LYSOSOMAL INTEGRAL MEMBRANE-PROTEIN-II BINDS THROMBOSPONDIN-1 - STRUCTURE-FUNCTION HOMOLOGY WITH THE CELL-ADHESION MOLECULE CD36 DEFINES A CONSERVED RECOGNITION MOTIF

LIMPII (lysosomal integral membrane protein II) is one of a family of proteins structurally related to the cell surface glycoprotein CD36. W e recently defined a single structural domain on CD36 that mediates bi nding to adhesive glycoprotein thrombospondin-l (TSP1). The CD36-TSP1 interaction is known to play a role in platelet-tumor and platelet-mon ocyte adhesion, angiogenesis, and in monocyte uptake of apoptotic cell s. To test whether LIMPII also binds TSP1, a LIMPII peptide correspond ing to the TSP1 binding domain of CD36 was expressed as a recombinant glutathione S-transferase (GST) fusion protein. In solid phase binding assays, purified I-125-TSP1 bound to immobilized GST/LIMPII in a time -dependent and saturable manner. Inhibition by excess unlabeled TSP1 o r EDTA demonstrated specificity. LIMPII.TSP1 complex formation was spe cifically blocked by soluble LIMPII fusion protein, by monospecific ra bbit IgG directed against the LIMPII peptide and by CD36 fusion protei ns containing the TSP1 binding domain. Transfection of Bowes melanoma cells with a chimeric LIMPII cDNA that targets expression to the plasm a membrane conferred the ability to bind I-125-TSP1 and to adhere to T SP1-coated surfaces. This study defines a TSP1 binding site conserved between LIMPII and CD36 and suggests that cell surface LIMPII may func tion in some circumstances as an adhesion receptor for TSP1. Computer- assisted homology searches suggest that the TSP1 recognition motif ide ntified from study of CD36 family members may be widely expressed in n ature.