In recent years considerable progress has been made in the understanding of the structure and function of the red blood cell membrane. The protein spectrin, of high molecular weight and propensity for self-association, appears to play a major role, in concert with actin, in maintaining the shape and integrity of the membrane. A study of the physical-chemical properties of spectrin, and its size, shape, self-association pattern, and its interaction with other components, leads to a plausible model for the way this protein performs its biological role. The evidence from the structure and interactions of spectrin suggests a structure which is relatively symmetrical yet highly expanded, and which allows extensive, two-dimensional network formation with actin. In these respects, the structure of spectrin is quite different from that of myosin, to which it has often been likened.
The self-association of human spectrin between 21 and 35 degrees C and between pH 6.5 and 9.5 has been studied at sedimentation equilibrium. For a given set of solution conditions between pH 6.5 and 8.5, coincidence of omega function plots as a function of total spectrin concentration (0-2 g/L) indicated that equilibrium was attained and that no significant concentration of solute was incapable of participating in the self-association reaction. Above pH 8.5, however, irreversible aggregation occurred, inferred from a failure of overlap in the omega function and molecular weight distributions. The behavior of spectrin can best be described by a cooperative isodesmic model, in which the promoter for association is the heterodimer and for which K12 is between 10(6) and 10(7) M-1 (depending on pH and temperature) and all other K are approximately 10(6) M-1. The returned values of the second viral coefficient for this model fall within the range calculated from the charge and Stokes radius of spectrin. Association appears to be favored slightly by decreased temperature and by decreased pH. The pH dependence resides only in K12 and is consistent with the presence of a single group, possibly histidine, displaying a slightly higher pKa value in the tetramer than in the dimer. The association reaction appears to be driven by the loss of enthalpy associated with release of strain in the heterodimer. The association sites appear to be conserved in the association reactions, consistent with the images from electron microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)
Seeking to resolve conflicting literature on cytoskeletal structure in mammalian "primitive" generation erythrocytes, we have utilized the circulating blood of developing marsupials. In young of the Tammar Wallaby (Macropus eugenii) and the Gray Short-tailed Opossum (Monodelphis domestica), relatively large, nucleated primitive erythrocytes constituted nearly 100% of the circulating population at birth (= day 0) and in fetuses (Tammar) several days before birth. These cells were discoidal or elliptical, and flattened except for a nuclear bulge. Their cytoskeletal system, consisting of a marginal band of microtubules enclosed within a cell surface-associated network (membrane skeleton), closely resembled that of non-mammalian vertebrate erythrocytes. By day 2 or 3, much smaller anucleate erythrocytes of "definitive" morphology, lacking marginal bands, appeared in abundance. These accounted for greater than 90% of the circulating population of both species by day 6-8. Non-nucleated erythrocytes of a different type, constituting 1-6% of the cells in most blood samples up to day 7, were identified as anucleate primitives on the basis of size, shape, and presence of a marginal band. Thus, loss of erythrocyte nuclei in mammals appears to begin earlier than generally recognized, i.e., in the primitive generation. Counts of these anucleate primitives in young of various ages implicated nucleated primitives as their probable source. Pointed erythrocytes, occasionally found in younger neonates of both species, occurred in greatest number in fetuses (Tammar) prior to birth. This is in accord with previous work on non-mammalian vertebrates suggesting that such cells are morphogenetic intermediates. The results confirm the long-suspected similarity between mammalian primitive erythrocytes and the nucleated erythrocytes of all non-mammalian vertebrates.
The proteins of erythrocyte membranes from the red kangaroo, western grey kangaroo, eastern grey wallaroo (euro), red-necked wallaby, Tammar wallaby, and brush-tail possum have been fractionated on acrylamide gels in the presence of sodium dodecyl sulfate. The pattern of proteins was remarkably similar between the different marsupial species. The pattern of Coomassie blue-staining proteins in the membranes of these species was also very similar to that of the human erythrocyte membrane. However, the glycoproteins in the marsupial erythrocyte membranes were markedly less conspicuous than those of the human erythrocyte membrane. Furthermore, the mobilities of the glycoproteins from the marsupials were different from those of the human erythrocyte membrane.
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