FORMATION OF ONE OR MORE INTRACHAIN DISULFIDE BONDS IS REQUIRED FOR THE INTRACELLULAR PROCESSING AND TRANSPORT OF CD36

In monocytes/macrophages, CD36 is thought to have a role as a scavenge r receptor, mediating the phagocytosis of apoptotic cells and the endo cytic uptake of oxidized low-density lipoproteins and fatty acids. The proposed topology of CD36 predicts that, of ten cysteine residues, si x lie in the extracellular domain, whereas four are equally distribute d in the two short terminal tails flanking the N-terminal and C-termin al hydrophobic stretches. Here we investigate the formation of intrach ain disulphide bonds, on the basis of the assumption that the cysteine residues present in the luminal domains are generally oxidized, where as those in the cytosol are reduced. As revealed by gel mobility-shift assays, disulphide bonds are present in the extracellular domain of t he CD36 molecule. The formation of these bonds is required for the tra nsport of CD36 from endoplasmic reticulum to Golgi. Furthermore reacti ve thiol groups are present in the CD36 sequence, which upon lysis for m an intrachain extra loop as an artifact. This disulphide bond is not formed in either (1) truncated CD36 lacking the two C-terminal cystei ne residues or (2) Triton X-100-insoluble wild-type CD36 molecules, su ggesting that, in this fraction, the C-terminal thiol groups are modif ied.