Title Skb 5 , an SH 3 adaptor protein , regulates Pmk 1 MAPK signaling by controlling the intracellular localization of Mkh 1 MAPKKK Authors

The MAPK cascade is a highly conserved signaling module composed of MAPK/MAPKK/MAPKKK. MAPKKK Mkh1 is an initiating kinase in Pmk1 MAPK signaling, which regulates cell integrity in fission yeast. Our genetic screen for regulators of Pmk1 signaling identified Skb5 (Shk1 kinase binding protein 5), an SH3 domain-containing adaptor protein. Here, we showed that Skb5 serves as an inhibitor of Pmk1 MAPK signaling activation by downregulating Mkh1 localization to cell tips via its interaction with the SH3 domain. Consistently, the Mkh1 mutant protein, with impaired Skb5 binding, remained in the cell tips, even when Skb5 was overproduced. Intriguingly, Skb5 needs Mkh1 to localize to the growing ends as Mkh1 deletion and disruption of Mkh1 binding impairs Skb5 localization. Deletion of Pck2, an upstream activator of Mkh1, impaired the cell tip localization of Mkh1 and Skb5 as well as Mkh1/Skb5 interaction. Interestingly, both Pck2 and Mkh1 localized to the cell tips at the G1/S phase, which coincided with Pmk1 MAPK activation. Altogether, Mkh1 localization to cell tips is important for transmitting upstream signaling to Pmk1 and Skb5 spatially regulates this process.