A calsequestrin-like protein in the endoplasmic reticulum of the sea urchin: localization and dynamics in the egg and first cell cycle embryo

Characterization of a calsequestrin-like protein from sea-urchin eggs

Following our studies on the identification of a calsequestrin-like protein (CSLP) from sea-urchin eggs [Oberdorf, Lebeche, Head & Kaminer (1988) J. Biol Chem. 263, 6806-6809], we have characterized its Ca(2+)-binding properties and identified it as a glycoprotein. The molecule binds 23 mol of Ca2+/mol of protein, as determined by equilibrium dialysis. This is in the range reported for cardiac calsequestrin but is about half the binding capacity of striated muscle calsequestrin. The affinities of the CSLP for Ca2+ are decreased by increasing KCl concentrations (20-250 mM) and the presence of Mg2+ (3 mM) in the medium: the half-maximal binding values varied from 1.62 to 5.77 mM. Hill coefficients indicated mild co-operativity in the Ca2+ binding. Ca2+ (1-8 mM)-induced u.v. difference spectra and intrinsic fluorescence changes suggest a net exposure of aromatic residues to an aqueous environment. C.d. measurements showed minor Ca(2+)-induced changes in alpha-helical and beta-sheet content of less than 10%. These spectral changes are distinctly different from those found in muscle calsequestrin. Immunoblotting studies showed that the CSLP is distinct from calreticulin, a low-affinity Ca(2+)-binding protein.

The repetitive calcium waves in the fertilized ascidian egg are initiated near the vegetal pole by a cortical pacemaker

Ascidian eggs respond to fertilization with a series of repetitive calcium waves that originate mostly from the vegetal/contraction pole region (J. E. Speksnijder, C. Sardet, and L. F. Jaffe, 1990, Dev. Biol. 142, 246-249), where the myoplasm is concentrated during the first phase of ooplasmic segregation. This suggests that the myoplasm may be involved in initiating these calcium waves. To test this possibility, the starting position of the calcium waves was determined in eggs that had the subcortical, mitochondria-rich part of the myoplasm displaced by centrifugation. Such centrifuged eggs display four cytoplasmic layers: a large centrifugal yolk zone, a narrow clear zone, a mitochondria-rich layer, and a small clear zone at the centripetal pole. Imaging of the cytosolic calcium in centrifuged eggs that were injected with the calcium-specific photoprotein aequorin reveals a series of repetitive calcium waves after fertilization. About 70% of these waves start in the vegetal/contraction pole area, which is similar to the number of waves previously found to start in this area in uncentrifuged eggs. In contrast, only about 25% of the waves start close to the displaced mitochondria-rich layer. From this result it is concluded that the main wave initiation site is not displaced by the centrifugal forces that displace the subcortical, mitochondria-rich part of the myoplasm. Moreover, the observation that the animal-vegetal polarity of cortical components such as actin filaments and the endoplasmic reticulum has been retained after centrifugation further suggests that a cortical component located in the vegetal hemisphere--most likely the endoplasmic reticulum network in the cortical region of the myoplasm--is involved in initiating the repetitive calcium waves in the fertilized ascidian egg.

Cortical localization of a calcium release channel in sea urchin eggs

We have used an antibody against the ryanodine receptor/calcium release channel of skeletal muscle sarcoplasmic reticulum to localize a calcium release channel in sea urchin eggs. The calcium release channel is present in less than 20% of immature oocytes, where it does not demonstrate a specific cytoplasmic localization, while it is confined to the cortex of all mature eggs examined. This is in contrast to the cortical and subcortical localization of calsequestrin in mature and immature eggs. Immunolocalization of the calcium release channel reveals a cortical reticulum or honeycomb staining network that surrounds cortical granules and is associated with the plasma membrane. The network consists of some immunoreactive electron-dense material coating small vesicles and elongate cisternae of the endoplasmic reticulum. The fluorescent reticular staining pattern is lost when egg cortices are treated with agents known to affect sarcoplasmic reticulum calcium release and induce cortical granule exocytosis (ryanodine, calcium, A-23187, and caffeine). An approximately 380-kD protein of sea urchin egg cortices is identified by immunoblot analysis with the ryanodine receptor antibody. These results demonstrate: (a) the presence of a ryanodine-sensitive calcium release channel that is located within the sea urchin egg cortex; (b) an altered calcium release channel staining pattern as a result of treatments that initiate the cortical granule reaction; and (c) a spatial and functional dichotomy of the ER which may be important in serving different roles in the mobilization of calcium at fertilization.

