The Novel ATP-Binding Cassette Protein ARB1 Is a Shuttling Factor That Stimulates 40S and 60S Ribosome Biogenesis

Abstract

ARB1 is an essential yeast protein closely related to members of a subclass of the ATP-binding cassette (ABC) superfamily of proteins that are known to interact with ribosomes and function in protein synthesis or ribosome biogenesis. We show that depletion of ARB1 from Saccharomyces cerevisiae cells leads to a deficit in 18S rRNA and 40S subunits that can be attributed to slower cleavage at the A0, A1, and A2 processing sites in 35S pre-rRNA, delayed processing of 20S rRNA to mature 18S rRNA, and a possible defect in nuclear export of pre-40S subunits. Depletion of ARB1 also delays rRNA processing events in the 60S biogenesis pathway. We further demonstrate that ARB1 shuttles from nucleus to cytoplasm, cosediments with 40S, 60S, and 80S/90S ribosomal species, and is physically associated in vivo with TIF6, LSG1, and other proteins implicated previously in different aspects of 60S or 40S biogenesis. Mutations of conserved ARB1 residues expected to function in ATP hydrolysis were lethal. We propose that ARB1 functions as a mechanochemical ATPase to stimulate multiple steps in the 40S and 60S ribosomal biogenesis pathways.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/.

ACKNOWLEDGMENTS

We thank Leos Valasek, Sander Granneman, Susan Baserga, and Sungpil Yoon for suggestions and advice; Boots Quimby for technical assistance with confocal microscopy; Maurice Swanson for RPL39 antibodies; Jan van't Riet for RPS22 antibodies; Thorsten Schafer and Herbert Tschochner for the RPL25-GFP and RPS2-GFP plasmids; Yoshiko Kikuchi for the NOP1-GFP plasmid; and Kate Abruzzi and Michael Rosbash for the crm1-T539C strain.

J.L.J. is supported by NIH grants GM64779 and HL68744. A.J.L. is supported by NIH grants GM64779, HL68744, ES11993, and CA098131.