Ca channel gating during cardiac action potentials.

How do Ca channels conduct Ca ions during the cardiac action potential? We attempt to answer this question by applying a two-microelectrode technique, previously used for Na and K currents, in which we record the patch current and the action potential at the same time (Mazzanti, M., and L. J. DeFelice. 1987. Biophys. J. 12:95-100, and 1988. Biophys. J. 54:1139-1148; Wellis, D., L. J. DeFelice, and M. Mazzanti. 1990. Biophys. J. 57:41-48). In this paper, we also compare the action currents obtained by the technique with the step-protocol currents obtained during standard voltage-clamp experiments. Individual Ca channels were measured in 10 mM Ca/1 Ba and 10 mM Ba. To describe part of our results, we use the nomenclature introduced by Hess, P., J. B. Lansman, and R. W. Tsien (1984. Nature (Lond.). 311:538-544). With Ba as the charge carrier, Ca channel kinetics convert rapidly from long to short open times as the patch voltage changes from 20 to -20 mV. This voltage-dependent conversion occurs during action potentials and in step-protocol experiments. With Ca as the charge carrier, the currents are brief at all voltages, and it is difficult to define either the number of channels in the patch or the conductance of the individual channels. Occasionally, however, Ca-conducting channels spontaneously convert to long-open-time kinetics (in Hess et al., 1984, notation, mode 2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Steroids, intracellular sodium levels, and Na+/K+-ATPase regulation

In outer medullary kidney tubules, both specific mineralocorticoid, and specific glucocorticoid Na+/K+-ATPase activation in vitro were inhibitable by amiloride, an inhibitor of a number of Na+-transporting mechanisms (Bentley, P.J. (1968) J. Physiol. (Lond.) 195, 317-330; Kinsella, J. L., and Aronson, P. S. (1980) Am. J. Physiol. 238, F461-F469). In addition, dexamethasone raised, whereas amiloride reduced, intracellular Na+ levels. These observations are consistent with the possibility that the steroidal responses are mediated by changes in intracellular Na+ ion activity. However, when intracellular Na+ levels were increased by the incubation of tubule segments in medium containing ouabain (10(-4) M), no Na+/K+-ATPase activation was observed, over incubation periods of up to 6 h. As mineralocorticoid and glucocorticoid effects are maximal within 2 h (Rayson, B.M., and Lowther, S.O. (1984) Am. J. Physiol. 246, F656-F662), these results suggest that the Na+ ion per se does not mediate the steroidal effects observed, directly. Incubation of tubule segments in medium containing 10(-4) M ouabain, at 37 degrees C, for longer periods (18 h), however, did indeed increase Na+/K+-ATPase activity, markedly. Thus, a potential homeostatic mechanism was demonstrable, where a chronic increase in intracellular Na+ level, measured after 2-4 h of treatment, resulted in an increase in Na+/K+-ATPase activity, such that the intracellular Na+ level was restored after 18-20 h of incubation to one not significantly different from the control value. This mechanism, however, appears to be clearly distinguishable from that which mediates steroidal Na+/K+-ATPase activation.     

Subcellular localization and sequence of sea urchin kinesin heavy chain: evidence for its association with membranes in the mitotic apparatus and interphase cytoplasm [published erratum appears in J Cell Biol 1991 Aug;114(4):following 863]

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.     

Identification of a MAP 2-like ATP-binding protein associated with axoplasmic vesicles that translocate on isolated microtubules

