Fig. 1. The absence of inhibitory Cdk1 tyrosine phosphorylation causes severe morphological defects in U. maydis. (A) Cdk1 is phosphorylated at Tyr¹⁵. Protein extracts from wild-type and TAU17 cells expressing a FLAG-tagged version of Cdk1 (cdk1-1) were subjected to immunoblotting with anti-phospho-Cdc2 (Tyr15) and anti-PSTAIRE antibodies. Cdk2 is the U. maydis homolog of S. cerevisiae Pho85, which is also recognized by the anti-PSTAIRE antibody. (B) Expression of the ectopic myc-tagged Cdk1. Protein extracts from wild-type, and UMC5 and UMC6 cells expressing myc-tagged Cdk1 (P_tef1:cdk1-myc) and the constitutively unphosphorylated Cdk1 allele (P_tef1:cdk1^AF-myc), respectively, were immunoblotted with anti-PSTAIRE antibodies. (C) UMC5 and UMC6 cells were grown to mid-exponential phase in rich medium (YPD) and then stained with DAPI and FITC-WGA, to visualize nuclei and cell walls, respectively. Scale bars: 20 µm.