Mak21p of Saccharomyces cerevisiae, a Homolog of Human CAATT-binding Protein, Is Essential for 60 S Ribosomal Subunit Biogenesis

Mak21-1 mutants are unable to propagate M1 double-stranded RNA, a satellite of the L-A double-stranded RNA virus, encoding a secreted protein toxin lethal to yeast strains that do not carry M1. We clonedMAK21 using its map location and found that Mak21p is homologous to a human and mouse CAATT-binding protein and open reading frames in Schizosaccharomyces pombe and Caenorhabditis elegans. Although the human protein regulates Hsp70 production, Mak21p is essential for growth and necessary for 60 S ribosomal subunit biogenesis. mak21-1mutants have decreased levels of L-A coat protein and L-A double-stranded RNA. Electroporation with reporter mRNAs shows thatmak21-1 cells cannot optimally express mRNAs which, like L-A viral mRNA, lack 3′-poly(A) or 5′-cap structures but can normally express mRNA with both cap and poly(A). The virus propagation phenotype of mak21-1 is suppressed byski2 or ski6 mutations, each of which derepresses translation of non-poly(A) mRNA.