Axoplasmic vesicles were purified and observed to translocate on isolated microtubules in an ATP-dependent, trypsin-sensitive manner, implying that ATP-binding polypeptides essential for force generation were present on the vesicle surface. To identify these proteins [alpha 32P]8-azidoadenosine 5'-triphosphate ([alpha 32P]8-N3ATP), a photoaffinity analogue of ATP, was used. The results presented here identify and characterize a vesicle-associated polypeptide having a relative molecular mass of 292 kD that bound [alpha 32P]8-N3ATP. The incorporation of label is ultraviolet light-dependent and ATP-sensitive. Moreover, the 292-kD polypeptide could be isolated in association with vesicles or microtubules, depending on the conditions used, and the data indicate that the 292-kD polypeptide is similar to mammalian brain microtubule-associated protein 2 (MAP 2) for the following reasons: The 292-kD polypeptide isolated from either squid axoplasm or optic lobe cross-reacts with antiserum to porcine brain MAP 2. Furthermore, it purifies with taxol-stabilized microtubules and is released with salt. Based on these characteristics, the 292-kD polypeptide is distinct from the known force-generating molecules myosin and flagellar dynein, as well as the 110-130-kD kinesin-like polypeptides that have recently been described (Brady, S. T., 1985, Nature (Lond.), 317:73-75; Vale, R. D., T. S. Reese, and M. P. Sheetz, 1985b, Cell, 42:39-50; Scholey, J. M., M. E. Porter, P. M. Grissom, and J. R. McIntosh, 1985, Nature (Lond.), 318:483-486). Because the 292-kD polypeptide binds ATP and is associated with vesicles that translocate on purified MAP-free microtubules in an ATP-dependent fashion, it is therefore believed to be involved in vesicle-microtubule interactions that promote organelle motility.     

Taxol-induced microtubule asters in mitotic extracts of Xenopus eggs: requirement for phosphorylated factors and cytoplasmic dynein

Taxol, a microtubule stabilizing drug, induces the formation of numerous microtubule asters in the cytoplasm of mitotic cells (De Brabander, M., G. Geuens, R. Nuydens, R. Willebrords, J. DeMey. 1981. Proc. Natl. Acad. Sci. USA. 78:5608-5612). The center of these asters share with spindle poles some characteristics such as the presence of centrosomal material and calmodulin. We have recently reproduced the assembly of taxol asters in a cell-free system (Buendia, B., C. Antony, F. Verde, M. Bornens, and E. Karsenti. 1990. J. Cell Sci. 97:259-271) using extracts of Xenopus eggs. In this paper, we show that taxol aster assembly requires phosphorylation, and that they do not grow from preformed centers, but rather by a reorganization of microtubules first crosslinked into bundles. This process seems to involve sliding of microtubules along each other and we show that cytoplasmic dynein is required for taxol aster assembly. This result provides a possible functional basis to the recent findings, that dynein is present in the spindle and enriched near spindle poles (Pfarr, C. M., M. Cove, P. M. Grissom, T. S. Hays, M. E. Porter, and J. R. McIntosh. 1990. Nature (Lond.). 345:263-265; Steuer, E. R., L. Wordeman, T. A. Schroer, and M. P. Sheetz. 1990. Nature (Lond.). 345:266-268).     

Characterization of the microtubule-activated ATPase of brain cytoplasmic dynein (MAP 1C)

We recently found that the brain cytosolic microtubule-associated protein 1C (MAP 1C) is a microtubule-activated ATPase, capable of translocating microtubules in vitro in the direction corresponding to retrograde transport. (Paschal, B. M., H. S. Shpetner, and R. B. Vallee. 1987b. J. Cell Biol. 105:1273-1282; Paschal, B. M., and R. B. Vallee. 1987. Nature [Lond.]. 330:181-183.). Biochemical analysis of this protein (op. cit.) as well as scanning transmission electron microscopy revealed that MAP 1C is a brain cytoplasmic form of the ciliary and flagellar ATPase dynein (Vallee, R. B., J. S. Wall, B. M. Paschal, and H. S. Shpetner. 1988. Nature [Lond.]. 332:561-563). We have now characterized the ATPase activity of the brain enzyme in detail. We found that microtubule activation required polymeric tubulin and saturated with increasing tubulin concentration. The maximum activity at saturating tubulin (Vmax) varied from 186 to 239 nmol/min per mg. At low ionic strength, the Km for microtubules was 0.16 mg/ml tubulin, substantially lower than that previously reported for axonemal dynein. The microtubule-stimulated activity was extremely sensitive to changes in ionic strength and sulfhydryl oxidation state, both of which primarily affected the microtubule concentrations required for half-maximal activation. In a number of respects the brain dynein was enzymatically similar to both axonemal and egg dyneins. Thus, the ATPase required divalent cations, calcium stimulating activity less effectively than magnesium. The MgATPase was inhibited by metavandate (Ki = 5-10 microM for the microtubule-stimulated activity), 1 mM NEM, and 1 mM EHNA. In contrast to other dyneins, the brain enzyme hydrolyzed CTP, TTP, and GTP at higher rates than ATP. Thus, the enzymological properties of the brain cytoplasmic dynein are clearly related to those of other dyneins, though the brain enzyme is unique in its substrate specificity and in its high sensitivity to stimulation by microtubules.     

