Proteins That Interact with the Mucin-Type Glycoprotein Msb2p Include Regulators of the Actin Cytoskeleton

Transmembrane mucin-type glycoproteins can regulate signal transduction pathways. In yeast, signaling mucins regulate mitogen-activated protein kinase (MAPK) pathways that induce cell differentiation to filamentous growth (fMAPK pathway) and the response to osmotic stress (HOG pathway). To explore regulatory aspects of signaling mucin function, protein microarrays were used to identify proteins that interact with the cytoplasmic domain of the mucin-like glycoprotein, Msb2p. Eighteen proteins were identified that comprised functional categories of metabolism, actin filament capping and depolymerization, aerobic and anaerobic growth, chromatin organization and bud growth, sporulation, ribosome biogenesis, protein modification by iron-sulfur clusters, RNA catabolism, and DNA replication and DNA repair. A subunit of actin capping protein, Cap2p, interacted with the cytoplasmic domain of Msb2p. Cells lacking Cap2p showed altered localization of Msb2p and increased shedding of Msb2ps N-terminal glycosylated domain. Consistent with its role in regulating the actin cytoskeleton, Cap2p, and another Msb2p-interacting protein, Aip1p, were required for the enhanced cell polarization during filamentous growth. Our study identifies proteins that connect a signalling mucin to diverse cellular processes and may provide insight into new aspects of mucin function.