The Pseudokinase Domain of Saccharomyces cerevisiae Tra1 Is Required for Nuclear Localization and Incorporation into the SAGA and NuA4 Complexes

Tra1 is an essential component of the SAGA/SLIK and NuA4 complexes in S. cerevisiae, recruiting these co-activator complexes to specific promoters. As a PIKK family member, Tra1 is characterized by a C-terminal phosphoinositide 3-kinase (PI3K) domain. Unlike other PIKK family members (e.g. Tor1, Tor2, Mec1, Tel1), Tra1 has no demonstrable kinase activity. We identified three conserved arginine residues in Tra1 that reside proximal or within the cleft between the N- and C-terminal subdomains of the PI3K domain. To establish a function for Tra1's PI3K domain and specifically the cleft region, we characterized a tra1 allele where these three arginine residues are mutated to glutamine. The half-life of the Tra1_Q3 protein is reduced but its steady state level is maintained at near wild-type levels by a transcriptional feedback mechanism. The tra1_Q3 allele results in slow growth under stress and alters the expression of genes also regulated by other components of the SAGA complex. Tra1_Q3 is less efficiently transported to the nucleus than the wild-type protein. Likely related to this, Tra1_Q3 associates poorly with SAGA/SLIK and NuA4. The ratio of Spt7^SLIK to Spt7^SAGA increases in the tra1_Q3 strain and truncated forms of Spt20 become apparent upon isolation of SAGA/SLIK. Intragenic suppressor mutations of tra1_Q3map to the cleft region further emphasizing its importance. We propose that the PI3K domain of Tra1 is directly or indirectly important for incorporating Tra1 into SAGA and NuA4 and thus the biosynthesis and/or stability of the intact complexes.

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