Preferential retention of paternal alleles in human retinoblastoma: evidence for genomic imprinting

The origins of the initial mutations in sporadic retinoblastoma were explored using polymorphic markers from chromosome 13q. The paternal chromosome was maintained in 3 of 3 informative bilateral tumors which had undergone reduction to homozygosity for regions of this chromosome. The paternal chromosome was maintained in 7 of 8 informative unilateral tumors which likewise demonstrated a reduction of homozygosity. These data are in contrast to previously published studies of chromosome retention in unilateral retinoblastoma [Dryja, T. P., Mukai, S., Petersen, R., Rapaport, J. M., Walton, D., and Yandel, D. W. Nature (Lond.), 339: 556-558, 1989; Zhu, Z., Dunn, J. M., Phillips, R. A., Goddard, A. D., Paton, K. E., Becker, A., and Gallie, B. L. Nature (Lond.), 340: 312-313, 1989] and provide the first evidence that genomic imprinting may play a role in this disease.     

Membrane binding characteristics of two forms of transforming growth factor-beta

Cartilage-inducing factors A and B (CIF-A and CIF-B) from bovine bone have recently been identified as transforming growth factor-beta (TGF-beta) (Seyedin, S.M., Thompson, A. Y., Bentz, H., Rosen, D. M., McPherson, J. M., Conti, A., Siegel, N. R., Galluppi, G. R., and Piez, K. A. (1986) J. Biol. Chem., 261, 5693-5695) and a unique protein homologous to TGF-beta (Seyedin S. M., Segarini, P. R., Rosen, D. M., Thompson, A. Y., Bentz, H., and Graycar, J. (1987) J. Biol. Chem., 262, 1946-1949), respectively. Although the biological activities of TGF-beta and CIF-B are similar, the divergence of CIF-B from the highly conserved amino acid sequence of TGF-beta prompted an investigation of its receptor binding properties. Three classes of cell surface binding components were identified. Class A has exclusive affinity for TGF-beta; class B has greater affinity for CIF-B; and class C has equal affinity for both proteins. A high molecular weight component, the predominant binding species, was further characterized and shown to consist of two components that are either class B or class C. The differential binding properties of TGF-beta and CIF-B to cell surface components suggest that there are biological activities unique to each of the proteins.     

Comparison of the data, analysis, and results of X-ray absorption studies of cytochrome c oxidase

Differences in the methods of analysis of X-ray absorption data used by Powers et al. [Powers, L., Blumberg, W. E., Chance, B., Barlow, C., Leigh, J., Jr., Smith, J., Yonetani, T., Vik, S., & Peisach, J. (1979) Biochim. Biophys. Acta 547, 520-538; Powers, L., Chance, B., Ching, Y., & Angiolillo, P. (1981) Biophys. J. 34, 465-498] and Scott et al. [Scott, R., Schwartz, J., & Cramer S. (1986) Biochemistry 25, 5546-5555] are clarified. In addition, we compare the X-ray absorption data and results for resting cytochrome c oxidase reported by both groups using the same analysis method and conclude apart from any assumptions that the data are not identical.     

Sodium pump-catalyzed sodium-sodium exchange associated with ATP hydrolysis

Inside-out red cell membrane vesicles have been used to study sodium interactions with the ATP-dependent sodium pump at sites accessible to both membrane surfaces. ATP-dependent 22Na+ influx (equivalent to efflux from cells) shows sigmoid dependence on extravesicular Na+ concentration. This is observed both in the absence of intravesicular cations and in the presence of intravesicular K or Rb ions. The kinetic behavior is similar to that observed earlier with intact cells, (Garay, R. P., and Garrahan, P. J. (1973) J. Physiol. (Lond.) 231, 297-325) and is consistent with a ratio of close to three Na ions transported per molecule of ATP hydrolyzed. With vesicles having relatively high intravesicular sodium concentration, (approximately 50 mM NaCl), the sodium pump effects an ATP-dependent sodium efflux coupled to sodium influx and to strophanthidin-sensitive ATP hydrolysis. The influx:efflux stoichiometry is approximately 1:1, and the influx:ATP hydrolysis ratio is close to 3. This ATP-dependent exchange has a higher affinity for vanadate than ATP plus ADP-dependent sodium exchange. It is concluded that this sodium-sodium exchange mode resembles sodium-potassium exchange whereby intravesicular sodium, i.e. sodium at the extracellular surface, at relatively high concentration, behaves like potassium.     

