin

The relationship between Ca2+ and lens fiber cell communication was investigated in the isolated intact rat lens by using radiotracer and electrophysiological techniques. The lens internal calcium was increased by adding the SH oxidant diamide (1 mM), by incubating in a sodium-free (n-methylglucamine) solution or by increasing external calcium from 1 to 10 mM. A 12 hours incubation in diamide produced a ten-fold increase in 45Ca uptake into the lens which was accompanied by a ten-fold increase in internal resistance. Incubation in Na-free solution or in 10 mM Ca2+ both produced a 5-fold increase in 45Ca content, while the increase in internal resistance was five and six fold respectively. This uncoupling was prevented in the diamide and Na-free treated lenses by omitting Ca2+ from the incubation medium. Fiber cell uncoupling was noticed in each of these experimental conditions after approximately 5 hours incubation, and good recovery was obtained in the high calcium solution if the stress was removed. The calmodulin antagonists calmidazolium (3 microM) and W7 (100 microM) both prevented uncoupling in the high calcium solution, provided there was a 2 hours preincubation period in calcium-free solution containing antagonist before the stress was applied. These data indicate that lens fiber cell communication is required by Ca2+ and calmodulin.

Molecular portrait of lens gap junction protein MP70

A 70-kDa membrane protein (MP70) is a component of the lens fiber gap junctions. Its membrane topology and its N-terminal sequence are similar to those of the connexin family of proteins. Some features of MP70 containing fiber gap junctions are, however, distinct from gap junctions in other mammalian tissues: (i) Lens connexons form crystalline arrays only after cleavage of junctional proteins in vitro. These hexagonal arrays have a periodicity of 13.6 nm which is significantly larger than the 8- 9-nm spacing of liver and heart gap junctions. (ii) Lens fiber gap junctions dissociate in low concentrations of nonionic detergent and this provides an avenue to purify MP70 directly from a membrane mixture. Isolated MP70 in the form of 17 S structures has an appearance consistent with connexon pairs. (iii) The C-terminal half of MP70 is cleaved in situ by a lens endogenous calcium-dependent protease. The processed from MP38 remains in the membrane and is abundant in the central region of the lens. A testable hypothesis for MP70 function is presented.

Freeze-fracture cytochemistry: review of methods

"Freeze-fracture cytochemistry" encompasses a diversity of recently developed techniques in which freeze-fracture and cytochemistry are combined. Cytochemical labeling may, in principle, be integrated into one of three basic points in the standard freeze-fracture procedure; 1) before the specimen is frozen, 2) after it has been fractured, or 3) after it has been platinum shadowed and/or carbon coated. Visualization of the labeled cellular structures can be achieved by a variety of different methodologies. For example, the markers (usually colloidal gold particles) may be viewed embedded within a replica, or attached to it via fragments of membrane (or other cellular components). Sectioning is a central strategy in a number of techniques, either in combination with or in place of replication. The different combinations of methods that have been devised are not, for the most part, alternative ways of arriving at the same result; each provides quite distinct information about specific classes of membrane component or other structure in the cell. The purpose of this review is to present, within a single article, a systematic survey of the full range of techniques currently available in freeze-fracture cytochemistry. Emphasis is placed on explaining the principles underlying the methods and on illustrating their applications. With the success recently achieved, freeze-fracture cytochemistry has moved from the phase of experimental development to a position in which it may be expected increasingly to make significant contributions across a wide spectrum of problems in cell and membrane biology.

Bovine lens 23, 21 and 19 kDa intrinsic membrane proteins have an identical amino-terminal amino acid sequence

We have isolated bovine lens intrinsic membrane proteins (MP) having molecular masses of 19, 21 and 23 kDa. Limited amino acid sequence analysis of the amino-terminal portion of each of these polypeptides revealed a 100% homology in sequence for the number of residues determined (20 amino acids). Northern blot analysis of bovine lens mRNA using a labeled antisense oligonucleotide probe common to the amino acid sequence of these three peptides revealed a single band having an apparent molecular size of 0.8 kb. Taken together, these findings suggest a genetic commonality between these polypeptides.

Distribution of MP17 in isolated lens fibre membranes

MP17 is the second most abundant integral membrane protein in the mammalian lens. It has some common features with the major intrinsic polypeptide MIP26, but amino terminal sequencing shows that MP17 is a separate gene product. Both MP17 and MIP26 are abundant in isolated lens fibre membrane vesicles and are not detectable in the fibre gap junctions.

