Characterization and quantitation of fatty acids covalently bound to erythrocyte membrane proteins: anion transporter contains 1 mol of fatty acid thiol ester

Palmitoylation is not required for trafficking of human anion exchanger 1 to the cell surface

AE1 (anion exchanger 1) is a glycoprotein found in the plasma membrane of erythrocytes, where it mediates the electroneutral exchange of chloride and bicarbonate, a process important in CO2 removal from tissues. It had been previously shown that human AE1 purified from erythrocytes is covalently modified at Cys-843 in the membrane domain with palmitic acid. In this study, the role of Cys-843 in human AE1 trafficking was investigated by expressing various AE1 and Cys-843Ala (C843A) mutant constructs in transiently transfected HEK-293 cells. The AE1 C843A mutant was expressed to a similar level to AE1. The rate of N-glycan conversion from high-mannose into complex form in a glycosylation mutant (N555) of AE1 C843A, and thus the rate of trafficking from the endoplasmic reticulum to the Golgi, were comparable with that of AE1 (N555). Like AE1, AE1 C843A could be biotinylated at the cell surface, indicating that a cysteine residue at position 843 is not required for cell-surface expression of the protein. The turnover rate of AE1 C843A was not significantly different from AE1. While other proteins could be palmitoylated, labelling of transiently transfected HEK-293 cells or COS7 cells with [3H]palmitic acid failed to produce any detectable AE1 palmitoylation. These results suggest that AE1 is not palmitoylated in HEK-293 or COS7 cells and can traffic to the plasma membrane.

Kx, a quantitatively minor protein from human erythrocytes, is palmitoylated in vivo

Kx is a quantitatively minor blood group protein of human erythrocytes which is thought to be a membrane transporter. In the red cell membrane, Kx forms a complex stabilized by a disulfide bond with the Kell blood group membrane protein which might function as a metalloprotease. The palmitoylation status of these proteins was studied by incubating red cells with [3H] palmitic acid. Purification of the Kell-Kx complex, by immunochromatography on an immobilized human monoclonal antibody of Kell blood group specificity demonstrated that the Kx but not the Kell protein is palmitoylated. Six cysteines in Kx are predicted to be intracytoplasmic and might be targets for palmitoylation. Three of these cysteines are present in a portion of sequence which is predicted to form an amphipathic alpha helix. Palmitoylation of one or several of these cysteines might contribute to anchor the cytoplasmic portion of the Kx protein to the inner surface of red cell membrane.

Palmitoylation of endogenous and viral acceptor proteins by fatty acyltransferase (PAT) present in erythrocyte ghosts and in placental membranes

Human erythrocyte ghosts were shown to have palmitoylating activity which acylates both endogenous ghost polypeptides and exogenous proteins derived from Semliki Forest virus (SFV). Cell-free fatty acid transfer from [3H]palmitoyl-CoA to endogenous protein was greatly enhanced in ghosts when pre-existing fatty acids linked to the endogenous acyl proteins were removed by hydroxylamine treatment prior to the transfer reaction. In contrast to erythrocyte acyl proteins acceptor proteins present in human placental membranes were palmitoylated in vitro to a similar extent with or without prior deacylation by hydroxylamine treatment. This indicates the presence of large pools of non-acylated proteins in placenta and small pools in erythrocytes. In testing for the protein substrate specificity of the palmitoyl transferase (PAT) present in ghosts we found that the SFV acceptor proteins, which are totally unrelated to erythrocytes, competed with the palmitoylation of endogenous ghost protein acceptors. This palmitoylating enzyme is inhibited by Cibacron Blue, SDS, and heat treatment, but stimulated in the presence of low concentrations of mild detergent (TX-100). Since PAT operating at the surface membrane of red blood cells has properties very similar to those of PAT present in human placental microsomes [1], we suggest that only one type of PAT may transfer fatty acids to various acylproteins that occur at multiple locations in different tissues [2].

Anion transport function of mouse erythroid band 3 protein (AE1) does not require acylation of cysteine residue 861

Cys-861 of mouse band 3 is equivalent to Cys-843 of human band 3, the only acylated cysteine residue in the anion exchanger AE1 of the red blood cell (Hamasaki et al. (1992) Progress Cell Res. 2, 65-71). Mutation of Cys-861 to serine or methionine caused no significant changes of band 3-mediated anion exchange as measured after expression of the appropriate cRNAs in Xenopus oocytes. Susceptibility to inhibition of transport by 4,4'-dinitrostilbene-2,2'-disulfonate and PCMBS was not affected. We conclude that palmitoylation is not an absolute requirement for the successful execution the anion transport function by the hydrophobic domain of band 3 in the plasma membrane.

Fatty acylation of a 55 kDa membrane protein of human erythrocytes

The major palmitoylated human erythrocyte membrane protein has an M(r) of 55,000. It is distinct from the glucose transporter and is not derived from band 3 or ankyrin. It resists salt extraction suggesting a high affinity for the membrane. Pulse chase experiments demonstrate that palmitoylation is a dynamic process, and it may therefore have regulatory significance in membrane protein-protein or protein-lipid interaction. Slower dynamics of palmitoylation in erythrocytes from patients suffering from chronic myelogenous leukemia, which are less stable than normal erythrocytes, strengthen this view.

Thiol-fatty acylation of the glucose transport protein of human erythrocytes

Incubation of intact human erythrocytes with [3H]palmitate labeled a protein with electrophoretic characteristics of the glucose transporter. This labeling occurred via a thioester linkage, since it was unaffected by organic solvent extraction, but was substantially removed as the hydroxamate upon treatment with neutral hydroxylamine. Immunoprecipitation of the labeled protein with a monoclonal antibody to the glucose transporter confirmed its identity.

The rapidly phosphorylated 25 kDa polypeptide of the light- harvesting complex of photosystem II is encoded by the type 2 cab-II genes

The main light-harvesting complex of Photosystem II (LHC II) in higher plants consists of two sub-populations. The 'inner' pool consists only of a 27 kDa polypeptide, whereas in the 'outer' pool both the 27 kDa and a 25 kDa polypeptide are found. We purified the 25 and the 27 kDa LHC II polypeptides from Scots pine and 25 kDa LHC II polypeptide from spinach. Protein sequencing after cleavage with endoproteinase Lys-C showed that the 25 kDa polypeptide is encoded by the Type 2 cab-II genes and the 27 kDa polypeptide by the Type I cab-II genes. A fatty acid was not covalently attached to the peptides assembled into the pigment-protein complex. Our results show that the different polypeptides seen on a gel are different gene products, and not the result of different processing.