Blastocoel expansion in the preimplantation mouse embryo: stimulation of sodium uptake by cAMP and possible involvement of cAMP-dependent protein kinase

Elevating cAMP levels in mouse blastocysts increases the rate of blastocoel expansion (F. Manejwala, E. Kaji, and R. M. Schultz, 1986, Cell, 46, 95-103), which requires extracellular sodium (F. Manejwala, E. J. Crago, Jr., and R. M. Schultz, 1989, Dev. Biol. 133, 210-220). We report that cAMP analogs that can activate the cAMP-dependent protein kinase stimulate 22Na+ uptake by cavitating mouse blastocysts and that inhibitors of cAMP-dependent protein kinase activity inhibit the cAMP-stimulated increase in both the rate of blastocoel expansion and 22Na+ uptake.     

Modeling the cellular basis of altered excitation-contraction coupling in heart failure

Ca transients measured in failing human ventricular myocytes exhibit reduced amplitude and slowed relaxation [Beuckelmann, D.J., Nabauer, M., Erdmann, E., 1992. Intracellular calcium handling in isolated ventricular myocytes from patients with terminal heart failure. Circulation 85, 1046-1055; Gwathmey, J.K., Copelas, L., MacKinnon, R., Schoen, F.J., Feldman, M.D., Grossman, W., Morgan, J.P., 1987. Abnormal intracellular calcium handling in myocardium from patients with end-stage heart failure. Circ. Res. 61, 70-76; Kaab, S., Nuss, H. B., Chiamvimonvat, N., O'Rourke, B., Pak, P.H., Kass, D.A., Marban, E., Tomaselli, G.F., 1996. Ionic mechanism of action potential prolongation in ventricular myocytes from dogs with pacing-induced heart failure. Circ. Res. 78(2); Li, H.G., Jones, D.L., Yee, R., Klein, G.J., 1992. Electrophysiologic substrate associated with pacing-induced hert failure in dogs: potential value of programmed stimulation in predicting sudden death. J. Am. Coll. Cardiol. 19(2), 444-449; Vermeulen, J.T., McGuire, M.A., Opthof, T., Colonel, R., Bakker, J.M.T.d., Klopping, C., Janse, M.J., 1994. Triggered activity and automaticity in ventricular trabeculae of failing human and rabbit hearts. Cardiovasc. Res. 28, 1547-1554.] and blunted frequency dependence [Davies, C.H., Davia, K., Bennett, J.G., Pepper, J.R., Poole-Wilson, P.A., Harding, S.E., 1995. Reduced contraction and altered frequency response of isolated ventricular myocytes from patients with heart failure. Circulation, 92, 2540-2549; Hasenfuss, G., Reinecke, H., Studer, R., Meyer, M., Pieske, B., Holtz, J., Holubarsch, C., Posival, H., Just, H., Drexler, H., 1994. Relation between myocardial function and expression of sarcoplasmic reticulum Ca-ATPase in failing and nonfailing human myocardium. Circ. Res. 75, 434-442; Hasenfuss, G., Reinecke, H., Studer, R., Pieske, B., Meyer, M., Drexler, H., Just, H., 1996. Calcium cycling proteins and force-frequency relationships in heart failure. Basic Res. Cardiol. 91, 17-22; Monte, F.D., O'Gara, P., Poole-Wilson, P.A., Yacoub, M., Harding, S.E., 1995. Cell geometry and contractile abnormalities of myocytes from failing human left ventricle. Cardiovasc. Res. 30, 281-290; Philips, P.J., Gwathmey, J.K., Feldman, M.D., Schoen, F.J., Grossman, W., Morgan, J.P., 1990. Post-extrasystolic potentiation and the force-frequency relationships: differential augmentation of myocardial contractility in working myocardium from patients with end-stage heart failure. J. Mol. Cell. Cardiol. 22, 99-110; Pieske, B., Hasenfuss, G., Holubarsch, C., Schwinger, R., Bohm, M., Just, H., 1992. Alterations of the force-frequency relationship in the failing human heart depend on the underlying cardiac disease. Basic Res. Cardiol. 87, 213-221.]. Analyses of protein levels in these failing hearts reveal that the SR Ca-ATPase is down-regulated on average by 50% and that the Na/Ca exchanger is up-regulated on average by a factor of two. In this paper, we test the hypothesis that this altered pattern of expression of Ca handling proteins is sufficient to account for changes in excitation-contraction coupling properties measured experimentally at the cellular level. To do this, we present an integrated model of excitation-contraction coupling in the guinea pig ventricular cell. The model is used to determine the effects of SR Ca-ATPase down-regulation and Na/Ca exchanger up-regulation on action potential duration, Ca transient shape and amplitude, and isometric force. Model analyses demonstrate that changes in Ca handling proteins play a direct and critical role in prolongation of action potential duration, and in reduction of contractile force in heart failure.     

