Structural Divergence of the Group I Intron Binding Surface in Fungal Mitochondrial Tyrosyl-tRNA Synthetases That Function in RNA Splicing
Year: 2016
Type: article
Abstract: The mitochondrial tyrosyl-tRNA synthetases (mtTyrRSs) of Pezizomycotina fungi, a subphylum that includes many pathogenic species, are bifunctional proteins that both charge mitochondrial tRNA Tyr and act as splicing cofactors for autocatalytic group I introns.Previous studies showed that one of these proteins, Neurospora crassa CYT-18, binds group I introns by using both its N-terminal catalytic and C-terminal anticodon binding domains and that the catalytic domain uses a newly evolved group I intron binding surface that includes an N-terminal extension and two small insertions (insertions 1 and 2) with distinctive features not found in non-splicing mtTyrRSs.To explore how this RNA binding surface diverged to accommodate different group I introns in other Pezizomycotina fungi, we determined x-ray crystal structures of C-terminally truncated Aspergillus nidulans and Coccidioides posadasii mtTyrRSs.Comparisons with previous N. crassa CYT-18 structures and a structural model of the Aspergillus fumigatus mtTyrRS showed that the overall topology of the group I intron binding surface is conserved but with variations in key intron binding regions, particularly the Pezizomycotina-specific insertions.These insertions, which arose by expansion of flexible termini or internal loops, show greater variation in structure and amino acids potentially involved in group I intron binding than do neighboring protein core regions, which also function in intron binding but may be more constrained to preserve mtTyrRS activity.Our results suggest a structural basis for the intron specificity of different Pezizomycotina mtTyrRSs, highlight flexible terminal and loop regions as major sites for enzyme diversification, and identify targets for therapeutic intervention by disrupting an essential RNA-protein interaction in pathogenic fungi.Aminoacyl-tRNA synthetases are a class of ancient, essential enzymes that catalyze the ligation of amino acids onto cognate tRNAs.In addition to this essential role, many aminoacyl-tRNA synthetases have evolved secondary functions, frequently by the acquisition of new domains or sequence expansions (1-3).Among the best-studied examples of such gain-of-function are the mitochondrial tyrosyl-tRNA synthetases (mtTyrRS) of fungi belonging to the subphylum Pezizomycotina, which evolved to promote the splicing of mitochondrial (mt) group I introns (4).Group I introns are ribozymes that catalyze their own splicing via guanosine-initiated transesterification reactions (5).Although some self-splice in vitro, most have acquired mutations that impair self-splicing and have thus become dependent upon intron-encoded or cellular proteins to promote formation of the catalytically active ribozyme structure (6).The Pezizomycotina mtTyrRS 2 evolved to function in splicing via a series structural adaptations, which occurred during or after the divergence of Pezizomycotina from Saccharomycotina and resulted in a new group I intron binding surface distinct from that which binds tRNA Tyr (4, 7).These structural adaptations included acquisition of a longer N-terminal extension and several small insertions (insertions (Ins) 1-5) resulting from the expansion of flexible terminal or loop regions (4).Thus, these enzymes provide an ideal model system for investigating how proteins in general and aminoacyl-tRNA synthetases in particular acquire new RNA binding functions.Thus far most studies of how fungal mtTyrRSs function in splicing group I introns have focused on the Neurospora crassa mtTyrRS, denoted CYT-18 protein, which was identified as a group I intron splicing factor in an early genetic screen for splicing-deficient mutants (8).Temperature-sensitive mutants in the cyt-18 gene were found to be defective in splicing the N. crassa mt large subunit rRNA (mtLSU) intron and two other group I introns (8, 9).Further studies showed that CYT-18 is by itself sufficient to promote the splicing of these introns as well as diverse group I introns from other organisms in vivo and in vitro (10 -12).CYT-18 promotes splicing by binding specifically to group I intron RNAs and stabilizing their catalytically active RNA structure (13).Group I introns have minimal sequence conservation but fold into a conserved tertiary structure that is required for ribozyme activity (14 -17).This conserved structure consists of two extended helical domains, denoted the P4-P6 and P3-P9 domains, whose juxtaposition creates an active-site cleft that binds the splice sites and the guanosine * This work was supported by National Institutes of Health Grant GM37951.
Source: Journal of Biological Chemistry
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: hybrid
Estimated APC paid: $3,430
Awards: G1286236842, G1286251239, G2539837713, G2880384052, G30714459, G3083819904, INVOLVEMENT OF PROTEINS IN SPLICING GROUP I INTRONS, G382641700, G419881185, G42916606, Mobile Group II Introns and Their Reverse Transcriptases, G4501827968, G465867758, G498139845, G5363686239, G650090426, G6608191692, Involvement of Proteins in Splicing Group I and II Introns, G9000242233, G969749505, G969889393
OpenAlex ID: W2335475622