Using antibodies against Dictyostelium membranes to identify an actin-binding membrane protein

Polyclonal antibodies made against Dictyostelium discoideum membranes were used to block the interaction of those membranes with actin. As expected, actin interacted mostly with the internal surface of the membrane, demonstrated by the fact that whole cells could only absorb out a minor fraction of the blocking antibody. The antibody was used to show that the membrane component(s) which interacted with actin were probably integral; they could be extracted with detergent but not with solutions designed to extract peripheral membrane proteins. To identify the responsible protein(s), Western transfers of membranes were cut into fractions which were tested for their ability to absorb out the blocking activity of the antibody. We observed a single peak at a molecular weight of approximately 20,000, and thus conclude that a 20,000-mol-wt protein is a major integral membrane actin-binding protein in Dictyostelium. This approach to the identification of proteins involved in actin-membrane interaction has allowed us to make the first identification of an actin-binding membrane protein which is based on its activity in native membranes.

Isolation of an actin-binding protein from membranes of Dictyostelium discoideum

We prepared a probe of radiolabeled, glutaraldehyde cross-linked filamentous actin (F-actin) to study binding of actin to membranes of Dictyostelium discoideum. The probe bound to membranes or detergent extracts of membranes with a high affinity and in a saturable manner. The binding could be reduced by boiling of either the actin probe or the membranes, or by addition of excess native F-actin, but not by addition of an equivalent amount of bovine serum albumin, to the assay. The probe labeled several proteins when used to overlay sodium dodecyl sulfate gels of Dictyostelium membranes. One of these labeled proteins was a 24,000-mol-wt protein (p24), which was soluble only in the presence of a high concentration of sodium deoxycholate (5%, wt/vol) at room temperature or above. The p24 was purified by selective detergent extraction and column chromatography. When tested in a novel two-phase binding assay, p24 bound both native monomeric actin (G-actin) and F-actin in a specific manner. In this assay, G-actin bound p24 with a submicromolar affinity.

A Ca2+ insensitive actin-crosslinking protein from Dicytostelium discoideum

We have isolated a 30,000-dalton protein from Dictyostelium which cosedimented with and affected the low shear viscosity of actin. At low concentrations, this protein increased the low shear viscosity to greater than that of the actin control, whereas higher concentrations decreased viscosity. The viscosity decrease correlated with the formation of actin filament bundles, as seen electron microscopically. This protein resembled a previously reported actin-binding protein from Dictyostelium [Fechheimer and Taylor, 84, J Biol Chem 259:4514] in electrophoretic mobility, Stokes radius, and ability to crosslink filaments, but was shown to be different by peptide mapping, lack of immunologic crossreactivity, and lack of sensitivity to calcium.

Connectin: cell surface protein that binds both laminin and actin

A purified cell surface receptor protein for laminin (Mr = 70,000) isolated from mouse fibrosarcoma cells binds to actin with specificity and high affinity. This binding was demonstrated both by cosedimentation of the receptor with actin and binding of the receptor to actin immobilized on nitrocellulose filters. Specificity was demonstrated by displacement of 35S-labeled receptor by unlabeled receptor. Scatchard analysis of receptor binding to actin yielded a Kd of 6 X 10(-7) M. The receptor was observed to reduce the viscosity of actin filaments. It also caused the formation of bundles of parallel filaments. This observation and the stoichiometry of binding suggest that the receptor binds along the sides of actin filaments. Based on the ability of this receptor to bind both extracellular laminin and intracellular actin, we have named this protein "connectin." Connectin may be an example of a transmembrane protein that is capable of mediating the interaction of a cell with its extracellular matrix.

Academic health centers

There are 123 academic health centers in the United States, and they are markedly diverse in organization and function. Some have large research programs, others emphasize the education of nurses and allied health professionals, but all have one characteristic in common--namely, the dominant role of the medical school-teaching hospital combination. Their evolution has been shaped to a great degree by four federal initiatives: funding of research and research training by the National Institutes of Health, legislation that permitted close relations between Veterans Administration hospitals and medical schools, health-manpower legislation, and Medicare and Medicaid. Although academic health centers were created to foster the integration of structure and function, federal funding has always been categorical in support of research, teaching, or patient care. No federal funding was ever intended to stabilize the overall academic health center as an institution. This mattered little during a period of expansion, but the future of academic health centers is now uncertain in a period of federal cutbacks, rising health-care costs, and worry about an oversupply of physicians. Academic health centers must enter a new phase of institutional planning for which they are ill equipped. Special interests must be submerged for the good of the whole, diversity must be encouraged, and each center should exploit its own special strengths.

