Complement receptor 3 of macrophages is associated with galectin-1-like protein

We have previously identified a 16-kDa protein with a pI of 5.1 (P16/5.1) that is associated with macrophage CR3. Microsequencing of P16/5.1 indicated exclusive homology to the beta-galactoside-binding lectin, galectin-1. Abs specific to a galectin-1 unique peptide reacted with P16/5.1. The association of P16/5.1 with CR3 was specifically inhibited by lactose, which binds with high affinity to galectin-1. These data together with similarities in molecular mass and pI suggest that P16/5.1 is galectin-1. Two-color immunofluorescence staining revealed the expression of galectin-1 on the macrophage surface and its colocalization with CR3. However, a surplus of CR3 was free of galectin-1, and some galectin-1 molecules were associated with cell surface receptors other than CR3. Based on these results we propose two models depicting the functional significance of CR3-galectin-1 association: 1) homodimeric galectin-1 possessing a divalent sugar binding site may act as an extracellular adapter molecule that cross-links CR3 with other receptors; and 2) association of galectin-1 with beta-galactosides on the extracellular domain of CR3 may modify the binding affinity of the receptor to its ligand. These possibilities are not mutually exclusive and can clarify the mode by which CR3 transmits signals in macrophages.