Phosphorylation of lens fiber cell membrane proteins

Phosphorylation determines the calmodulin-mediated Ca2+ response and water permeability of AQP0

In Xenopus oocytes, the water permeability of AQP0 (P(f)) increases with removal of external calcium, an effect that is mediated by cytoplasmic calmodulin (CaM) bound to the C terminus of AQP0. To investigate the effects of serine phosphorylation on CaM-mediated Ca(2+) regulation of P(f), we tested the effects of kinase activation, CaM inhibition, and a series of mutations in the C terminus CaM binding site. Calcium regulation of AQP0 P(f) manifests four distinct phenotypes: Group 1, with high P(f) upon removal of external Ca(2+) (wild-type, S229N, R233A, S235A, S235K, K238A, and R241E); Group 2, with high P(f) in elevated (5 mm) external Ca(2+) (S235D and R241A); Group 3, with high P(f) and no Ca(2+) regulation (S229D, S231N, S231D, S235N, and S235N/I236S); and Group 4, with low P(f) and no Ca(2+) regulation (protein kinase A and protein kinase C activators, S229D/S235D and S235N/I236S). Within each group, we tested whether CaM binding mediates the phenotype, as shown previously for wild-type AQP0. In the presence of calmidazolium, a CaM inhibitor, S235D showed high P(f) and no Ca(2+) regulation, suggesting that S235D still binds CaM. Contrarily, S229D showed a decrease in recruitment of CaM, suggesting that S229D is unable to bind CaM. Taken together, our results suggest a model in which CaM acts as an inhibitor of AQP0 P(f). CaM binding is associated with a low P(f) state, and a lack of CaM binding is associated with a high P(f) state. Pathological conditions of inappropriate phosphorylation or calcium/CaM regulation could induce P(f) changes contributing to the development of a cataract.

Regulation of TRP channel TRPM2 by the tyrosine phosphatase PTPL1

TRPM2, a member of the transient receptor potential (TRP) superfamily, is a Ca(2+)-permeable channel, which mediates susceptibility to cell death following activation by oxidative stress, TNFalpha, or beta-amyloid peptide. We determined that TRPM2 is rapidly tyrosine phosphorylated after stimulation with H(2)O(2) or TNFalpha. Inhibition of tyrosine phosphorylation with the tyrosine kinase inhibitors genistein or PP2 significantly reduced the increase in [Ca(2+)](i) observed after H(2)O(2) or TNFalpha treatment in TRPM2-expressing cells, suggesting that phosphorylation is important in TRPM2 activation. Utilizing a TransSignal PDZ domain array blot to identify proteins which interact with TRPM2, we identified PTPL1 as a potential binding protein. PTPL1 is a widely expressed tyrosine phosphatase, which has a role in cell survival and tumorigenesis. Immunoprecipitation and glutathione-S-transferase pull-down assays confirmed that TRPM2 and PTPL1 interact. To examine the ability of PTPL1 to modulate phosphorylation or activation of TRPM2, PTPL1 was coexpressed with TRPM2 in human embryonic kidney-293T cells. This resulted in significantly reduced TRPM2 tyrosine phosphorylation, and inhibited the rise in [Ca(2+)](i) and the loss of cell viability, which follow H(2)O(2) or TNFalpha treatment. Consistent with these findings, reduction in endogenous PTPL1 expression with small interfering RNA resulted in increased TRPM2 tyrosine phosphorylation, a significantly greater rise in [Ca(2+)](i) following H(2)O(2) treatment, and enhanced susceptibility to H(2)O(2)-induced cell death. Endogenous TRPM2 and PTPL1 was associated in U937-ecoR cells, confirming the physiological relevance of this interaction. These data demonstrate that tyrosine phosphorylation of TRPM2 is important in its activation and function and that inhibition of TRPM2 tyrosine phosphorylation reduces Ca(2+) influx and protects cell viability. They also suggest that modulation of TRPM2 tyrosine phosphorylation is a mechanism through which PTPL1 may mediate resistance to cell death.