Cell type specific inhibition of cAMP phosphodiesterase activity during terminal differential in Dictyostelium discoideum

Cyclic AMP phosphodiesterase (PDE) activity reaches a peak during the aggregation stage of development where it functions to regulate extracellular levels of cAMP. During the subsequent differentiation of the two cell types at the culmination stage, the activity reappears but only in stalk cells. We found that extracts from the culmination stage contained PDE which could be activated by preincubation with Mg2+ and dithiothreitol (DTT), a treatment which is known to release an endogenous inhibitor from the aggregation stage enzyme. When the culmination stage extracts were subjected to chromatography on Biogel P300, two peaks of activity were eluted, PDE-I (Mr greater than 260,000) and PDE-II (Mr 100,000). Treatment of the fractions with Mg-DTT did not affect the low-molecular-weight enzyme but caused activation of the high-molecular-weight enzyme and the appearance of a third, intermediate form. Kinetic analysis of the two peaks revealed Km values for cAMP of 2 mM and 10 microM for PDE-I and PDE-II, respectively. We tested the possibility that these forms of the enzyme might be distributed differently in the two cell types by measuring the Km for cAMP and the effect of Mg-DTT treatment on isolated sections of stalk and spore cells. The spore sections contained a high Km form of the enzyme (0.3 mM) which was activated by preincubation with Mg . DTT whereas stalk sections contained a low Km form (3 microM) which was not affected by the activation treatment. We conclude that both cell types contain enzyme protein and that the apparent localization of PDE activity in stalk cells is due to the inhibition of activity in spore cells.

Purification and properties of a cyclic-AMP phosphodiesterase that is active in only one cell type during the multicellular development of Dictyostelium discoideum

Cyclic-AMP phosphodiesterase (PDE) accumulates during the aggregation stage of Dictyostelium where it functions in maintaining extracellular levels of cyclic AMP (cAMP). The activity decreases during the subsequent multicellular slug stage and then accumulates again during sorocarp construction, but the enzyme is active only in the developing stalk. Because of the possible significance of this localized activity in only one of the two cell types, we have purified the enzyme from the multicellular stage in order to understand its mode of regulation in vivo. We find that the enzyme which is localized in the prestalk cells is similar in many respects to the extracellular PDE which is active at the aggregation stage. The enzyme from both stages is inhibited by a low molecular weight protein. The mechanism of this inhibition is through a shift in the apparent Km for cAMP from micromolar to millimolar levels. The inhibited form of the enzyme can be activated by preincubation with MgSO4 and dithiothreitol (DTT). This activation treatment releases the inhibitor from the enzyme, thus restoring the low Km form, changes the molecular weight of the culmination stage enzyme from 95 000-100 000 to 68 000 by releasing the Mr 35 000-40 000 inhibitor protein, and causes irreversible loss of inhibitor activity. Although the inhibitor could be obtained in high yield from the aggregation stage by simply heating the extracellular fluid, it could not be detected from culmination stage extracts when prepared by this method. However, inclusion of calcium in the extraction buffer resulted in release of inhibitor from both heated and nonheated samples. The results indicate that the stalk cell specific PDE is regulated similarly to the aggregation stage PDE and opens the possibility of differential regulation of PDE in the two cell types.

A 40,000-dalton protein from Dictyostelium discoideum affects assembly properties of actin in a Ca2+-dependent manner

A 40,000-dalton protein that affects the assembly properties of actin in a Ca2+-dependent manner has been purified from Dictyostelium discoideum. Gel filtration chromatography indicates that the native form of this protein is a monomer. A major effect of this protein is to reduce the sedimentability of F-actin in a stoichiometric fashion. Nearly complete loss of sedimentability is observed at ratios of the 40,000-dalton protein to actin of greater than 1:10. At low stoichiometries, this protein can accelerate the rate of actin assembly under certain experimental conditions. These effects of the 40,000-dalton protein on the actin assembly properties described above require calcium ion. The 40,000-dalton protein does not exert its effects by proteolyzing actin. Furthermore, peptide maps demonstrate that this protein is not a proteolytic fragment of actin.

Mechanism of action of cytochalasin: evidence that it binds to actin filament ends

To test the idea that cytochalasin retards actin assembly by binding to filament ends, we have designed a new assay for cytochalasin binding in which the number of filament ends can be varied independently of the total actin concentration. Actin is reacted with polylysine-coated polystyrene beads to make filament ends (Brown and Spudich, 1979, J. Cell Biol. 80:499-504) and then reacted with [3H]cytochalasin B. We have found that cytochalasin B binds to beads in the presence of actin, and that the number of cytochalasin B binding sites can be varied as a function of the number of filament ends independent of the total actin concentration by varying the bead concentration.

