Separate RNA-binding surfaces on the multifunctional La protein mediate distinguishable activities in tRNA maturation

Abstract

By sequence-specific binding to 3' UUU-OH, the La protein shields precursor (pre)-RNAs from 3' end digestion and is required to protect defective pre-transfer RNAs from decay. Although La is comprised of a La motif and an RNA-recognition motif (RRM), a recent structure indicates that the RRM beta-sheet surface is not involved in UUU-OH recognition, raising questions as to its function. Progressively defective suppressor tRNAs in Schizosaccharomyces pombe reveal differential sensitivities to La and Rrp6p, a 3' exonuclease component of pre-tRNA decay. 3' end protection is compromised by mutations to the La motif but not the RRM surface. The most defective pre-tRNAs require a second activity of La, in addition to 3' protection, that requires an intact RRM surface. The two activities of La in tRNA maturation map to its two conserved RNA-binding surfaces and suggest a modular model that has implications for its other ligands.

Molecular Chaperones and Quality Control in Noncoding RNA Biogenesis

“…Since the typical RNA interaction surface of the central RRM, a four-stranded antiparallel β-sheet, does not contact the 3´ uridylates (Teplova et al 2006), it could stabilize other pre-tRNA elements. Consistent with this hypothesis, conserved residues in the RRM are necessary for maturation of several structurally impaired pre-tRNAs in Schizosaccharomyces pombe (Huang et al 2006). Similarly, as the usual surface used by winged helix domain proteins to recognize double-stranded DNA is not involved in binding of La to 3´ ends (Dong et al 2004;Teplova et al 2006), this surface may be involved in other interactions.…”

Partitioning of the Nuclear and Mitochondrial tRNA 3′-End Processing Activities between Two different Proteins in Schizosaccharomyces pombe

“…Because trz1 is essential, we used a strain carrying a temperature-sensitive allele of trz1 (trz1-1) (39) to evaluate the role of trz1 in nuclear tRNA 3Ј-end processing. We examined the processing of two well characterized nuclear tRNAs (tRNA AAC Val and tRNA CUU Lys ) in the trz1-1 and wild-type strains by Northern blotting using the intron-specific and mature oligonucleotide probes as described previously (5,39,(42)(43)(44)(45). These two tRNAs are encoded by the most abundant intron-containing tRNA genes in S. pombe, and thus, their processing intermediates can be easily detected.…”

“…tRNA processing and modification steps include removal of 5Ј-and 3Ј-extensions, splicing of introns (for only a subset of nuclear pre-tRNAs), base modifications, and addition of 3Ј-CCA sequence (2)(3)(4). For most nuclear pre-tRNAs, 5Ј-end processing occurs prior to 3Ј-end processing, and binding by the conserved La protein to pre-tRNAs is required for the orderly processing of tRNA and for the endonucleolytic processing of pre-tRNA 3Ј-ends (5,6). In the absence of the yeast La protein, the 3Ј-end of nuclear tRNA is matured by exonuclease Rex1p (7,8).…”

Competition between the Rex1 exonuclease and the La protein affects both Trf4p-mediated RNA quality control and pre-tRNA maturation

“…Studies of tRNA maturation in budding and fission yeasts have resulted in a model in which La sequesters pre-tRNA 39 ends from exonucleases, favoring trailer removal by an endonuclease (Van Horn et al 1997;Yoo and Wolin 1997;Huang et al 2006). Although the identity of the endonuclease that matures the trailer in yeast has not been reported, work in other organisms has revealed that pre-tRNA trailers are removed by the endonuclease RNase Z (Redko et al 2007;Spath et al 2007).…”