TRPM2 is an ion channel that modulates hematopoietic cell death through activation of caspases and PARP cleavage

TRPM2 is a Ca(2+)-permeable channel activated by oxidative stress or TNF-alpha, and TRPM2 activation confers susceptibility to cell death. The mechanisms were examined here in human monocytic U937-ecoR cells. This cell line expresses full-length TRPM2 (TRPM2-L) and several isoforms including a short splice variant lacking the Ca(2+)-permeable pore region (TRPM2-S), which functions as a dominant negative. Treatment with H(2)O(2), a model of oxidative stress, or TNF-alpha results in reduced cell viability. Expression of TRPM2-L and TRPM2-S was modulated by retroviral infection. U937-ecoR cells expressing increased levels of TRPM2-L were treated with H(2)O(2) or TNF-alpha, and these cells exhibited significantly increased intracellular calcium concentration ([Ca(2+)](i)), decreased viability, and increased apoptosis. A dramatic increase in cleavage of caspases-8, -9, -3, and -7 and poly(ADP-ribose)polymerase (PARP) was observed, demonstrating a downstream mechanism through which cell death is mediated. Bcl-2 levels were unchanged. Inhibition of the [Ca(2+)](i) rise with the intracellular Ca(2+) chelator BAPTA blocked caspase/PARP cleavage and cell death induced after activation of TRPM2-L, demonstrating the critical role of [Ca(2+)](i) in mediating these effects. Downregulation of endogenous TRPM2 by RNA interference or increased expression of TRPM2-S inhibited the rise in [Ca(2+)](i), enhanced cell viability, and reduced numbers of apoptotic cells after exposure to oxidative stress or TNF-alpha, demonstrating the physiological importance of TRPM2. Our data show that one mechanism through which oxidative stress or TNF-alpha mediates cell death is activation of TRPM2, resulting in increased [Ca(2+)](i), followed by caspase activation and PARP cleavage. Inhibition of TRPM2-L function by reduction in TRPM2 levels, interaction with TRPM2-S, or Ca(2+) chelation antagonizes this important cell death pathway.

Characterization of the ovine-lens plasma-membrane protein-kinase substrates

The cAMP-dependent protein-kinase-catalyzed phosphorylation of the two major intrinsic lens fiber cell plasma membrane proteins, MP20 and MP26, is likely restricted to the inner cortical and nuclear regions of the lens in vivo. The ovine-lens-specific connexin, MP70, that has been identified as Cx50 in mice and Cx45.6 in the chick, is also a protein kinase substrate although it does not appear to be phosphorylated by a number of protein kinases including cAMP-dependent protein kinase, calmodulin-dependent protein kinase or protein kinase C. Rather, an extrinsic lens membrane fraction was isolated which contained protein kinase activity that catalyzed the phosphorylation of MP70; this protein kinase activity was cAMP-independent, Ca(2+)-independent, Mg(2+)-dependent, phosphorylated MP70 on a serine residue(s) and migrated with a molecular mass of 35 kDa on a gel filtration column. Both MP70 phosphorylation and the endogenous protein kinase activity were restricted to the lens outer cortical region. This membrane-associated protein kinase activity represents the first reported partial characterization of an endogenous lens fiber cell protein kinase activity that catalyzes the phosphorylation of a lens connexin protein. The phosphatase-induced shift in the electrophoretic mobility of MP70 is not reversed by this protein kinase, indicating that MP70 is likely phosphorylated on different residues by two or more protein kinases.

Cloning, characterization, and chromosomal mapping of human aquaporin of collecting duct

We recently cloned a cDNA of the collecting duct apical membrane water channel of rat kidney, which is important for the formation of concentrated urine (Fushima, K., S. Uchida, Y. Hara, Y. Hirata, F. Marumo, and S. Sasaki. 1993. Nature [Lond.]. 361:549-552). Since urine concentrating ability varies among mammalian species, we examined whether an homologous protein is present in human kidney. By screening a human kidney cDNA library, we isolated a cDNA clone, designated human aquaporin of collecting duct (hAQP-CD), that encodes a 271-amino acid protein with 91% identity to rat AQP-CD. mRNA expression of hAQP-CD was predominant in the kidney medulla compared with the cortex, immunohistochemical staining of hAQP-CD was observed only in the collecting duct cells, and the staining was dominant in the apical domain. Functional expression study in Xenopus oocytes confirmed that hAQP-CD worked as a water channel. Western blot analysis of human kidney medulla indicated that the molecular mass of hAQP-CD is 29 kD, which is the same mass expected from the amino acid sequence. Chromosomal mapping of the hAQP-CD gene assigned its location to chromosome 12q13. These results could be important for future studies of the pathophysiology of human urinary concentration mechanisms in normal and abnormal states.

