A carboxyl terminal truncation mutant of CD36 is secreted and binds thrombospondin: evidence for a single transmembrane domain

CD36 has been implicated in several intracellular signalling events, including platelet and monocyte activation, and receptor-mediated internalization of bound ligands such as oxidized low-density lipoprotein and apoptotic neutrophils. These processes are presumably mediated by the intracytoplasmic domain(s) of the molecule. By analysis of hydrophobicity plots and by analogy to rat LIMPII, which has a 60% homology to CD36, a two-transmembrane domain model has been proposed. To characterize the structure-function relationships of CD36 involved in transducing the signal, we have defined the number of transmembrane and intracellular domains experimentally using a mutagenesis approach. A truncated CD36 cDNA was constructed that encodes a protein that terminates just proximal to the putative C-terminal transmembrane domain. This mutant was cloned into eukaryotic expression plasmid vectors to generate short-term and stable transfected cells. Our results indicate that the truncated mutant is secreted by the transfectants into the postculture medium, indicating that there is only one transmembrane domain in CD36, which is present at the C- terminal end. The soluble secreted protein from all of these cells is functional as indicated by its binding to thrombospondin.