Mucin glycoproteins comprise a family of cellular adhesions that provide a layer of protection to oral epithelial tissues. Mucin proteins are secreted from the cell where they function in the oral cavity by creating a layer of resistance to microbes, by preventing the demineralization of teeth, and by promoting general lubrication. Mucins are also transcriptionally up-regulated in oral carcinomas (as well as other tumors), which become enriched for mucin production, making mucins ideal molecules for cancer detection and immunological cancer therapies. A recent series of advances demonstrates that mucins can function as signal transduction molecules that activate G-protein and MAP kinase dependent signaling pathways. This new aspect of mucin function is relatively uncharacterized. In this proposal we employ a genetically tractable model organism to understand mucin function and regulation. Specifically, we discovered a mucin receptor in the versatile model organism budding yeast. This mucin, called Msb2, is required for yeast cell-to-cell contact and for biofilm formation. In addition, Msb2 operates at the head of a Cdc42-dependent MAPK pathway. That is, Msb2 is a signaling mucin. We have also found that Msb2, like its mammalian mucin counterparts, is secreted from the cell, and we have identified a family of aspartyl proteases, called yapsins, that are required for liberation of Msb2 from the cell surface. As the proteases required for mucin processing in mammalian cells have yet to be identified, this finding is of broad significance and may be a general feature of mammalian mucins. In this proposal, we seek to understand at a detailed molecular level, the signaling and adhesion functions of this model mucin. In the first aim of this proposal, we propose to dissect the signaling and secretion properties of Msb2. We will confirm that yapsins cleave Msb2, and we will determine if the cleavage event is the cue that triggers receptor activation. In the second aim of this proposal we will purify the secreted form of Msb2 and determine its role in cell adherence and biofilm formation. Our objective is to fully characterize a fungal mucin using a model genetic system. As part of our long-term goals, we will extend our findings to mucin regulation in other systems. ? ? Mucins are an important class of adhesion molecules in oral tissues that have recently been shown to function as signaling molecules. Their ability to promote microbial resistance in oral tissues, and their prominence in metastasizing cancers makes these extremely relevant molecules from the perspective of human health. We use a model genetic system - budding yeast - to investigate mucin function in MAP kinase pathway activation, cell adherence, and biofilm formation. ? ? ?
