SYNTHESIS, PROCESSING, AND INTRACELLULAR-TRANSPORT OF CD36 DURING MONOCYTIC DIFFERENTIATION

CD36 is an integral membrane glycoprotein expressed by several cell ty pes, including endothelial cells of the microvasculature, erythrocytes , platelets, and monocytes, In the monocytic lineage, CD36 is expresse d during the late stages of differentiation in the bone marrow, in cir culating monocytes, and in some tissue resident macrophages, and it is thought to mediate the phagocytosis of apoptotic cells and the endocy tic uptake of modified lipoproteins. Here we analyze the synthesis, pr ocessing, and intracellular transport of CD36 in U937 and THP-1, two h uman cell lines representing different stages of monocytic maturation. In both cell lines, phorbol 12-myristate 13-acetate induces the expre ssion of CD36. A 74-kDa intracellular precursor is first synthesized t hat has the hallmarks of a resident protein of the endoplasmic reticul um, The precursor protein is later processed into a mature form of 90- 105 kDa which is transported to the cell surface, The kinetics of proc essing differ significantly in U937 and THP-1. These differences are s pecific for the CD36, as two unrelated proteins (CD11b and CD45R) are processed and transported to the surface at similar rates in the two c ell lines. A 33-kDa endoglycosidase H-sensitive glycoprotein specifica lly associates with the 74-kDa precursor. Coprecipitation of gp33 corr elates with slow processing of CD36 precursor, suggesting that gp33 ma y play a role in regulating the intracellular transport of CD36, durin g monocyte maturation.