The human lens fiber-cell intrinsic membrane protein MP19 gene: isolation and sequence analysis

DNA sequence analysis of overlapping shotgun and restriction fragments have revealed the entire sequence of the human lens fiber cell intrinsic membrane protein MP19 gene (also termed MP17, MP18, and MP20). The 8,056 bp MP19 gene contains 5 exons encoding a mature protein of 173 amino acids, which displayed a very high degree of identity (91%) with that of bovine MP19, deduced from a bovine cDNA sequence. The exon range in size from 52 bases (exon 1) to about 340 bases (exon 5). The introns consist of two large segments (introns B and C) of about 4,700 bases and 1,800 bases, respectively, and two small segments (intron A and D) of about 450 and 250 bases each. Seven Alu family DNA repeats are found within the human MP19 gene. The sequenced gene includes 100 bases of 5' flanking sequence.

Assignment of the lens intrinsic membrane protein MP19 structural gene to human chromosome 19

We have isolated and characterized a bovine cDNA clone encoding the bovine lens intrinsic membrane protein, MP19. This cDNA was used as a probe to analyze a panel of Southern blots of human-Chinese hamster somatic cell hybrid DNAs to assign the gene coding for MP19 to its human chromosome. Control human and Chinese hamster DNAs displayed a distinct EcoR1 restriction fragment pattern when hybridized with the bovine MP19 cDNA. When somatic cell hybrid DNAs were restricted with Eco R1 and Southern blots hybridized with the bovine MP19 cDNA, the characteristic human restriction pattern was observed only when human chromosome 19 was present in the hybrid panel. This assignment was confirmed using a human chromosome 19-specific genomic library. A clone from this human chromosome 19-specific library was identified and further characterized. This clone contained a 7.9 kilobase fragment that contained identical DNA sequences with that of the authentic bovine MP19 cDNA, and with a separate human genomic clone containing the MP19 gene.

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.

In vitro and in vivo phosphorylation of chicken beta B3-crystallin

Incubations of chicken lens homogenates with [32P]-ATP revealed the phosphorylation of a 28 kDa protein, and phosphoamino acid analysis of the phosphorylated protein showed the presence of phosphoserine. The protein is present in the beta-crystallin fraction and after purification and partial sequence determination, by way of peptide mapping and subsequent amino acid analyses and Edman degradation, this 28 kDa protein was identified as the beta B3-crystallin subunit, based on its homology with the bovine and rat orthologue. From phosphate content determination it could be concluded that this chicken beta B3 subunit contains in vivo 2 mol phosphate/mol polypeptide.

Phosphorylation of MP26, a lens junction protein, is enhanced by activators of protein kinase C

MP26, a protein thought to form gap junctional channels in the lens, and other lens proteins were phosphorylated under conditions that activate protein kinase C. Phosphorylation was detected both in lens fiber cell fragments in an "in vivo" labeling procedure with 32P-phosphate and in cell homogenates with 32P-ATP. In these experiments, both calcium and 12-O-tetradecanoylphorbol 13-acetate (TPA) were necessary for maximal phosphorylation of MP26. Calcium stimulated the phosphorylation of MP26 approximately fourfold and TPA with calcium led to a sevenfold increase. If TPA was present, 1 microM calcium was sufficient for maximal labeling. Phosphoamino acid analysis demonstrated approximately 85% phosphoserine, 15% phosphothreonine, and no phosphotyrosine when MP26 was phosphorylated in lens homogenates in the presence of TPA and calcium and then electrophoretically purified. Phosphorylation occurred near the cytoplasmic, C-terminal of MP26. The possible involvement of other kinases was also examined. The Walsh inhibitor, which affects cAMP-dependent protein kinases, had no influence on the TPA-mediated increase in phosphorylation. In studies with isolated membranes and added kinases, MP26 was also found to not be a substrate for calcium/calmodulin-dependent protein kinase II. Thus, protein kinase C may have phosphorylated MP26 in a direct manner.

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.

cAMP-dependent protein phosphorylation of membrane proteins in the parotid gland, platelets and liver. Comparison of a 22-kDa phosphoprotein from rat parotid microsomes (protein III) with phosphoproteins of similar molecular size from platelet and liver membranes

Stimulation of secretion in exocrine secretory glands leads to the phosphorylation of a 22-kDa membrane protein (protein III) whose function is still unknown [Jahn et al. (1980) Eur. J. Biochem. 112, 345-352; Jahn & Söling (1980) Proc. Natl Acad. Sci. USA 78, 6903-6906]. This report describes the comparison of this protein with phosphorylated membrane proteins of similar molecular mass in platelets and liver. Incubation of platelets with agents which raise the intracellular cAMP concentration results in the phosphorylation of a 22-kDa protein which is also phosphorylated in membrane preparations by endogenous kinases or by exogenous cAMP-dependent protein kinase. It is shown that this protein is distinct from protein III although both proteins have the same molecular mass and are substrates of cAMP-dependent protein kinase. In contrast to platelets, protein III could be demonstrated in liver microsomes. This indicates that the function of protein III is not exclusively linked to the stimulus-secretion coupling in exocrine cells.