Confocal microscopy of fertilization-induced calcium dynamics in sea urchin eggs

Although confocal microscopy has typically been utilized in studies of fixed specimens, its potential for exploring dynamic processes in living cells is rapidly being realized. In this report, confocal laser scanning microscopy is used to analyze the calcium wave that occurs following fertilization in living sea urchin eggs microinjected with the calcium-sensitive fluorescent probes fluo-3 or calcium green. Time-lapse recordings of optical sections depicting calcium dynamics within the eggs are also subjected to volumetric reconstructions. Such analyses indicate that (1) cytoplasmic free calcium levels become elevated throughout the fertilized egg, (2) fertilization also causes the egg nucleus to undergo a transient increase in free calcium, and (3) normal cleavage can be obtained following time-lapse imaging of the calcium waves.

Microtubule motors in the early sea urchin embryo

Sea urchin gametes and early embryos have proven to be a useful system for studying the roles of microtubule (MT)-associated motors in axonemal motility and cytoplasmic MT-based movements in dividing cells. In this brief article, known and potential sea urchin MT motors are listed and their possible biological functions are discussed.

Demonstration of calcium uptake and release by sea urchin egg cortical endoplasmic reticulum

The calcium indicator dye fluo-3/AM was loaded into the ER of isolated cortices of unfertilized eggs of the sea urchin Arbacia punctulata. Development of the fluorescent signal took from 8 to 40 min and usually required 1 mM ATP. The signal decreased to a minimum level within 30 s after perfusion with 1 microM InsP3 and increased within 5 min when InsP3 was replaced with 1 mM ATP. Also, the fluorescence signal was lowered rapidly by perfusion with 10 microM A23187 or 10 microM ionomycin. These findings demonstrate that the cortical ER is a site of ATP-dependent calcium sequestration and InsP3-induced calcium release. A light-induced wave of calcium release, traveling between 0.7 and 2.8 microns/s (average speed 1.4 microns/s, N = 8), was sometimes observed during time lapse recordings; it may therefore be possible to use the isolated cortex preparation to investigate the postfertilization calcium wave.

Characterization of sea urchin egg endoplasmic reticulum in cortical preparations

The cortical endoplasmic reticulum (ER) of sea urchin eggs was localized on isolated egg cortices by staining with aqueous suspensions of the dicarbocyanine "DiI." Immunofluorescence localization of a calsequestrin-like protein was essentially identical; this is consistent with a role for the ER in calcium regulation. The ER often encircles cortical granules, making it well-suited for initiating fusion and propagating the calcium wave. Thiazole orange and Hoechst dye 33258 at pH 2 stain ribosomes bound to the ER, providing evidence that the cortical ER is rough ER. High chloride concentrations were found to disrupt ER continuity.

Organization of the membrane skeleton in spreading mouse blastomeres. I. Morphological analysis

Differentiation in the mouse embryo begins at the 8-cell stage when the blastomeres spread against each other in a process called compaction. The spreading behavior of blastomeres on lectin-coated coverslips mimics that of blastomeres in the embryo, and we have utilized this model system to obtain an en face view of the membrane skeleton in the spreading blastomeres. Embryos were cultured on the coverslips for periods ranging from 20 sec to 6 hr, and the cells were disrupted to expose the cytoplasmic face of the adherent membranes and their associated filaments. The "membrane lawn" preparations were fixed, critical point dried, rotary shadowed, and the replicas examined by transmission electron microscopy. Using this technique we found that the plasmalemma of rounded blastomeres is associated with a lacy 3-dimensional filamentous meshwork that is transformed into a thin mat of densely woven filaments when the cells flatten. The overall organization of the membrane skeleton is similar in flattening 2- and 8-cell embryos, but there are significant differences in the time required for spreading to take place, in the means whereby the membrane skeletons are reorganized, and in the extent of maximal flattening. The significance of these observations for the compaction process is discussed.

