Component of the KEOPS complex that is probably involved in the transfer of the threonylcarbamoyl moiety of threonylcarbamoyl-AMP (TC-AMP) to the N6 group of A37, a step in the formation of a threonylcarbamoyl group on adenosine at position 37 (t(6)A37) in tRNAs that read codons beginning with adenine. Cgi121 stimulates Bud32 kinase activity via an activation of Bud32 autophosphorylation. Component of the KEOPS complex that consists of Kae1, Bud32, Cgi121 and Pcc1; the whole complex dimerizes. Belongs to the CGI121/TPRKB family.