A photoactivable phospholipid analogue that specifically labels membrane cytoskeletal proteins of intact erythrocytes

Functional involvement of proteins, interacting with sphingolipids, in sphingolipid transport to the canalicular membrane in the human hepatocytic cell line, HepG2?

A photoreactive sphingolipid precursor was used to investigate the potential involvement of protein-lipid interactions that may convey specificity to sphingolipid transport in the human hepatoma cell line, HepG2. A 125I-labeled, photoreactive ceramide, 125I-N3-Cer, was incubated with the cells and became incorporated into two sphingolipid products. The major product was photoreactive sphingomyelin (125I-N3-SM) (25% of total radioactivity), while only minor amounts of photoreactive glucosylceramide (125I-N3-GlcCer) were formed (< 2%). After photoactivation, a restricted number of proteins was labeled. Given the absolute amounts of the newly synthesized, photoreactive lipids and their precursor present in the cells, labeling of the proteins can be assumed to be derived from interaction with either ceramide (Cer) or sphingomyelin (SM), or both. To discriminate between these possibilities, photoactivation and protein analysis was performed in cells treated with D-threo-1-phenyl-2-decanoyl amino-3-morpholino-1-propanol (PDMP), an inhibitor of sphingolipid biosynthesis. In treated cells, the radioactive SM pool was reduced by approximately 80%. Concomitantly, labeling of a 60-kd protein, seen in control cells, decreased. Furthermore, the 60-kd protein is membrane-associated and insoluble in detergent at low temperature. Moreover, when cells containing photoreactive sphingolipids after a preincubation with the photoreactive Cer were photoactivated and subsequently incubated with fluorescent sphingolipid analogs, transport of the latter to the bile canalicular membrane, as observed in control cells, was inhibited. Taken together, the data suggest that distinct proteins, among them a 60-kd protein, may play a specific and functional role in sphingolipid transport to the bile canalicular membrane.

The role of inositol phospholipids in the association of band 4.1 with the human erythrocyte membrane

Band 4.1 is a major protein of the erythrocyte membrane skeleton. It promotes the binding of spectrin to F-actin and may anchor the skeletal network to the plasma membrane via its association with integral membrane proteins. Here, we have investigated the involvement of inositol phospholipids in the binding of band 4.1 to erythrocyte membranes using membrane vesicles stripped of all peripheral proteins at alkaline pH. Trypsinization of these vesicles allows the discrimination of two classes of band 4.1 binding sites: trypsin-sensitive sites (60-65% of the total), largely or exclusively on band 3, and trypsin-resistant sites (35-40% of the total), composed, at least in part, of the glycophorins. ATP depletion or activation of erythrocyte phosphoinositol phospholipase C led to a reduction in membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] content by 20-70% in different experiments. The resulting decrease of band 4.1 binding to vesicles by was variable, but averaged about 15-20%. The same treatments led to an average decrease in the band 4.1 binding capacity of trypsinized vesicles of 55%. Since this is equivalent to a 20% decrease in the binding capacity of non-trypsinized vesicles (consistent with the above result), it indicates that PtdIns(4,5)P2 regulates the binding of band 4.1 only to trypsin-resistant binding sites (and to only a subset of these) accounting for about 15-20% of total band 4.1 binding sites on membranes. We found that hydrolysis of > 95% of PtdIns(4,5)P2 with exogenous phospholipase C-delta (PLC delta) resulted in no further decrease in band 4.1 binding to vesicles than did hydrolysis of 65-70% of PtdIns(4,5)P2 which is accessible to erythrocyte phosphoinositol phospholipase C. This suggests that only 65-70% of total membrane PtdIns(4,5)P2 is involved in regulating band 4.1 binding. Significantly, the pool of PtdIns(4,5)P2 involved is the same pool which can be hydrolysed by erythrocyte phosphoinositol phospholipase C, and which has been shown to be metabolically labile in erythrocytes. The membrane binding capacity for band 4.1 found in this study (averaging 1000 micrograms/mg vesicle protein) is considerably higher than that found in previous studies. The results are consistent with the existence of a binding site for band 4.1 on each copy of the major transmembrane proteins (band 3 and the glycophorins). These results provide new insights into the involvement of membrane inositol phospholipids in cytoskeletal-membrane interactions.

Saturable binding of halothane to rat brain synaptosomes

The hypothesis that volatile anesthetics act directly on or bind specifically to membrane proteins remains controversial. In earlier in situ electron probe microanalysis studies in cardiac muscle we showed preferential partitioning of halothane into mitochondria. To determine whether partitioning represents saturable binding or simple solubility, a photoaffinity labeling method was developed for halothane to examine binding in rat brain synaptosomes. Radioligand binding assays were then used to determine binding parameters for this important inhalational anesthetic. UV-light exposure of synaptosomes incubated with clinical concentrations of [14C]halothane resulted in sufficient labeling to allow characterization of binding sites. Analysis of saturation and competition curves showed that greater than 60% of [14C]halothane photolysis product binding to synaptosomes was specific, with low affinity (Kd = 0.49 +/- 0.16 mM) and high binding site concentration (Bmax = 1.87 +/- 0.75 nmol/mg of protein). Halothane photoaffinity labeling was partially inhibited by isoflurane (20%), chloroform (44%), 2-bromotrifluoroethane (20%), and dichlorotrifluoroethane (20%) but not by ethanol. The Kd measured with this photoaffinity approach is similar to the concentration of halothane required to produce anesthesia in rats.

Bilayer/cytoskeleton interactions in lipid-symmetric erythrocytes assessed by a photoactivable phospholipid analogue

Two mechanisms have been proposed for maintenance of transbilayer phospholipid asymmetry in the erythrocyte plasma membrane, one involving specific interactions between the aminophospholipids of the inner leaflet of the bilayer and the cytoskeleton, particularly spectrin, and the other involving the aminophospholipid translocase. If the former mechanism is correct, then erythrocytes which have lost their asymmetric distribution of phospholipids should display altered bilayer/cytoskeleton interactions. To test this possibility, normal erythrocytes, erythrocytes from patients with chronic myelogenous leukemia or sickle disease, and lipid-symmetric and -asymmetric erythrocyte ghosts were labeled with the radioactive photoactivable analogue of phosphatidylethanolamine, 2-(2-azido-4-nitrobenzoyl)-1-acyl-sn-glycero-3-phospho[14C]ethanolamine ([14C]AzPE), previously shown to label cytoskeletal proteins from the bilayer. The labeling pattern of cytoskeletal proteins in pathologic erythrocytes and lipid-asymmetric erythrocyte ghosts was indistinguishable from normal erythrocytes, indicating that the probe detects no differences in bilayer/cytoskeleton interactions in these cells. In contrast, in lipid-symmetric erythrocyte ghosts, labeling of bands 4.1 and 4.2 and actin, and to a lesser extent ankyrin, by [14C]AzPE was considerably reduced. Significantly, however, labeling of spectrin was unaltered in the lipid-symmetric ghosts, suggesting that its relationship with the bilayer is normal in these lipid-symmetric cells. These results do not support a model in which spectrin is involved in the maintenance of an asymmetric distribution of phospholipids in erythrocytes.