Signaling Mucin in Yeast

Abstract

In two separate screens for genes involved in the filamentous growth (FG) pathway in yeast, Cullen et al. identified MSB2 , which encodes a cell surface protein with a single transmembrane domain and a large extracellular domain with Ser-Thr-Pro-rich repeats typical of mammalian glycoproteins called mucins. MSB2 expression was stimulated by treatments that activated the FG pathway and was dependent on the FG pathway-specific transcription factor Ste12. Yeast with mutations in msb2 were defective in agar invasion and single-cell invasion assays, as well as pseudohyphal growth and activation of FG target genes, which confirmed the involvement of Msb2 as a component in the FG pathway. Sho1 is a plasma membrane protein involved in both the FG pathway and the osmosensing pathway. Overexpression of Msb2 or Sho1 causes hyperinvasive growth, and yeast with mutant sho1 or msb2 show the same phenotype. Cullen et al. observed an interaction between Sho1 and Msb2 using coimmunoprecipitation assays. In various assays, the cytoplasmic domain of Msb2 interacted with Cdc42, the guanosine triphosphatase that is downstream of Sho1 in the FG pathway. Deletion of the mucin domain or an N-terminal domain distinct from the mucin domain of Msb2 resulted in constitutive activation of the FG pathway. Glycosylation of Msb2 may be important for its negative effect on the FG pathway, and glycosylation-defective yeast show activation of the FG pathway that is Msb2- dependent. Yeast with deletion of the mucin domain of Msb2 (Msb2*) exhibit hyperactivity in the FG pathway, including increased phosphorylation and abundance of the FG-specific mitogen-activated protein kinase (MAPK) Kss1. Responsiveness to mating factor mediated by the mating pathway-specific MAPK Fus3 was not affected in yeast with mutant msb2 . Msb2 was also not required for activation of the osmosensing pathway. Thus, Msb2 appears to be a FG pathway-specific protein that controls the activity of the FG pathway by interacting with both Sho1 and Cdc42. P. J. Cullen, W. Sabbagh Jr., E. Graham, M. M. Irick, E. K. van Olden, C. Neal, J. Delrow, L. Bardwell, G. F. Sprague Jr., A signaling mucin at the head of the Cdc42- and MAPK-dependent filamentous growth pathway in yeast. Genes Dev. 18 , 1695-1708 (2004). [Abstract] [Full Text]