The structural organization and protein composition of lens fiber junctions

The structural organization and protein composition of lens fiber junctions isolated from adult bovine and calf lenses were studied using combined electron microscopy, immunolocalization with monoclonal and polyclonal anti-MIP and anti-MP70 (two putative gap junction-forming proteins), and freeze-fracture and label-fracture methods. The major intrinsic protein of lens plasma membranes (MIP) was localized in single membranes and in an extensive network of junctions having flat and undulating surface topologies. In wavy junctions, polyclonal and monoclonal anti-MIPs labeled only the cytoplasmic surface of the convex membrane of the junction. Label-fracture experiments demonstrated that the convex membrane contained MIP arranged in tetragonal arrays 6-7 nm in unit cell dimension. The apposing concave membrane of the junction displayed fracture faces without intramembrane particles or pits. Therefore, wavy junctions are asymmetric structures composed of MIP crystals abutted against particle-free membranes. In thin junctions, anti-MIP labeled the cytoplasmic surfaces of both apposing membranes with varying degrees of asymmetry. In thin junctions, MIP was found organized in both small clusters and single membranes. These small clusters also abut against particle-free apposing membranes, probably in a staggered or checkerboard pattern. Thus, the structure of thin and wavy junctions differed only in the extent of crystallization of MIP, a property that can explain why this protein can produce two different antibody-labeling patterns. A conclusion of this study is that wavy and thin junctions do not contain coaxially aligned channels, and, in these junctions, MIP is unlikely to form gap junction-like channels. We suggest MIP may behave as an intercellular adhesion protein which can also act as a volume-regulating channel to collapse the lens extracellular space. Junctions constructed of MP70 have a wider overall thickness (18-20 nm) and are abundant in the cortical regions of the lens. A monoclonal antibody raised against this protein labeled these thicker junctions on the cytoplasmic surfaces of both apposing membranes. Thick junctions also contained isolated clusters of MIP inside the plaques of MP70. The role of thick junctions in lens physiology remains to be determined.

The 43-kD polypeptide of heart gap junctions: immunolocalization, topology, and functional domains

Analysis by SDS-PAGE of gap junction fractions isolated from heart suggests that the junctions are comprised of a protein with an Mr 43,000. Antibodies against the electroeluted protein and a peptide representing the 20 amino terminal residues bind specifically on immunoblots to the 43-kD protein and to the major products arising from proteolysis during isolation. By immunocytochemistry, the protein is found in ventricle and atrium in patterns consistent with the known distribution of gap junctions. Both antibodies bind exclusively to gap junctions in fractions from heart examined by EM after gold labeling. Since only domains of the protein exposed at the cytoplasmic surface should be accessible to antibody, we conclude that the 43-kD protein is assembled in gap junctions with the amino terminus of the molecule exposed on the cytoplasmic side of the bilayer, that is, on the same side as the carboxy terminus as determined previously. By combining proteolysis experiments with data from immunoblotting, we can identify a third cytoplasmic region, a loop of some 4 kD between membrane protected domains. This loop carries an antibody binding site. The protein, if transmembrane, is therefore likely to cross the membrane four times. We have used the same antisera to ascertain if the 43-kD protein is involved in cell-cell communication. The antiserum against the amino terminus blocked dye coupling in 90% of cell pairs tested; the antiserum recognizing epitopes in the cytoplasmic loop and cytoplasmic tail blocked coupling in 75% of cell pairs tested. Preimmune serum and control antibodies (one against MIP and another binding to a cardiac G protein) had no or little effect on dye transfer. Our experimental evidence thus indicates that, in spite of the differences in amino acid sequence, the gap junction proteins in heart and liver share a general organizational plan and that there may be several domains (including the amino terminus) of the molecule that are involved in the control of junctional permeability.

Antisera directed against connexin43 peptides react with a 43-kD protein localized to gap junctions in myocardium and other tissues

