Spb1p is a putative methyltransferase required for 60S ribosomal subunit biogenesis in Saccharomyces cerevisiae

Several mutants (spb1-spb7) have been previously identified as cold-sensiti ve extragenic suppressors of loss-of-function mutations in the poly(A)C-bin ding protein 1 of Saccharomyces cerevisiae. Cloning, sequence and disruptio n analyses revealed that SPB1 (YCL054W) encodes an essential putative S-ade nosyl-methionine-dependent methyltransferase, Polysome analyses showed an u nder-accumulation of 60S ribosomal subunits in the spb1-1 mutant and in a s train genetically depleted of Spb1p, Northern and primer extension analyses indicated that this was due to inhibition of processing of the 27SB precur sors, which results in depletion of the mature 25S and 5.8S rRNAs, At later time points of Spb1p depletion, the stability of 40S ribosomal subunits is also affected, These results suggest that Spb1p is involved in 60S ribosom al subunit biogenesis and associates early with the pre-ribosomes. Consiste nt with this, hemagglutinin epitope-tagged Spb1p localizes to the nucleus w ith nucleolar enrichment. Despite the expected methyltransferase activity o f Spb1p, global methylation of pre-rRNA is not affected upon Spb1p depletio n. We propose that Spb1p is required for proper assembly of pre-ribosomal p articles during the biogenesis of 60S ribosomal subunits.