Evidence for the involvement of microtubules, ER, and kinesin in the cortical rotation of fertilized frog eggs

During the first cell cycle, the vegetal cortex of the fertilized frog egg is translocated over the cytoplasm. This process of cortical rotation creates regional cytoplasmic differences important in later development, and appears to involve an array of aligned microtubules that forms transiently beneath the vegetal cortex. We have investigated how these microtubules might be involved in generating movement by analyzing isolated cortices and sections of Xenopus laevis and Rana pipiens eggs. First, the polarity of the cortical microtubules was determined using the "hook" assay. Almost all microtubules had their plus ends pointing in the direction of cortical rotation. Secondly, the association of microtubules with other cytoplasmic elements was examined. Immunofluorescence revealed that cytokeratin filaments coalign with the microtubules. The timing of their appearance and their position on the cytoplasmic side of the microtubules suggested that they are not involved directly in generating movement. ER was visualized with the dye DiIC16(3) and by immunofluorescence with anti-BiP (Bole, D. G., L. M. Hendershot, and J. F. Kearney, 1986. J. Cell Biol. 102:1558-1566). One layer of ER was found closely underlying the plasma membrane at all times. An additional, deeper layer formed in association with the microtubules of the array. Antibodies to sea urchin kinesin (Ingold, A. L., S. A. Cohn, and J. M. Scholey. 1988. J. Cell Biol. 107:2657-2667) detected antigens associated with both the ER and microtubules. On immunoblots they recognized microtubule associated polypeptide(s) of approximately 115 kD from Xenopus eggs. These observations are consistent with a role for kinesin in creating movement between the microtubules and ER, which leads in turn to the cortical rotation.

Organization of the sea urchin egg endoplasmic reticulum and its reorganization at fertilization

The ER of eggs of the sea urchin Lytechinus pictus was stained by microinjecting a saturated solution of the fluorescent dicarbocyanine DiIC18(3) (DiI) in soybean oil; the dye spread from the oil drop into ER membranes throughout the egg but not into other organelles. Confocal microscopy revealed large cisternae extending throughout the interior of the egg and a tubular membrane network at the cortex. Since diffusion of DiI is confined to continuous bilayers, the spread of the dye supports the concept that the ER is a cell-wide, interconnected compartment. In time lapse observations, the internal cisternae were seen to be in continuous motion, while the cortical ER was stationary. After fertilization, the internal ER appeared to become more finely divided, beginning as a wave apparently coincident with the calcium wave and becoming most marked by 2-3 min. By 5-8 min the ER returned to an organization similar to that of the unfertilized egg. The cortical network also changed at fertilization; it became disrupted and eventually recovered. DiI labeling allowed continuous observations of the ER during pronuclear migration and mitosis. DiI-stained membranes accumulated in the region of the microtubule array surrounding the sperm nucleus and centriole (the sperm aster) as it migrated to the center of the egg; this accumulation persisted near the centrosomes and zygote nucleus throughout pronuclear fusion and the first two mitotic cycles. We have used a new method to observe the spatial and temporal organization of the ER in a living cell, and we have demonstrated a striking reorganization of the ER at fertilization.

The calcium content of cortical granules and the loss of calcium from sea urchin eggs at fertilization

In many species, fertilization triggers a wave of cortical granule exocytosis in the egg that is the consequence of an increase in intracellular free calcium concentration. We have measured the total calcium content of cortical granules from two species of sea urchins by quantitative X-ray microanalysis and spectrometric measurements. Our results show that cortical granules: (1) contain a high concentration of total calcium (around 30 and 95 mM for Paracentrotus lividus and Arbacia lixula, respectively), (2) represent a major cortical storage site of calcium in the egg (5 and 11% of total egg calcium for P. lividus and A. lixula, respectively), and (3) exchange part of their accumulated calcium by an ATP dependent mechanism. In addition we have confirmed that at fertilization, sea urchin eggs lose a sizeable amount of their calcium (7% for P. lividus and 15% for A. lixula). The kinetics and magnitude of the loss suggest that some of this calcium could be provided by cortical granules during exocytosis.

Subcellular localization and sequence of sea urchin kinesin heavy chain: evidence for its association with membranes in the mitotic apparatus and interphase cytoplasm

Kinesin was previously immunolocalized to mitotic apparatuses (MAs) of early sea urchin blastomeres (Scholey, J.M., M.E. Porter, P.M. Grissom, and J.R. McIntosh. 1985. Nature [Lond.]. 318:483-486). Here we report evidence that this MA-associated motor protein is a conventional membrane-bound kinesin, rather than a kinesin-like protein. Our evidence includes the observation that the deduced amino acid sequence of this sea urchin kinesin heavy chain is characteristic of a conventional kinesin. In addition, immunolocalizations using antibodies that distinguish kinesin from kinesin-like proteins confirm that conventional kinesin is concentrated in MAs. Finally, our immunocytochemical data further suggest that conventional kinesin is associated with membranes which accumulate in MAs and interphase asters of early sea urchin embryos, and with vesicles that are distributed in the perinuclear region of coelomocytes. Thus kinesin may function as a microtubule-based vesicle motor in some MAs, as well as in the interphase cytoplasm.

