A bona fide La protein is required for embryogenesis in Arabidopsis thaliana

Abstract

Searches in the Arabidopsis thaliana genome using the La motif as query revealed the presence of eight La or La-like proteins. Using structural and phylogenetic criteria, we identified two putative genuine La proteins (At32 and At79) and showed that both are expressed throughout plant development but at different levels and under different regulatory conditions. At32, but not At79, restores Saccharomyces cerevisiae La nuclear functions in non-coding RNAs biogenesis and is able to bind to plant 3′-UUU-OH RNAs. We conclude that these La nuclear functions are conserved in Arabidopsis and supported by At32, which we renamed as AtLa1. Consistently, AtLa1 is predominantly localized to the plant nucleoplasm and was also detected in the nucleolar cavity. The inactivation of AtLa1 in Arabidopsis leads to an embryonic-lethal phenotype with deficient embryos arrested at early globular stage of development. In addition, mutant embryonic cells display a nucleolar hypertrophy suggesting that AtLa1 is required for normal ribosome biogenesis. The identification of two distantly related proteins with all structural characteristics of genuine La proteins suggests that these factors evolved to a certain level of specialization in plants. This unprecedented situation provides a unique opportunity to dissect the very different aspects of this crucial cellular activity.

Processing of a Dicistronic tRNA-snoRNA Precursor: Combined Analysis in Vitro and in Vivo Reveals Alternate Pathways and Coupling to Assembly of snoRNP

“…In addition, it is also in agreement with kinetics reported for processing of tRNA precursors in other eukaryotes, in which 5# leader cleavage by RNase P is the first event and precedes 3#end processing (O'Connor and Peebles, 1991;Wolin and Matera, 1999). Processing of tsnoRNA 3#ends probably implicates the recently identified Arabidopsis La protein, which was found associated with the 3#end of the tsnoR43 precursor (Fleurdepine et al, 2007). The La protein, Lhp1p in yeast, is a conserved protein in eukaryotes that binds to the U extension and is required for processing of the 3#ends of RNA polymerase III transcripts.…”

A comprehensive analysis of the La-motif protein superfamily

“…1). With the exception of groups A and E, each of these clusters contains previously recognized functional genuine La proteins (HsLa,DmLa,TbLa,Lhp1p [ScLa], Sla1p [SpLa], and AtLa1) (Yoo and Wolin 1994;Van Horn et al 1997;Wolin and Cedervall 2002;Foldynova-Trantirkova et al 2005;Fleurdepine et al 2007). The human, fly, and trypanosome proteins are found in group B, yeast proteins in group C, and plant proteins in group D. Proteins falling into these four groups are between 263 and 770 amino acids long with an average size of z400 amino acids and adopt two types of domain arrangements: LAM-RRM1 or LAM-RRM1-RRM2.…”

“…), a single genuine La protein was found per organism. For A. thaliana, despite the presence of two proteins with characteristics of genuine La, only one (AtLa1) is able to bind to polymerase III transcripts and fulfill nuclear La functions (Fleurdepine et al 2007). Therefore, it remains to be seen if two functional genuine La proteins can coexist in the same organism.…”

“…Co-crystal structure analysis of the human La NTD showed that in the presence of an RNA ligand, while keeping their overall structure unchanged, LAM-RRM1 domains synergize and fold back together to form a tight RNA-binding pocket around the 39-UUU-OH motif (Teplova et al 2006;Kotik-Kogan et al 2008). With the exception of genuine La homologs from a few species (Wolin and Cedervall 2002;Fleurdepine et al 2007), the COOH-terminal domain (CTD) of genuine La proteins contains the atypical RRM (RRM2) followed by an unstructured terminal region that was shown in HsLa and Sla1p (Schizosaccharomyces pombe La) to contain several phosphorylation sites, a short basic motif (SBM), a nuclear localization signal (NLS), and a nuclear retention element (NRE) (Simons et al 1996;Van Horn et al 1997;Rosenblum et al 1998;Maraia and Intine 2001;Intine et al 2002). Based on crystal structure data, the HsLa RRM2 domain was shown to adopt a typical RRM fold with an additional long terminal a-helix that folds back on the b-sheet surface (Jacks et al 2003).…”

“…Based on crystal structure data, the HsLa RRM2 domain was shown to adopt a typical RRM fold with an additional long terminal a-helix that folds back on the b-sheet surface (Jacks et al 2003). The RRM2 motif is conserved in the CTD of most La homologs, albeit not at the primary sequence level but in terms of secondary structure motifs (Fleurdepine et al 2007). At present, it seems that the RRM2 domain does not display the ability to bind RNA (Jacks et al 2003), but several reports suggest that the C-terminal domain is involved in the regulation of the complex nuclear and cytoplasmic activities of genuine La proteins, at least in mammals (Park et al 2007).…”