UCSF

To survive, cells must be able to properly sense and respond to information in a fluctuating environment. Inside a cell, information flows through discrete signaling pathways built around specific networks of protein interactions. However, it is difficult for the cell to ensure signaling fidelity in the face of a broad range of stimuli, a high internal concentration of proteins - many of which are related, and signaling pathways that use shared elements. For example in the yeast mating mitogen-activated protein kinase (MAPK) pathway, extracellular input activates a three-tiered kinase cascade (MAPKKK Ste11 phosphorylates MAPKK Ste7 which in turn phosphorylates MAPK Fus3), and leads to the mating response. However a similar, but distinct pathway, the filamentation pathway also uses the same MAPKKK Ste11 and MAPKK Ste7 to activate a related MAPK, Kss1. How are these pathways assembled via protein interactions and what prevents signals from crossing over to the wrong pathway?

Cells utilize a general strategy of subcellular compartmentalization to direct the flow of information. Spatial regulation is achieved by targeting proteins to organelles and membranes, or at a molecular level by assembling proteins into distinct signaling complexes. Modular docking interactions are the basis for wiring most three-tiered MAPK kinase cascades. However, docking interactions alone are not sufficient for the MAPKK Ste7 to discriminate between Fus3 and Kss1 MAPKs; they bind competitively to the same motif on Ste7. Instead a scaffold protein, Ste5, is required to selectively activate Fus3 during the mating response. Unexpectedly, the scaffold functions not only as a passive tethering platform but also actively controls signaling by co-catalyzing phosphorylation between the MAPKK Ste7 and the MAPK Fus3 (enhancing kcat nearly 5000-fold). This unique scaffold mechanism answers a long-standing question in the field about why there is limited signal crosstalk between two pathways that share the same upstream components, Ste11 and Ste7.