Crystal structure of Rcl1, an essential component of the eukaryal pre-rRNA processosome implicated in 18s rRNA biogenesis.

Rcl1 is an essential nucleolar protein required for U3 snoRNA-guided pre-rRNA processing at sites flanking the 18S rRNA sequence. A potential catalytic role for Rcl1 during pre-rRNA cleavage has been suggested based on its primary structure similarity to RNA 3'-terminal phosphate cyclase (Rtc) enzymes, which perform nucleotidyl transfer and phosphoryl transfer reactions at RNA ends. Here, we report the 2.6 Å crystal structure of a biologically active yeast Rcl1, which illuminates its modular 4-domain architecture and overall homology with RNA cyclases while revealing numerous local differences that account for why Rtcs possess metal-dependent adenylyltransferase activity and Rcls do not. A conserved oxyanion-binding site in Rcl1 was highlighted for possible catalytic or RNA-binding functions. However, the benign effects of mutations in and around the anion site on Rcl1 activity in vivo militate against such a role.