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.

Calmodulin-like proteins and communicating junctions. Electrical uncoupling of crayfish septate axons is inhibited by the calmodulin inhibitor W7 and is not affected by cyclic nucleotides

The effects of W7, a calmodulin (CaM)-inhibitor, and cyclic nucleotides on electrical coupling and uncoupling are studied in crayfish lateral giant axons (septate axons). The septate axons provide a relatively simple two cell system in which both surface membrane and junctional resistance can be measured independently. Four microelectrodes are inserted into a septate axon, two on each side of the septum. Hyperpolarizing current pulses (150 nA) are injected alternatively in the caudal and rostral axon segment and the resulting electrotonic potentials are recorded. The axons are uncoupled at regular intervals by superfusing them with acetate-containing saline solution (pH 6.3) in the presence or absence of W7 (50-100 microM) or, as a control, its nonchlorinated form (W5). W7 strongly inhibits the acetate-induced increased in junctional resistance, while W5 is ineffective. The uncoupling inhibition does not appear to be caused by an increase in cyclic nucleotide concentration, because in preliminary experiments exposure to db-cAMP or db-cGMP (up to 1 mM) does not seem to influence either the basic values of Rj or their changes with acetate. The data confirm previous evidence for a participation of CaM-like proteins in cell-to-cell channel gating.

Adrenergic stimulation of lens cytoskeletal phosphorylation

Stimulation of lens fiber cytoskeletal phosphorylation by adrenergic drugs is described. The effect of isoproterenol on phosphorylation of the 47 Kd beaded filament protein is dose-dependent, detectable as soon as one minute after treatment and blocked by propranolol. Epinephrine increases the phosphorylation of both 47 Kd and the intermediate filament protein, vimentin. 47 Kd phosphorylation is also increased by norepinephrine, dibutyryl-cAMP or forskolin. The results indicate that lens fiber cytoskeletal phosphorylation is regulated, at least in part, via a beta-adrenergic receptor coupled to cyclic AMP production.

Some aspects of the phosphorylation of alpha-crystallin A

The cAMP-dependent phosphorylation of alpha-crystallin was investigated. The major products of in vitro phosphorylation of total bovine lens homogenate are the alpha A1 and alpha B1 polypeptides, but in addition a minor labeled spot is present which might correspond with a double phosphorylated alpha B chain. It is demonstrated that the A1 and B1 subunits of alpha-crystallin from bovine eye lenses are solely the result of phosphorylation of the primary gene products alpha A2 and alpha B2, respectively, as judged from the stoichiometry of the phosphate content of these polypeptides. Both the in vitro and in vivo phosphorylation sites of the A chain of bovine alpha-crystallin were determined and found to be the same. After in vitro incubation the majority of the 32P label was found in the tryptic peptides T17a and T16-17a, the latter being the result of incomplete tryptic cleavage between T16 and T17a. The in vivo phosphorylation site is also located in T17a, as could be concluded from the retention times on reversed-phase HPLC of T16-17a and T17a from alpha A1 as compared to those from alpha A2, and from the differences in their mobilities on high-voltage paper electrophoresis at pH 6.5. Furthermore, both T17a and T16-17a of alpha A1 contain approximately 1 mol phosphate/mol peptide. Thermolytic digestion of T16-17a of both alpha A2 and alpha A1, followed by separation on RP-HPLC, demonstrated that Ser-122 is the phosphorylation site of the A chain of bovine lens alpha-crystallin. The replacement of this phosphorylation site or the lack of basic amino acids at the N-terminal side of Ser-122 in some vertebrate species apparently results in the absence of phosphorylation of alpha-crystallin A both in vitro and in vivo.