Rat heart and other organs contain mRNA coding for connexin43, a polypeptide homologous to a gap junction protein from liver (connexin32). To provide direct evidence that connexin43 is a cardiac gap junction protein, we raised rabbit antisera directed against synthetic oligopeptides corresponding to two unique regions of its sequence, amino acids 119-142 and 252-271. Both antisera stained the intercalated disc in myocardium by immunofluorescence but did not react with frozen sections of liver. Immunocytochemistry showed anti-connexin43 staining of the cytoplasmic surface of gap junctions in isolated rat heart membranes but no reactivity with isolated liver gap junctions. Both antisera reacted with a 43-kD polypeptide in isolated rat heart membranes but did not react with rat liver gap junctions by Western blot analysis. In contrast, an antiserum to the conserved, possibly extracellular, sequence of amino acids 164-189 in connexin32 reacted with both liver and heart gap junction proteins on Western blots. These findings support a topological model of connexins with unique cytoplasmic domains but conserved transmembrane and extracellular regions. The connexin43-specific antisera were used by Western blots and immunofluorescence to examine the distribution of connexin43. They demonstrated reactivity consistent with gap junctions between ovarian granulosa cells, smooth muscle cells in uterus and other tissues, fibroblasts in cornea and other tissues, lens and corneal epithelial cells, and renal tubular epithelial cells. Staining with the anti-connexin43 antisera was never observed to colocalize with antibodies to other gap junctional proteins (connexin32 or MP70) in the same junctional plaques. Because of limitations in the resolution of the immunofluorescence, however, we were not able to determine whether individual cells ever simultaneously express more than one connexin type.

Strategy and tactics in electron microscopy of cell surfaces

Over the past decade new methods have been developed to visualize both the external and the protoplasmic surfaces of cultured cells in the electron microscope. In this review the emphasis is on cell monolayers, though some of the techniques are also applicable to cells in suspension. There is no universal method which would satisfy all our requirements i.e. the preservation of native structure and antigenicity and the visualization of the whole cell surface at high resolution. While surface replicas of freeze-dried or critical point-dried cells are eminently suited for high resolution studies including gold immunolabelling, scanning electron microscopy provides a view of the whole cell and a large sample for 'statistical' evaluation. Whole mount preparations of cleaved cells prove useful in studies of plasma membrane associated structures such as the cytoskeleton. A series of new procedures have been developed for studies of cytoskeleton/membrane interactions, identification of intramembrane particles and their contacts with the glycocalyx, to mention some of the biological problems. Although the lysis-squirting technique appears most suitable for the visualization and immunolabelling of protoplasmic surfaces of ventral membranes, dry- or wet-cleaving represent a useful alternative for studies of the protoplasmic surfaces of dorsal membranes and of the ventral membrane associated cytoplasmic domains. An assessment of the methods is given though this should only serve as guidance and it is up to the experimentor to choose the most useful technique for the project under study. Briefly the aim of the project determines the choice of the method. A multi-methodical approach is recommended when one method does not provide satisfactory results.

Structural and molecular biology of the eye lens membranes

Lens transparency is associated with a unique design in tissue development and architecture. The fiber plasma membrane has domains which link with the cytoskeleton, thus maintaining cell shape. Other membrane regions form processes which interlock adjacent lens fibers, and intercellular junctions contain transmembrane pores which allow passage of metabolites between cells. Much interest has recently focused on the study of lens membrane structure and function, mainly because membrane dysfunction may be associated with cataract formation. This article reviews what is known about the structure of membrane domains, about the identification of domain-specific proteins, and describes current attempts to relate these results to function. Much of the presently available data is controversial, and an attempt will be made to reconcile them in revised models and testable hypotheses.

Synthesis of lens capsule and plasma membrane glycoproteins by lens epithelial cells and fibers in the rat

The lens of the eye possesses a capsule which is a greatly hypertrophied basement membrane. To investigate the synthesis of glycoproteins destined for this capsule, 3H-fucose was injected into the vitreous body of intact rats weighing approximately 200 gm. The animals were killed from 10 min to 14.5 months later, and their lenses were processed for electron microscope radioautography. At 10 min after injection, more than 58% of the silver grains were localized to the Golgi apparatus of the lens epithelial cells. By day 1, the heaviest sites of reaction were the plasma membrane (more than 50% of total label), the basal cytoplasm, and the adjacent lens capsule, where a heavy band of reaction was seen. The remainder of the capsule exhibited a lighter diffuse reaction. In the lens fibers, the label was at first localized to clusters of vesicles but then migrated to the plasma membrane and to the region of the capsule adjacent to the basal surface of these fibers. Light microscope radioautographs of the lens capsule at later time intervals revealed that by 1 month after injection the diffuse reaction had disappeared, and only the strongly labeled band remained. By 14.5 months after injection, this band had migrated partially across the lens capsule, but the capsule itself had increased considerably in thickness. On the other hand, the distance between the labeled band and the free edge of the capsule had decreased from that seen at the time of injection.

The gap junction: a channel for multiple functions?

