Ustilago maydis rak1 mediated gene expression during exponential growth

In mammalian cells RACK1 serves as a scaffold protein that has a role in integrating inputs from different signaling pathways and affects translation through association with ribosomes. U. maydis contains a seven-WD40 repeat motif protein designated Rak1, which shows 68% identity to RACK1 and is homologous to Asc1p of Saccharomyces cerevisiae. The asc1 mutant could be complemented by introduction of U. maydis rak1. The deletion of rak1 affected cell growth, cell wall integrity and specifically attenuated cell fusion. This latter defect was caused by reduced expression of prf1 encoding the regulator for pheromone and pheromone-receptor genes. Rak1 interacts with a variety of ribosomal proteins and microarray analysis revealed that the deletion of rak1 led to severely reduced expression of rop1, an activator prf1. The constitutive expression of rop1 could rescue the defect of mfa1 expression as well as conjugation tube formation in response to pheromone induction in the rak1 mutant. Moreover, a solopathogenic rak1 mutant failed to respond to plant derived stimuli, resulting in attenuated filamentation and pathogenicity. This could be partially rescued by constitutive expression of the b heterodimer. These data suggest that rak1 is a regulator of rop1 expression with additional roles after cell fusion. Ustilago maydis strains FB1 and FB1∆rak1 were grown on a rotary shaker (200 rpm) in liquid Ustilago complete medium with 1% glucose at 28°C to an OD600 of 0.5. The experiment was performed using three independent biological replicates per strain.