Intracellular Ca2+ stores in chicken Purkinje neurons: differential distribution of the low affinity-high capacity Ca2+ binding protein, calsequestrin, of Ca2+ ATPase and of the ER lumenal protein, Bip

To identify intracellular Ca2+ stores, we have mapped (by cryosection immunofluorescence and immunogold labeling) the distribution in the chicken cerebellar cortex of an essential component, the main low affinity-high capacity Ca2+ binding protein which in this tissue has been recently shown undistinguishable from muscle calsequestrin (Volpe, P., B. H. Alderson-Lang, L. Madeddu, E. Damiani, J. H. Collins, and A. Margreth. 1990. Neuron. 5:713-721). Appreciable levels of the protein were found exclusively within Purkinje neurons, distributed to the cell body, the axon, and the elaborate dendritic tree, with little labeling, however, of dendritic spines. At the EM level the protein displayed a dual localization: within the ER (rough- and smooth-surfaced cisternae, including the cisternal stacks recently shown [in the rat] to be highly enriched in receptors for inositol 1,4,5-triphosphate) and, over 10-fold more concentrated, within a population of moderately dense, membrane-bound small vacuoles and tubules, identified as calciosomes. These latter structures were widely distributed both in the cell body (approximately 1% of the cross-sectional area, particularly concentrated near the Golgi complex) and in the dendrites, up to the entrance of the spines. The distribution of calsequestrin was compared to those of another putative component of the Ca2+ stores, the membrane pump Ca2+ ATPase, and of the ER resident lumenal protein, Bip. Ca2+ ATPase was expressed by both calciosomes and regular ER cisternae, but excluded from cisternal stacks; Bip was abundant within the ER lumena (cisternae and stacks) and very low within calciosomes (average calsequestrin/Bip immunolabeling ratios were approximately 0.5 and 36.5 in the two types of structure, respectively). These results suggest that ER cisternal stacks do not represent independent Ca2+ stores, but operate coordinately with the adjacent, lumenally continuous ER cisternae. The ER and calciosomes could serve as rapidly exchanging Ca2+ stores, characterized however by different properties, in particular, by the greater Ca2+ accumulation potential of calciosomes. Hypotheses of calciosome biogenesis (directly from the ER or via the Golgi complex) are discussed.

Two Ca2(+)-binding proteins in the mitotic apparatus of sea urchin eggs

Two Ca2(+)-binding proteins of sea urchin eggs were purified and partially characterized. They showed Ca2(+)-dependent binding to actin filaments and Ca2(+)-dependent changes of fluorescence intensity which was used to estimate the affinity constant of these proteins to Ca2+ ions. Ca2+ ions did not increase phospholipid binding ability of these proteins. Therefore these proteins are distinguished from the calpactin family. Staining of sections of metaphase eggs embedded in paraffin showed their localization in the mitotic apparatus. Furthermore, staining of whole mount eggs with anti-tubulin and antibodies against these proteins, followed by observations with confocal laser-scanning microscopy showed their co-localization with microtubules more clearly. In vitro co-sedimentation assay of microtubules with these proteins, however, showed no interaction between them. This suggested that some structures surrounding the mitotic apparatus microtubules are responsible for their localization.

Multiple intracellular signals coordinate structural dynamics in the sea urchin egg cortex at fertilization

Fertilization of the sea urchin egg is accompanied by a sequence of structural changes in the egg cortex that include exocytosis, endocytosis, and microvillar growth. This architectural reorganization is coordinated by two intracellular signals: a rapid, transient rise in cytosolic free calcium and a slower, longer lasting increase in cytoplasmic pH. In this report we provide ultrastructural views of these events in quick-frozen eggs and discuss their relationship to the calcium and pH signals.

