PubMed · 11676932
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A Lee. 2001-10-16. Membrane structure.. https://doi.org/10.1016/s0960-9822(01)00491-2
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Recently discovered macrocyclic carbon suboxide (MCS) factors with the general formula (C(3)O(2))(n) were found to strongly inhibit rabbit and rat Na,K-ATPase as well as SR Ca-ATPase. Highly active MCS factors were obtained by a base/acid treatment of their lipophilic precursor isolated from plants. In the ESI-MS spectra, the dominant molar mass ion of 431 Da corresponds to a 1:1 complex of the carbon suboxide hexamer (n=6; M(r)=408 Da) with a Na(+) ion. Additional mass ions identified in positive and negative ion mode were assigned as complexes of the MCS hexamer (n=6) and octamer (n=8) with Na(+) or with TFA(-) in various ratios. The dominant mass ion values of these active MCS factors from plants are also found in mass spectra of previously described endogenous digitalis-like factors (EDLF) from animals. This would suggest that ubiquitously distributed MCS factors may function as putative endogenous regulatory substances of Na,K-ATPase and possibly of other ATPases. With the symmetric display of several equivalent carbonyl or hydroxy groups, the structure of MCS factors is particularly suited for interactions with proteins and other bio-molecules. This could explain the high biological activity and the unusual properties of the MCS factors.
Chaperone proteins have been initially identified by their ability to confer cellular resistance to various stress conditions. However, molecular chaperones participate also in many constitutive cellular processes. Mitochondria contain several members of the major chaperone families that have important functions in maintaining mitochondrial function. The major Hsp70 of the mitochondrial matrix (mtHsp70) is essential for the translocation of cytosolic precursor proteins across the two mitochondrial membranes. MtHsp70 interacts with the preprotein in transit in an ATP-dependent reaction as it emerges from the translocation channel of the inner membrane. Together with two essential partner proteins, Tim44 and Mge1, mtHsp70 forms a membrane-associated import motor complex responsible for vectorial polypeptide movement and unfolding of preprotein domains. Folding of newly imported proteins in the matrix is assisted by the soluble chaperone system formed by mtHsp70 and its partner protein Mdj1. For certain substrate proteins, the protected folding environment that is offered by the large oligomeric Hsp60 complex facilitates further folding reactions. The mitochondrial Hsp70 Ssq1 is involved in the assembly of mitochondrial Fe/S clusters together with another member of the DnaJ family, Jac1. Chaperones of the Clp/Hsp100 family mediate the prevention of aggregation under stress conditions and eventually the degradation of mitochondrial proteins. Together, the chaperones of the mitochondrial matrix form a complex interdependent chaperone network that is essential for most reactions of mitochondrial protein biogenesis.
An increase in intracellular Ca(2+) occurs during ageing of human erythrocytes in vivo. The aged cells show a reduced capacity for active Ca(2+) extrusion. Such a defect may arise from pump proteolysis, due to calpain activation by the raised intracellular Ca(2+). To test this possibility, Ca(2+) pump phosphorylation by [gamma-(32)P]ATP was studied on percoll-separated young and old human erythrocytes. After phosphorylation for 30 s with Ca(2+), the amount of phosphoenzyme produced by the young cell membranes was 50% that of the old cells. With Ca(2+) plus La(3+), in contrast, the phosphoenzyme level was nearly the same in both preparations. After a prolonged phosphorylation period (50-90 s), the phosphoenzyme reached almost identical equilibrium levels in both membrane preparations. On the other hand, a single Ca(2+)-dependent radioactive band of about 150 kDa was apparent in both preparations after acidic electrophoresis. Likewise, Western blotting using 5F10 monoclonal antibody also detected a single band of similar molecular weight. These results demonstrate that there is no alteration in either molecular mass or number of active Ca(2+) pump units during cell ageing, thus indicating that the reduced Ca(2+) pumping activity of aged cells does not arise from pump proteolysis.