Moss - What genes cant do - 2003 (522929), страница 24
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Whatis not immediately evident however, is whether and to what extent themembrane protein-cytoskeletal interactions are biologically specific andinformation-rich in nature. This question was more directly addressedby studies that examined the partitioning of membrane proteins intomobile and immobile fractions.Studies carried out on the NIH 3T3 mouse fibroblast cell line examined the mobility of two cell-surface receptors, the insulin receptor andreceptor for epidermal growth factor (EGF). In both cases the mobilefraction was between 40 and 80 percent of the total receptor population(Schlessinger 1978). Interestingly, the mobile fraction plummeted towardzero when the temperature was raised from 23°C to 37°C.
This wasattributed to the likelihood of receptors being aggregated or internalizedA Critique of Pure (Genetic) Information91at the elevated temperature. The ability of a chemical messenger, suchas insulin or a growth factor, to elicit biological responses in receptivecells is contingent on a cascade of events occurring after the chemicalmessenger is bound at the cell surface.
These often involve the internalization of the chemical messenger–cell surface receptor complex. Specificinternalization of bound receptors implies the presence of a cellularapparatus capable of selectively picking out the right molecules for internalization. Such mechanisms, which are the stock-in-trade of “signaltransduction” processes, have long since been well characterized. We willconsider further evidence for the specificity of membrane protein immobilization but first briefly comment on the possibly larger implications ofthe cases referred to above. It was reported that for both insulin andEGF receptors that raising the temperature from 23°C to 37°C resultedin a complete loss of diffusional mobility. What is of particular interestabout this is that based on thermodynamics the loss of diffusionalfreedom with an increase in temperature is the exact opposite of whatone would expect.
In order for heightened temperature to result in theloss of entropy, the cell pushes the process up a thermodynamic hill byexpending its own energy stores.The idea that an organism can buffer itself against noxious perturbations such as heat fluxes is of course nothing new. But how far down theorganizational hierarchy would one expect this capacity to be found?The immobilization of cell-surface-membrane proteins in response toheat is suggestive of a biological stress response resulting in the loss,rather than the gain, of entropy.
When considered down to the level ofmolecular dynamics within the cell, it suggests that biological resistanceto thermodynamically driven entropic heat decay obtains all the waydown to the most basic fabric of living matter.Schrödinger had no window on the negentropic dynamics of intracellular molecular processes. His hereditary code-script vision is one whichpartitions the source of entropy resistance to within the nucleus, andthe fact of such a partition continues to be implied by the rhetorical tradition that distinguishes the genome as the source of biological information.
Empirically, I suggest, there just is no such partition to be foundor any asymmetrical flow of “order.” Schrödinger’s order-from-orderdescriptor well characterizes the cell as a whole—but only as a whole.92Chapter 3The cell’s system of membrane-based compartmentalization, posttranslational modification, and transport provides perhaps the best subcellular analogue to Schrödinger’s clockwork mechanism that operatesas if it were invulnerable to thermal fluctuations. Like the clockworkthe membrane flow system of the cell is in constant motion.
Unlike theclockwork it resists heat decay not through the rigidity of its parts andsteric constraints on their motions but rather through a colossal systemof gated checkpoints. The cell in effect “plays off” of thermodynamicbarriers. It uses high-energy thresholds, such as that of membrane fusion,to limit the occurrence of fusion events to those in which cellular energystores are mobilized but under strict constraints delineated by the orderwhich is heritably embedded in the system itself. The goal of this chapterwill continue to be that of providing a purview on of how biologicalorder is multifaceted, distributed, and systematic.Subsequent articulation of the premise that immobilization ofmembrane proteins is associated with the biologically specific binding ofproteins to cytoskeletal or other constituents on the cytoplasmic surfacefocused on the use of the red blood cell for study.
Mammalian red bloodcells have been a mainstay of plasma membrane study because they areeasily obtained and the plasma membrane can be readily isolated. Redblood cells, which are already enucleated (devoid of a nucleus), can bepurged of their hemoglobin (their major cytoplasmic constituent) andyet will continue to retain the approximate size and shape of the original cell.
Such experimental preparations have been referred to as red-cell“ghosts.” The red-cell ghosts can be prepared in either normal orinverted, inside-out forms.Red blood cells are required during the course of their journey throughthe circulatory system to undergo major deformations. A biconcave diskof 8 mm, it is drawn through capillaries of less than 2 mm in diameter(Goodman et al. 1983). The red cell’s elasticity and resistance to mechanical damage is derived from a meshwork of protein filaments that adhereclosely to the cytoplasmic side of the plasma membrane.