Synaptic vesicle recycling in synapsin I knock-out mice

The synapsins are a family of four neuron-specific phosphoproteins that have been implicated in the regulation of neurotransmitter release. Nevertheless, knock-out mice lacking synapsin Ia and Ib, family members that are major substrates for cAMP and Ca2+/ Calmodulin (CaM)-dependent protein kinases, show limited phenotypic changes when analyzed electrophysiologically (Rosahl, T.W., D. Spillane, M. Missler, J. Herz, D.K. Selig, J.R. Wolff, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1995. Nature (Lond.). 375: 488-493; Rosahl, T.W., M. Geppert, D. Spillane, D., J. Herz, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1993. Cell. 75:661-670; Li, L., L.S. Chin, O. Shupliakov, L. Brodin, T.S. Sihra, O. Hvalby, V. Jensen, D. Zheng, J.O. McNamara, P. Greengard, and P. Andersen. 1995. Proc. Natl. Acad. Sci. USA. 92:9235- 9239; see also Pieribone, V.A., O. Shupliakov, L. Brodin, S. Hilfiker- Rothenfluh, A.J. Czernik, and P. Greengard. 1995. Nature (Lond.). 375:493-497). Here, using the optical tracer FM 1-43, we characterize the details of synaptic vesicle recycling at individual synaptic boutons in hippocampal cell cultures derived from mice lacking synapsin I or wild-type equivalents. These studies show that both the number of vesicles exocytosed during brief action potential trains and the total recycling vesicle pool are significantly reduced in the synapsin I- deficient mice, while the kinetics of endocytosis and synaptic vesicle repriming appear normal.     

Impulse responses of automaticity in the Purkinje fiber

We examined the effects of brief current pulses on the pacemaker oscillations of the Purkinje fiber using the model of McAllister , Noble, and Tsien (1975. J. Physiol. [Lond.]. 251:1-57). This model was used to construct phase-response curves for brief electric stimuli to find "black holes," where rhythmic activity of the Purkinje fiber ceases. In our computer simulation, a brief current stimulus of the right magnitude and timing annihilated oscillations in membrane potential. The model also revealed a sequence of alternating periodic and chaotic regimes as the strength of a steady bias current is varied. We compared the results of our computer simulations with experimental work on Purkinje fibers and pointed out the importance of modeling results of this kind for understanding cardiac arrhythmias.     

The thrombospondin motif of aggrecanase-1 (ADAMTS-4) is critical for aggrecan substrate recognition and cleavage