Gap junctions are specialized membrane structures that enable the intercytoplasmic exchange of small molecules and ions between contacting cells. During the past decade, biophysical and structural analyses of the junctional channel have considerably increased our understanding of the pharmacological properties and gating mechanisms of gap junctions. Despite this impressive amount of work, until recently the physiological role of these ubiquitous intercellular pathways has remained speculative in most tissues. This review summarizes the most recent information obtained on the structure of the gap junction by molecular cloning of the major protein components and emphasizes the growing evidence for their functional role in adult tissues formed by highly differentiated secretory cells. The relevance of cell-to-cell coupling for the co-ordinated function of the exocrine and endocrine pancreas is discussed.

Immunological characterization of rat cardiac gap junctions: presence of common antigenic determinants in heart of other vertebrate species and in various organs

Antibodies to the following synthetic peptide, SALGKLLDKVQAY, were purified by affinity chromatography and characterized by ELISA and immunoblotting. These antibodies, shown to be specific to the major protein constituent of isolated rat heart junctions: connexin 43, cross-reacted with a homologous protein in immunoreplicas of whole heart fractions of trout, frog, chicken, guinea pig, mouse and rat, suggesting a phylogenic conservation of connexin 43 in vertebrates. By immunoblotting of whole organ fractions it was also demonstrated that these antibodies cross-reacted with major proteins of Mr 32 and 22 kD in rat and mouse liver, of Mr 41 kD in rat cerebellum, of Mr 43 kD in uterus, stomach and kidney of rat, of Mr 46 and 70 kD in rat lens, suggesting that these proteins share common or related epitopes with the synthetic peptide and connexin 43.

Expression of the gene for main intrinsic polypeptide (MIP): separate spatial distributions of MIP and beta-crystallin gene transcripts in rat lens development

The main intrinsic polypeptide (MIP) is the major protein present in the lens fiber cell membrane and is the product of a gene which, as far as is known, is expressed only in the lens. We have used in situ hybridization and immunofluorescence microscopy to characterize the expression of this gene during the course of development in the rat. At progressive stages of lens morphogenesis, we find that synthesis of the protein is closely tied to the accumulation of MIP mRNA in cells that are committed to terminal differentiation, first in the elongating presumptive primary lens fibers and later in the secondary fibers as they differentiate from the anterior epithelial cells. The transcripts accumulate in the basal cytoplasm of the primary fibers and in the cytoplasm which surrounds the cell nucleus in the secondary fibers. We have compared this pattern of expression with that of a gene for a cytoplasmic protein, beta-crystallin beta-A1/A3. In sharp contrast to the localized concentrations seen for the MIP mRNA, beta-A1/A3 transcripts are relatively uniformly distributed throughout the cytoplasm. Neither MIP nor crystallin gene appears to be transcriptionally active in the undifferentiated epithelial cell, but transcripts from the beta-A1/A3 gene appear earlier in fiber cell differentiation than do those from the gene for MIP.

Homologies between gap junction proteins in lens, heart and liver

The cells in the mammalian lens are electrically and metabolically coupled with each other by a network of gap junctions. These are clusters of transmembrane channels by which the fibre cells situated deeper in the lens communicate through the epithelium with the aqueous humour, the source of nutrients for the lens. Hence gap junctions are important for lens transparency. The gap junction proteins in the mammalian lens have not yet been identified with certainty. A putative fibre gap junction protein of relative molecular mass 26,000 (26K) is not related to those from other tissues, such as the liver 28K junction component. Another lens membrane protein with Mr 70K (MP70) has also been localized in the lens fibre gap junctions. Here we demonstrate by amino-terminal sequence analysis that MP70 and its in vivo-processed form, MP38 (ref. 8), belong to a wider family of gap junction proteins. With this new data on the lens, homologies between gap junction proteins now extend to organs derived from all three embryonal layers, endoderm (liver), mesoderm (heart) and ectoderm (lens).

MP17, a fiber-specific intrinsic membrane protein from mammalian eye lens

A major protein with a molecular weight of 17,000, designated as MP17, has been identified in mammalian eye lens plasma membranes. Hydrophobic photolabeling experiments revealed that MP17 is a genuine intrinsic membrane protein. By using monoclonal antibodies we demonstrated that MP17 is not detectable in liver, heart, muscle, spleen and kidney, and thus can be considered, like MP26, as a lens-specific membrane protein. Furthermore, we showed that MP17 is a substrate for cAMP-dependent protein kinase and that it is a calmodulin-binding protein.