The majorprotein of the meshwork, comprising 75 percent of cytoplasmic protein,is the heterodimer1 “spectrin.” While red cells have certain unique features and requirements, subsequent work has continued to find a widevariety of cells with elements in common with the red-cell membrane.A Critique of Pure (Genetic) Information93Differences between the membrane structures of various cell types occuralong a continuum.The predominant transmembrane protein of red cells is an anionchannel referred to as “Band 3” and found to be present in approximately 106 copies per cell.
Band 3 contains a large region that extendsinto the cytoplasm. The principal components of the “membraneskeleton” found on the cytoplasmic face of red cells include the a & bspectrin subunits, actin—which is a ubiquitous component of cytoskeletal structures (as well as muscle tissue)—and a protein called ankyrin.Comparison of spectrin binding to inside-out versus right-side out“ghosts” demonstrated the 10-fold greater preference for binding to thecytoplasmic surface (accessible on the inside-out preparation). Treatmentof the cytoplasmic surface with a protein-degrading enzyme (chymotrypsin) resulted in the loss of 90 percent of the spectrin binding sites(Bennett 1978).
The protein fragments released by chymotrypsin digestion were assayed for their ability to inhibit, through competition, thebinding of spectrin to the cytoplasmic surface of the red-cell membranes.The fragment found to accomplish this was one derived from ankyrin(Luna 1979). Having established the principle linkage between spectrinand another (peripheral) component of the cytoplasmic membrane skeleton, similar methods were undertaken to establish that ankyrin is alsothe principal linkage between the transmembrane protein Band 3 and thecytoplasmic membrane skeleton (Hargreaves et al. 1980).The image that emerges from these findings is one in which spectrinheterodimers form an intricate latticework on the cytoplasmic surface ofthe membrane with periodic attachment points to ankryin.
Ankyrinserves as a kind of adapter molecule—itself being fastened to the membrane by linkages with Band 3—that traverses the membrane. The distinction between the mobile and immobile fraction of the Band 3molecules could then be addressed by consideration of the stoichiometry or numerical ratios of the Band 3 and ankryin molecules. Band 3, assuggested above, was found to be present in approximately 106 copiesper cell whereas ankyrin was found to be present in only about one-tenthof this amount.
Band 3, however, is thought to form dimers and higherorder aggregates. The binding of Band 3 in the form of tetramers (groupsof four) to single molecules of ankyrin would, for example, be94Chapter 3consistent with the partitioning of Band 3 into mobile and immobile fractions along the numerical lines observed. Further support for this modelwas derived from subsequent studies on the nature of diffusional constraints on Band 3.Without providing technical details, it was found that the rate of diffusion of the mobile Band 3 fraction and the ratio of mobile to immobile Band 3 fractions (i.e., the respective size of the fractions) could beuncoupled and modulated independently.
While the rate of diffusion wasshown to be a function of the steric hindrance of untethered Band 3 molecules by the spectrin lattice, the immobilization of Band 3 was a function of its being tethered by ankryin. This picture gained further credencethrough consideration of rotational, as opposed to lateral, diffusion.While the binding of Band 3 directly to ankyrin would inhibit rotationaldiffusion, the steric hindrance of the mobile Band 3 fraction would notbe expected to affect its rotational movement. Nigg and Cherry (1980)examined the affects of proteolytically cleaving the cytoplasmic portionof Band 3 on its rotational diffusion and found enhancement of only 40percent of the Band 3 molecules, a result consistent with the idea thatonly those Band 3 molecules (presumably about 40%) bound to ankyrinwere rotationally inhibited.
Since the time of these early pioneeringstudies on the structure of the red-cell membrane subsequent studies havedemonstrated that the movement of proteins within the plane of themembranes of all cell types are regulated by highly specific, biologicallysignificant mechanisms.The standard rationale for speaking of genes in the conflationarystyle—as the “information,” “blueprint,” “program,” “instructions,”and so forth for building an organism—is that DNA provides the template for synthesizing proteins and that proteins, as enzymes, regulate allof the chemical reactions of the cell. For this rationale to hold up it mustbe the case that either (1) spatial arrangements of enzymes in the cell areof no great consequence or (2) that spatial arrangement is somehow prefigured and predetermined by the one-dimensional array of nucleic acidsin the genes.
A principal objective of this chapter is to provide evidenceand an argument to the effect that neither of these is the case. Thiscurrent rationale stands in a close and not accidental proximity toSchrödinger’s argument. If Schrödinger was correct in assuming thatA Critique of Pure (Genetic) Information95thermodynamics prohibited anything outside of the aperiodic crystalfrom playing a central role in the continuity of living order, then thespatial arrangement of proteins in the cell could not in itself be of muchconsequence. Whether the movement from Schrödinger’s thermodynamicargument to this standard rationale has ever been made explicitly or not,it is at least implicit in the continuity of conflationary gene-centered talkfrom the hereditary code-script through present-day programs and blueprints.
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