Aggrecanase-1 (ADAMTS-4) is a member of the a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS) protein family that was recently identified. Aggrecanase-1 is one of two ADAMTS cartilage-degrading enzymes purified from interleukin-1-stimulated bovine nasal cartilage (Tortorella, M. D., Burn, T. C., Pratta, M. A. , Abbaszade, I., Hollis, J. M., Liu, R., Rosenfeld, S. A., Copeland, R. A., Decicco, C. P., Wynn, R., Rockwell, A., Yang, F., Duke, J. L., Solomon, K., George, H., Bruckner, R., Nagase, H., Itoh, Y., Ellis, D. M., Ross, H., Wiswall, B. H., Murphy, K., Hillman, M. C., Jr., Hollis, G. F., and Arner, E.C. (1999) Science 284, 1664-1666; 2 Abbaszade, I., Liu, R. Q., Yang, F., Rosenfeld, S. A., Ross, O. H., Link, J. R., Ellis, D. M., Tortorella, M. D., Pratta, M. A., Hollis, J. M., Wynn, R., Duke, J. L., George, H. J., Hillman, M. C., Jr., Murphy, K., Wiswall, B. H., Copeland, R. A., Decicco, C. P., Bruckner, R., Nagase, H., Itoh, Y., Newton, R. C., Magolda, R. L., Trzaskos, J. M., and Burn, T. C. (1999) J. Biol. Chem. 274, 23443-23450). The aggrecan products generated by this enzyme are found in cartilage cultures stimulated with cytokines and in synovial fluid from patients with arthritis, suggesting that aggrecanase-1 may be important in diseases involving cartilage destruction. Here we demonstrate that the thrombospondin type-1 (TSP-1) motif located within the C terminus of aggrecanase-1 binds to the glycosaminoglycans of aggrecan. Data from several studies indicate that this binding of aggrecanase-1 to aggrecan through the TSP-1 motif is necessary for enzymatic cleavage of aggrecan. 1) A truncated form of aggrecanase-1 lacking the TSP-1 motif was not effective in cleaving aggrecan. 2) Several peptides representing different regions of the TSP-1 motif effectively blocked aggrecanase-1 cleavage of aggrecan by preventing the enzyme from binding to the substrate. 3) Aggrecanase-1 was not effective in cleaving glycosaminoglycan-free aggrecan. Taken together, these data suggest that the TSP-1 motif of aggrecanase-1 is critical for substrate recognition and cleavage.     

Books Reviews

Book review in this article
Right Whales: Past And Present Status. R. L. Brownell, Jr., P. B. Best and J. H. Prescott, eds.
Research on Dolphins. M. M. Bryden and R. Harrison, eds.     

Books

Book Reviewed in this article:
A nimal T raining : S uccessful A nimal M anagement T hrough P ositive R inforcement . Ken Ramirez.
C onservation and M anagement of M arine M ammals . J. R. Twiss, Jr., R. R. Reeves and S. Montgomery, eds.
W ebster's W hales and D olphins of the W orld (CD-ROM).     

Intracellular transport and sorting of mutant human proinsulins that fail to form hexamers

Human proinsulin and insulin oligomerize to form dimers and hexamers. It has been suggested that the ability of prohormones to self associate and form aggregates may be responsible for the sorting process at the trans-Golgi. To examine whether insulin oligomerization is required for proper sorting into regulated storage granules, we have constructed point mutations in human insulin B chain that have been previously shown to prevent formation of insulin hexamers (Brange, J., U. Ribel, J. F. Hansen, G. Dodson, M. T. Hansen, S. Havelund, S. G. Melberg, F. Norris, K. Norris, L. Snel, A. R. Sorensen, and H. O. Voight. 1988. Nature [Lond.]. 333:679-682). One mutant (B10His----Asp) allows formation of dimers but not hexamers and the other (B9Ser----Asp) prevents formation of both dimers and hexamers. The mutants were transfected into the mouse pituitary AtT-20 cells, and their ability to be sorted into regulated secretory granules was compared to wild-type insulin. We found that while B10His----Asp is sorted somewhat less efficiently than wild-type insulin as reported previously (Carroll, R. J., R. E. Hammer, S. J. Chan, H. H. Swift, A. H. Rubenstein, and D. F. Steiner. 1988. Proc. Natl. Acad. Sci. USA. 85:8943-8947; Gross, D. J., P. A. Halban, C. R. Kahn, G. C. Weir, and L. Villa-Kumaroff. 1989. Proc. Natl. Acad. Sci. USA. 86:4107-4111). B9Ser----Asp is targeted to granules as efficiently as wild-type insulin. These results indicate that self association of proinsulin into hexamers is not required for its targeting to the regulated secretory pathway.     

Reviews

Plant Lectins. By A. P ustzai .
Techniques for the Rapid Detection of Plant Pathogens . Ed. by J. M. D uncan and L. T orrance .
Ecological Genetics and Air Pollution. Ed. by G. R. T aylor , Jr., L. F. P itelka and M. T. C legg .
Arctic Ecosystems in a Changing Climate. Ed. By F. S. C hapin III, R. L. J efferies , J. F. R eynolds , G. R. S haver , J. S voboda and E. W. C hu .