Phosphorylation of lens intrinsic membrane proteins by protein kinase C

Two intrinsic proteins of bovine lens membranes with apparent relative molecular masses (Mr, app) of 26,000 and 18,000 were phosphorylated in intact membranes by protein kinase C prepared from either bovine brain or lens. The kinase preparations exhibited histone H1 phosphorylation dependent on calcium and phospholipid but not on cAMP. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the lens membranes showed a major band at Mr, app = 26,000 (identified as MP26, the main intrinsic protein of lens fiber cells), an intermediate band at Mr, app = 18,000 and several minor bands. Autoradiography of complete assay mixture containing protein kinase C, calcium, magnesium and [gamma-32P]ATP showed major bands at Mr, app = 18,000 and 26,000. Several lines of evidence indicated that the label at Mr, app = 26,000 was associated with MP26, a protein which has been found in lens junctions and which may form cell-cell channels. Treatment of the phosphorylated membranes with chymotrypsin and V8 protease cleaved the major band at Mr, app = 26,000 to fragments of Mr, app .= 22,000 and 24,000. Label was not detected in the resulting Mr, app = 22,000 peptide, but the Mr, app = 24,000 peptide was found to be labeled. Phosphoamino acid analysis of MP26 indicated that approximately 75% of the label was on phosphoserine and 25% was on phosphothreonine. No label was found on phosphotyrosine. These results differ from those reported for cAMP-dependent phosphorylation of lens proteins. Phosphorylation by protein kinase C may account for some of the labeling of MP26 detected in vivo.

A lens intercellular junction protein, MP26, is a phosphoprotein

The major protein present in the plasma membrane of the bovine lens fiber cell (MP26), thought to be a component of intercellular junctions, was phosphorylated in an in vivo labeling procedure. After fragments of decapsulated fetal bovine lenses were incubated with [32P]orthophosphate, membranes were isolated and analyzed by SDS PAGE and autoradiography. A number of lens membrane proteins were routinely phosphorylated under these conditions. These proteins included species at Mr 17,000 and 26,000 as well as a series at both 34,000 and 55,000. The label at Mr 26,000 appeared to be associated with MP26, since (a) boiling the membrane sample in SDS led to both an aggregation of MP26 and a loss of label at Mr 26,000, (b) the label at 26,000 was resistant to both urea and nonionic detergents, and (c) two-dimensional gels showed that a phosphorylated Mr 24,000 fragment was derived from MP26 with V8 protease. Studies with proteases also provided for a localization of most label within approximately 20 to 40 residues from the COOH-terminus of MP26. Published work indicates that the phosphorylated portion of MP26 resides on the cytoplasmic side of the membrane, and that this region of MP26 contains a number of serine residues. The same region of MP26 was labeled when isolated lens membranes were reacted with a cAMP-dependent protein kinase prepared from the bovine lens. After the in vivo labeling of lens fragments, phosphoamino acid analysis of MP26 demonstrated primarily labeled serines, with 5-10% threonines and no tyrosines. Treatments that lowered the intracellular calcium levels in the in vivo system led to a selective reduction of MP26 phosphorylation. In addition, forskolin and cAMP stimulated the phosphorylation of MP26 and other proteins in concentrated lens homogenates. These findings are of interest because MP26 appears to serve as a protein of cell-to-cell channels in the lens, perhaps as a lens gap junction protein.

cAMP-dependent phosphorylation of bovine lens alpha-crystallin

This communication reports that the A1 and B1 chains of bovine lens alpha-crystallin are phosphorylated. The conclusion is based on the following evidence: (i) When soluble preparations from lens cortex are incubated with [gamma-32P]ATP, a cAMP-dependent labeling of a high molecular weight protein is obtained. (ii) After NaDodSO4/PAGE, the label is found in two bands with Mr 22,000 and 20,000, corresponding to the B and A chains of alpha-crystallin, respectively. (iii) Isoelectric focusing indicates that the radioactivity is almost exclusively in bands with pI values of 5.58 and 6.70, corresponding to the A1 and B1 chains, respectively. (iv) Similar results are obtained in experiments of [32P]orthophosphate incorporation in lens organ culture. (v) Analyses of the digested protein indicate the label is exclusively in phosphoserine. (vi) 31P NMR analyses of native, proteolytically digested, and urea-treated alpha-crystallin gives a chemical shift of 4.6 ppm relative to 85% H3PO4 at pH 7.4, suggesting that the phosphate is covalently bound to a serine in the protein. An abundance of approximately one phosphate per four or five monomer units was found. (vii) Similar results were obtained by chemical analyses of independently prepared alpha-crystallin samples. The results are consistent with the view that the A1 and B1 chains arise as result of the phosphorylation of directly synthesized A2 and B2 polypeptides. It is suggested that this metabolically controlled phosphorylation may be associated with the terminal differentiation of the lens epithelial cell and the intracellular organization of the lens fiber cell.