Subcellular localization of sea urchin egg spectrin: evidence for assembly of the membrane-skeleton on unique classes of vesicles in eggs and embryos

A recent study from our laboratory on the sea urchin egg suggested that spectrin was not solely restricted to the plasma membrane, but instead had a more widespread distribution on the surface of a variety of membranous inclusions. (E. M. Bonder et al., 1989, Dev. Biol. 134, 327-341). In this report we extend our initial findings and provide experimental and ultrastructural evidence for the presence of spectrin on three distinct classes of cytoplasmic vesicles. Immunoblot analysis of membrane fractions prepared from egg homogenates establishes that spectrin coisolates with vesicle-enriched fractions, while indirect immunofluorescence microscopy on cryosections of centrifugally stratified eggs demonstrates that spectrin specifically associates with cortical granules, acidic vesicles, and yolk platelets in vivo. Immunogold ultrastructural localization of spectrin on cortices isolated from eggs and early embryos details the striking distribution of spectrin on the cytoplasmic surface of the plasma membrane and the membranes of cortical granules, acidic vesicles, and yolk platelets, while quantitative studies show that relatively equivalent amounts of spectrin are present on the different membrane surfaces both before and after fertilization. These data, in combination with the localization of numerous spectrin crosslinks between actin filaments in surface microvilli, suggest that spectrin plays a pivotal role in structuring the cortical membrane-cytoskeletal complex of the egg and the embryo.

Differentiation of a calsequestrin-containing endoplasmic reticulum during sea urchin oogenesis

We have used light and electron microscopic immunolocalization to study the distribution of a sea urchin calsequestrin-like protein (SCS) during sea urchin oogenesis. SCS was localized exclusively in the lumen of the endoplasmic reticulum (ER) and in the nuclear envelope of oocytes of all maturation stages. Immunoelectron microscopy also revealed that SCS is not present in golgi complexes of oocytes. Double label immunofluorescent staining of frozen sections of ovary with the SCS antiserum and an antibody to the cortical granule protein hyalin indicated a dramatic morphogenesis of the SCS-containing ER (SCS-ER) coincident with oocyte maturation. This differentiation included an apparent increase in the amount and complexity of the cytoplasmic SCS-ER network, the transient appearance of stacks of SCS-ER cisternae in synthetically active vitellogenic oocytes, and the restructuring of the SCS-ER in the cortex. Immunofluorescence of isolated oocyte cortices showed a plasma membrane-associated SCS-ER which was much less dense and regular than that found surrounding the cortical granules in the mature unfertilized egg cortex. Cytoplasmic and cortical microtubule arrays are present in oocytes and may provide the basis for the SCS-ER distributional dynamics. The results of this study underscore the dynamic nature of ER and how it's organization reflects cellular functions. We suggest that the establishment during oogenesis of the dense SCS-ER tubuloreticulum provides the egg with the calcium sequestration and release apparatus that regulates calcium fluxes during egg activation and early development.

Simultaneous analysis of cell Ca2+ and Ca2(+)-stimulated chloride conductance in colonic epithelial cells (HT-29)

We used perforated patch, whole-cell current recordings and video-based fluorescence ratio imaging to monitor the relation of plasma membrane ionic conductances to intracellular free Ca2+ within individual colonic epithelial cells (HT-29). The Ca2(+)-mediated agonist, neurotensin, activated K+ and Cl- conductances that showed different sensitivities to [Ca2+]i. The Cl- conductance was sensitive to increases or decreases in [Ca2+]i around the resting value of 76 +/- 32 (mean +/- SD) nM (n = 46), whereas activation of the K+ conductance required at least a 10-fold rise in [Ca2+]i. Neurotensin increased [Ca2+]i by stimulating a transient intracellular Ca2+ release, which was followed by a sustained rise in [Ca2+]i due to Ca2+ influx from the bath. The onset of the initial [Ca2+]i transient, monitored at a measurement window over the cell interior, lagged behind the rise in Cl- current during agonist stimulation. This lag was not present when the [Ca2+]i rise was due to Ca2+ entry from the bath, induced either by the agonist or by the Ca2+ ionophore ionomycin. The temporal differences in [Ca2+]i and Cl- current during the agonist-induced [Ca2+]i transient can be explained by a localized Ca2+ release from intracellular stores in the vicinity of the plasma membrane Cl- channel. Chloride currents recover toward basal values more rapidly than [Ca2+]i after the agonist-induced [Ca2+]i transient, and, during a sustained neurotensin-induced [Ca2+]i rise, Cl- currents inactivate. These findings suggest that an inhibitory pathway limits the increase in Cl- conductance that can be evoked by agonist. Because this Cl- current inhibition is not observed during a sustained [Ca2+]i rise induced by ionomycin, the inhibitory pathway may be mediated by another agonist-induced messenger, such as diacylglycerol.

