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S J Singer

Publications and source records attributed to S J Singer.

At least 37 records · Page 2Linked to original sources
PubMed1994

A possible role for stable microtubules in intracellular transport from the endoplasmic reticulum to the Golgi apparatus.

The intracellular transport of secretory proteins involves at an early stage the formation of vesicles from transitional elements of the endoplasmic reticulum (ER) containing these proteins and the transfer of these vesicles to the cis-face of the Golgi apparatus. We propose that the latter transfer process does not occur by random diffusion, but is instead mediated by tracking along stable microtubules. To test this proposal, we have carried out double immunoelectron microscopic labeling experiments on frozen sections of HepG2 hepatoma cells secreting the protein human serum albumin (HSA). By a cycloheximide treatment protocol, the stage during which the transfer of newly synthesized HSA from the ER to the Golgi apparatus occurs in vivo was determined. Sections of the cells were then double immunolabeled using primary antibodies to HSA and to glu-tubulin, the latter specifically detecting stable microtubules. We observed a significantly high frequency of HSA-containing structures between the ER and the Golgi apparatus with which stable microtubules were closely associated. These results support the proposal that stable microtubules may play a critical role in directing the transfer process from the ER to the Golgi apparatus.

Biological Transport, Active↗
PubMed1994

A single-payer system in Jackson Hole clothing.

President Clinton's Health Security Act relies on government regulation, not market forces, to control costs. The act creates an entitlement to comprehensive benefits and places the federal budget at risk for total health care costs in order to achieve universal coverage; it creates a system of new state purchasing monopsonies; and it attempts to control costs with price controls on health plan premiums, set and administered by a National Health Board that would be part of the executive branch, not insulated from political considerations. We believe there is a better way.

Cost Control↗
PubMed1994

Choosing among health plans.

The health care reform system that is ultimately adopted, suggest the authors of this analysis, should rely on market forces, rather than government regulation, to reduce cost and improve quality in our health care delivery. A major portion of this paper compares the Health Security act (the Clinton proposal), the American Health Security Act (the McDermott bill), the Health Equity and Access Reform Today, HEART (the Chafee proposal), and the Managed Competition Act (the Cooper bill). The article focuses on the major areas of difference between these alternative proposals in the extent to which they would achieve true market-based reform, small group reform, universal coverage, and financing mechanisms.

Evaluation Studies as Topic↗
PubMed1993

A soluble secretory protein is first concentrated in the endoplasmic reticulum before transfer to the Golgi apparatus.

A soluble secretory protein is usually present at a much higher concentration in the Golgi apparatus than in the endoplasmic reticulum (ER) inside eukaryotic secretory cells in the steady state. We show by immunoelectron microscopic experiments with the soluble secretory protein serum albumin, inside Hep-G2 human hepatoma cells in culture, that the secretory protein is first concentrated at isolated sites within the ER before it is transferred to the cis face of the Golgi apparatus. This is contrary to expectations of the bulk-flow hypothesis of ER-to-Golgi transfer, and it suggests the involvement of concentration and transfer mechanisms within the ER that have not previously been recognized.

Carcinoma, Hepatocellular↗
PubMed1992

Intercellular communication and cell-cell adhesion.

In developmental biology, binary cell-cell interactions often determine the fate of one or both cell partners. The two cells must adhere to one another to allow chemical signals to be transmitted in one or both directions across the regions of cell-cell contact. The molecular mechanisms of cell-cell adhesion and intercellular communication, even if they are mediated by different cell surface components, may be functionally integrated in several different ways. Studies of helper T cells with antigen-presenting B cells in culture have illuminated such binary interactions. The possible application of similar mechanisms to other binary developmental systems is briefly explored.

Animals↗
PubMed1992

Cek5, a membrane receptor-type tyrosine kinase, is in neurons of the embryonic and postnatal avian brain.

Cek5 is a recently identified receptor-type tyrosine kinase of the Eph subclass that is nearly ubiquitously expressed during embryonic development (Pasquale, 1991). Cek5 is predominantly expressed in the avian CNS throughout development, and high levels remain apparent in adult neurons. By means of immunofluorescence microscopy and high-resolution immunoelectron microscopy, Cek5 was found to be expressed in many regions of the chicken brain at various developmental stages, most notably in the hippocampus and cerebellum. The highest concentration of Cek5 was observed in the molecular layer of the cerebellum, associated within the axons of mature granule cells (parallel fibers) and with the cell bodies of immature granule cells. In the axons of parallel fibers, Cek5 was concentrated in the fasciculated nonsynaptic portions. This localization, together with the "adhesion" motifs present in the Cek5 extracellular region suggest that Cek5 may interact with other cell surface-associated molecules and be involved in the growth, guidance, and/or bundling of certain unmyelinated axonal processes. Alternatively (or in addition), Cek5 may represent the receptor for a neurotrophic substance, similar to several other neuronal transmembrane tyrosine kinases.

Animals↗
PubMed1990

Sequence and domain structure of talin.

Talin is a high-molecular-weight cytoskeletal protein concentrated at regions of cell-substratum contact and, in lymphocytes, at cell-cell contacts. Integrin receptors are involved in the attachment of adherent cells to extracellular matrices and of lymphocytes to other cells. In these situations, talin codistributes with concentrations of integrins in the cell surface membrane. Furthermore, in vitro binding studies suggest that integrins bind to talin, although with low affinity. Talin also binds with high affinity to vinculin, another cytoskeletal protein concentrated at points of cell adhesion. Finally, talin is a substrate for the Ca2(+)-activated protease, calpain II, which is also concentrated at points of cell-substratum contact. To learn more about the structure of talin and its involvement in transmembrane connections between extracellular adhesions and the cytoskeleton, we have cloned and sequenced murine talin. We describe a model for the structure of talin based on this sequence and other data. Homologies between talin and other proteins define a novel family of submembranous cytoskeleton-associated proteins all apparently involved in connections to the plasma membrane.

