Saccharomyces cerevisiae is a generally accepted model organism to study the establishment and maintenance of cell polarity. In order to polarize, the yeast cells have to accumulate the GTPase Cdc42 in its active GTP bound form at the growth tip of the cell. The scaffold protein Bem1p performs an important function in this process by simultaneously binding to Cdc42GTP, its GEF Cdc24, and certain effector proteins of Cdc42GTP. We show that Bem1p can channel the flow of Cdc42GTP from Cdc24 to its effectors through a SH3 domain-based interaction network. Channelling becomes essential once the concentration of Cdc42GTP is artificially reduced. We further show that the composition of this network changes during the cell cycle. During bud growth Bem1p links Cdc42 and Cdc24 to PAK kinases and effectors of exocytosis and the actin cytoskeleton. Upon entry into mitosis and during cytokinesis Bem1 becomes actively isolated from these effectors. This behaviour of Bem1 reflects its changing localization during different stages of the cell cycle. Using isolated domains and mutants thereof we try to understand the localization of Bem1 to these different compartments.