Laboratory support for rural health care

Lack of technical facilities to support diagnostic and curative medicine discourages physicians from providing rural health care. In developing countries, therefore, the first decision on most health problems is often made by auxiliaries. Such staff, in conducting basic community medicine, collect specimens for appropriate investigations. Simple tests performed by the auxiliaries themselves can assist in routine surveillance, in establishing priorities of medical care and in systematizing referrals to health centres. The simplest screening tests--of weight, temperature and haemoglobin--are the most useful. Serial weighing in childhood can monitor malnutrition, acute or chronic infections and infestations; in young adults, it can point to ill-health requiring further investigation. The finding of fever and examination of a blood film may reveal acute bacterial infection, malnutrition, malaria, hookworm, bilharzia and sickling, and thus be followed up by a thin blood film, or tests for sickle-cell and stool hookworm ova, pus cells or amoeba. The repertoire of simple laboratory investigations is largely completed by examination of sputum for acid-fast bacilli, or diplococci; by urine microscopy, with haematuria; and tests for protein or sugar, in cases of oedema or polyuria. A limited range of more elaborate tests may be available in the laboratories of district or regional hospitals. Human and technological factors which bear on the modus operandi of rural laboratories are identified and discussed.

Localization of cyclic nucleotide phosphodiesterase in the multicellular stages of Dictyostelium discoideum

Cyclic 3',5'-adenosine monophosphate (cAMP) is secreted as the chemotactic signal by aggregating amoebae of the cellular slime mold Dictyostelium discoideum. We have used ultramicrotechniques in the biochemical analysis of cyclic nucleotide phosphodiesterase (PD) distribution in individual aggregates at various stages of development. With handmade constriction pipettes in microliter volumes, sections of lyophilized individuals weighing 20-100 ng could be assayed in a reaction coupled to 5'-nucleotidase. Phosphodiesterase activity was measured at pH 7.5 with 12 microM cAMP, cAMP-PD activity in aggregates ranged from 20-40 mmol/h/kg. In the pseudoplasmodium it had dropped to 5-10 mmol/h/kg and a difference in activity between the anterior prestalk cells and posterior prespore cells began to appear. The utmost posterior sections showed elevated phosphodiesterase from this stage onward. During culmination, activity rose to 40-60 mmol/h/kg associated with the developing stalk, while it declined in the spore mass. The papilla remained constant at 5-10 mmol/h/kg. The pattern of localization in the stalk was the same when cGMP was used as substrate. Extracellular phosphodiesterase inhibitor produced at the aggregation stage was found to reduce the localized activity in the culmination stage by 50-80%, with the most marked inhibition occurring in the center of the papilla. We found no evidence of endogenous heat-stable phosphodiesterase inhibitor within the culminating sorocarp.

Cytochalasin inhibits the rate of elongation of actin filament fragments

Submicromolar concentrations of cytochalasin inhibit the rate of assembly of highly purified dictyostelium discoideum actin, using a cytochalasin concentration range in which the final extent of assembly is minimally affected. Cytochalasin D is a more effective inhibitor than cytochalasin B, which is in keeping with the effects that have been reported on cell motility and with binding to a class of high-affinity binding sites from human erythrocyte membranes (Lin and Lin. 1978. J. Biol. CHem. 253:1415; Lin and Lin. 1979. Proc. Natl. Acad. Sci. U.S.A. 76:2345); 5x10(-7) M cytochalasin B lowers it to 70 percent of the control value, whereas 10(-7) M cytochalasin B lowers the rate to 25 percent. Fragments of F-actin were used to increase the rate of assembly fivefold by providing more filament ends on to which monomers could add. Under these conditions, cytochalasin has an even more dramatic effect on the assembly rate; the concentrations of cytochalasin B and cytochalasin D required for half-maximal inhibition are 2x10(-7) M and 10(-8) M, respectively. The assembly rate is most sensitive to cytochalasin when actin assembly is carried out in the absence of ATP (with 3 mM ADP present to stabilize the actin). In this case, the concentrations of cytochalasin B and cytochalasin D required for half-maximal inhibition are 4x10(-8) M and 1x10(-9) M, respectively. A scatchard plot has been obtained using [(3)H]cytochalasin B binding to F-actin in the absence of ATP. The K(d) from this plot (approximately 4x10(-8) M) agrees well with the concentration of cytochalasin B required for half-maximal inhibition of the rate of assembly under these conditions. The number of cytochalasin binding sites is roughly one per F-actin filament, suggesting that cytochalasin has a specific action on actin filament ends.