Amino Acid Sequence↗
PubMed1990

Embedded or not? Hydrophobic sequences and membranes.

The same translocation machinery appears to be responsible for both the translocation of soluble proteins across membranes and the insertion of integral membrane proteins into the bilayer. A single mechanism is proposed to accommodate these two functions. This model is also extended to explain the paradoxical translocation of mitochondrial and chloroplastic membrane proteins across one or more membranes before they are finally inserted into their target membranes.

Amino Acid Sequence↗
PubMed1990

Transfer of secretory proteins from the endoplasmic reticulum to the Golgi apparatus: discrimination between homologous and heterologous transfer in intact heterokaryons.

To examine aspects of the transfer of secretory proteins from the endoplasmic reticulum to the Golgi apparatus in situ, heterokaryons were formed between Hep G2 human hepatoma cells and WI-38 human fibroblasts. The cells were appropriately treated with cycloheximide before fusion, which emptied them of their respective secretory proteins, serum albumin for the Hep G2 cells and procollagen I for the WI-38 cells. After fusion was complete, the cycloheximide was washed out, protein synthesis was resumed, and the rates of reappearance of serum albumin and procollagen I in the two separated Golgi apparatuses within each heterokaryon were followed by immunofluorescence microscopy. Serum albumin was found to always reappear first in the Golgi apparatus contributed by the Hep G2 half of the heterokaryon, and procollagen I in the Golgi apparatus of the WI-38 half. These results suggest that the endoplasmic reticulum-to-Golgi apparatus transfer in situ is not simply a stochastic process but is either spatially restricted or exhibits cell-type specificity or both.

Carcinoma, Hepatocellular↗
PubMed1990

The early expression of myofibrillar proteins in round postmitotic myoblasts of embryonic skeletal muscle.

Previous reports on skeletal muscle myogenesis have shown that postmitotic spindle-shaped myoblasts express muscle-specific proteins, some of which are organized into nascent myofibrils. However, we show that, in skeletal muscle cultures derived from 12-day chick embryos, by 6 h after plating the predominant mononucleated cell type that expresses muscle-specific proteins is a round cell. These round myoblasts appear to precede spindle-shaped myoblasts in development, since the latter are more abundant in later cultures and contain larger amounts of muscle proteins and more highly organized myofibrils. By double immunofluorescence microscopy using antibodies specific for the muscle proteins titin, myosin heavy chain (MHC) and zeugmatin we find that 18 h after plating approximately 20% of the round myoblasts that are titin-positive are negative for myofibrillar MHC and zeugmatin. On the other hand, all spindle-shaped myocytes that are positive for titin are also positive for myofibrillar MHC and zeugmatin. These results suggest that titin expression precedes that of myofibrillar MHC and zeugmatin in the non-synchronized round myoblasts, and is consistent with earlier suggestions that titin may function as an initial organizer of myofibrillar proteins during myogenesis. Immunofluorescence data indicate that the earliest localization of the myofibrillar proteins titin, MHC, zeugmatin and alpha-actinin in the round myoblasts is surrounding the nucleus with no immunofluorescent labeling of the cytoplasm or near the plasma membrane. Furthermore, pairwise double immunofluorescence experiments show that these four myofibrillar proteins are all co-localized, at the light-microscopic level of resolution, in irregular patterns that may appear in either a punctate or a basket-like distribution. These labeling patterns around the nucleus are resistant to extraction with Triton X-100, suggesting that the proteins are associated in a stable array. These Triton X-100-resistant assemblies in round myoblasts appear to be composed solely of structural myofibrillar proteins, since the non-structural myofibrillar protein creatine kinase (CK) does not colocalize with the other myofibrillar proteins. These results indicate that in early myoblasts myofibrillar proteins form stable pre-myofibrillar assemblies surrounding the nucleus, and raise the possibility that these initial assemblies may play an organizing role during subsequent early stages of myofibrillogenesis.

Animals↗
PubMed1990

The PMA-induced specific association of LFA-1 and talin in intact cloned T helper cells.

Previous experiments with individual cell couples formed between cloned T helper (Th) cells and antigen-presenting cells have led us to suggest that the cytoskeletal protein talin may be associated with the cell surface protein LFA-1 in the Th cell. In order to examine this suggestion, we induced the surface capping of LFA-1 with suitable specific antibody reagents on the intact Th cells, and then determined by double immunofluorescence microscopic experiments, whether talin was co-clustered with the LFA-1 caps. With untreated Th cells, capping of LFA-1 did not result in any redistribution of intracellular talin. However, if the intact Th cells were treated with the phorbol ester PMA, the capping of LFA-1 resulted in a co-clustering of talin with the LFA-1 caps, but not a alpha-actinin. The capping of TCR or CD4 on the Th cells with or without pretreatment with PMA did not lead to any such co-clustering of talin with these caps. PMA treatment of the Th cells therefore induces a direct or indirect association of talin with LFA-1 underneath the Th cell surface. PMA treatment of the Th cells also increased their polarized spreading and adherence to substrata, as had been observed before. We found, furthermore, that this increased adherence upon PMA-treatment was inhibited by the presence of antibodies to LFA-1. The association of talin, and very likely also F-actin microfilaments, with LFA-1 appears to mediate a generalized increased adhesivity of the Th cells. The relevance of these findings with isolated Th cells to the interaction of Th cells with specific antigen-presenting cells is discussed.

Animals↗