Properties of Wnt-1 protein that enable cell surface association.

Members of the highly conserved Wnt gene family serve key functions in cellular growth and differentiation. Regulated expression of the murine Wnt-1 proto-oncogene is essential for normal development of the embryonic nervous system and when misexpressed in the mammary gland leads to hyperplasias and tumorigenic progression. Wnt-1 encodes a cysteine rich glycoprotein that is secreted and tightly associated with the cell surface and/or extracellular matrix. This restricted diffusion of Wnt-1 protein is important for specifying a spatially defined pattern of Wnt-activity necessary for cell to cell signaling in cell growth and differentiation. Since the receptors and cell surface molecules that bind Wnt proteins are as yet unknown we have sought to define the sequences within Wnt-1 protein that enable cell surface association and to define the nature of the cell surface structures with which Wnt-1 protein associates. First, site directed mutagenesis was used to identify a basic amino acid sequence motif within Wnt-1 protein that is required for this protein to accumulate on the cell surface. Second, wild type Wnt-1 protein was expressed in a series of proteoglycan-deficient CHO cell lines to show that, contrary to expectations, cellular glycosaminoglycans are not required for processing, secretion, cell surface association and accumulation of Wnt-1 protein.