The endoplasmic reticulum and calcium storage

Calcium storage is one of the functions commonly attributed to the endoplasmic reticulum (ER) in nonmuscle cells. Several recent studies have added support to this concept. Analysis of reticuloplasm, the luminal ER content, has shown that it contains several proteins (reticuloplasmins) which are prospective calcium storage proteins. One of these, calreticulin, is also present in the sarcoplasmic reticulum (SR). In sea urchin eggs, a calsequestrin-like protein has been clearly localised to the ER. The recent demonstration that the IP3 receptor, which has similarities with the calcium release channel in the SR is also localised in the ER membrane suggests that calcium stored in the ER is important for intracellular signalling. The alternative view, that the physiologically important calcium store is a specialised organelle, the calciosome, is not supported by these observations. Recent evidence also suggests that ER calcium might be important in ER structure and in the retention of the luminal ER proteins.

Identification and developmental expression of a chicken calsequestrin homolog

Calsequestrin (CAL) is a calcium-binding protein whose primary function is thought to involve sequestration of calcium in the muscle sarcoplasmic reticulum (SR). Little is known about the mechanisms regulating CAL expression, or about the role of this protein in muscle development. In addition, CAL may regulate calcium localization in some nonmuscle cells. We have identified an avian calsequestrin homolog. The predicted amino acid sequence of the avian CAL, first described as a laminin binding protein, and named aspartactin, is 70-80% identical to mammalian CAL sequences. We have used affinity-purified antibodies and cDNA probes to investigate expression in developing and adult chicken tissues. In adult chickens, the avian CAL homolog was expressed in slow and fast twitch skeletal muscle as well as in cardiac muscle. Surprisingly high levels of CAL protein were also detected in cerebellum. During development, CAL mRNA and protein were detected in Embryonic Day 5 (E-5) limb primordia, well before the initiation of myoblast fusion. In leg skeletal muscle, CAL protein and mRNA increase approximately 10-fold from E-8 to E-18 with a time course that just precedes myoblast fusion. This early expression pattern was also observed in cultured chicken pectoral myoblasts, and appears to be regulated at the level of mRNA abundance. The developmental profile of CAL expression is compared to that of other muscle proteins and possible additional functions of CAL are discussed.

Microinjection of a conserved peptide sequence of p34cdc2 induces a Ca2+ transient in oocytes

The product of the yeast cell cycle control gene cdc2, and its homologs in higher eukaryotes (p34cdc2), all contain a perfectly conserved sequence of 16 amino acids that has not been found in any other protein sequence. Microinjection of this peptide triggers a specific increase in the concentration of intracellular free Ca2+ that originates from intracellular stores in both starfish and Xenopus oocytes. Thus, p34cdc2 might interact through its conserved peptide domain with some component of the Ca2(+)-regulatory system.

Recent progress on structural interactions of the endoplasmic reticulum

Progress has been made recently in understanding certain aspects of the structure of the ER. It is very likely that kinesin, dynein, and myosin are associated with the ER and are responsible for distributing the fluid membranes of the ER by interaction of these motors with microtubules and actin filaments. Other kinds of structural protein associations with the ER are also likely, for instance, binding of cortical ER to subplasmalemmal regions or microtubule binding and/or polymerization along ER tubules.

Multiple zones in the sequence of calreticulin (CRP55, calregulin, HACBP), a major calcium binding ER/SR protein

The complete amino acid sequence of CRP55, the major 55 kd calcium binding protein of the ER lumen, was deduced from the murine cDNA nucleotide sequence. This was completed using a novel application of PCR amplification. The mature 399 residue protein encoded is preceded by a 17 amino acid leader sequence and ends in the ER signal sequence, KDEL. The protein contains no calcium binding motifs of the EF hand type or of the form seen in calelectrin-related proteins. The major region of potential low affinity calcium binding sites is a polyacidic stretch towards the C terminus. The primary structure of the protein is markedly zonal. The N-terminal region, of approximately neutral net charge and hydrophobicity, is followed by a central proline-rich zone with repeat sequences separated from the polyacidic C-terminal stretch by a short hydrophobic sequence. The general shape suggested is a globular domain attached to an extended tail. Immunofluorescence studies show that the protein is present in skeletal muscle and indicate that it is a sarcoplasmic reticulum protein. We propose that the protein be named calreticulin to reflect its calcium binding activity and location in the ER and SR.