Membrane Associated Proteins

Apart from the indications for the association of changes in band 3 conformation with NA, there are no structural data that might help explain the formation of acanthocytes in these patients. The known association of band 3 with other proteins [1, 12, 25], however, leads to the suggestion that the association of cytosolic proteins with the membrane might be specifically altered in the erythrocytes of patients with NA.

Preliminary findings support this suggestion. Membrane proteins were separated by gel electrophoresis, digested with trypsin, and peptide sequences were established using a nano-HPLC system connected to a LTQ-Fourier mass spectrometer. Proteins were identified by searching peak lists containing fragmentation spectra against the NCBI Homo sapiens protein sequence data-base, and annotated by function, biological process and/or cellular localization. A first analysis of a proteomic inventory of the membrane fraction of erythrocytes from HDL2 patients shows no obvious changes in the number of membrane and cytoskeleton proteins. However, these data do indicate a NA-associated increase in the membrane association of metabolic enzymes, components of the proteasome, and small G proteins (Fig. 5). These results are in general agreement with the growing awareness, based on comparable pro-teomic analyses, of the complexity of the erythrocyte proteome, and more particularly of the presence of an active signal transduction network in the erythrocyte membrane regulating erythrocyte homeostasis [20]. The increase in the number of various small G proteins in the patients' erythrocytes is especially intriguing since, in nucleated cells, small G proteins coordinate formation of vesicles, their motility and tethering to their target compartment [30]. In addition, recent data on erythrocytes of mice genetically lacking Rac GTPase show that these proteins are also involved in the dynamic regulation of red cell membrane network, subsequently affecting red cell morphology [11]. Thus, in erythrocytes, these proteins may mediate aging-related membrane restructuring and vesiculation (Bosman et al., unpublished observations). The latter process may be of relevance to acanthocyte formation as well as to neurodegeneration, in view of the postulated role of chorein and junctophilins in vesicle-mediated protein trafficking and/or internal membrane-plasma membrane interactions [26].

Fig. 5 Proteomic analysis shows an increased membrane recruitment of metabolic enzymes, components of the ubiquitin-proteasome system, and small G proteins in erythrocytes from an HDL2 patient. NA, neuroacanthocytosis; C-OLD, old erythrocytes (fraction V; see also the legend to Fig. 2) from a control donor

Fig. 5 Proteomic analysis shows an increased membrane recruitment of metabolic enzymes, components of the ubiquitin-proteasome system, and small G proteins in erythrocytes from an HDL2 patient. NA, neuroacanthocytosis; C-OLD, old erythrocytes (fraction V; see also the legend to Fig. 2) from a control donor

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