Conspectus
The function of cellular RNA
is modulated by a host of post-transcriptional
chemical modifications installed by dedicated RNA-modifying enzymes.
RNA modifications are widespread in biology, occurring in all kingdoms
of life and in all classes of RNA molecules. They regulate RNA structure,
folding, and protein–RNA interactions, and have important roles
in fundamental gene expression processes involving mRNA, tRNA, rRNA,
and other types of RNA species. Our understanding of RNA modifications
has advanced considerably; however, there are still many outstanding
questions regarding the distribution of modifications across all RNA
transcripts and their biological function. One of the major challenges
in the study of RNA modifications is the lack of sequencing methods
for the transcriptome-wide mapping of different RNA-modification structures.
Furthermore, we lack general strategies to characterize RNA-modifying
enzymes and RNA-modification reader proteins. Therefore, there is
a need for new approaches to enable integrated studies of RNA-modification
chemistry and biology.
In this Account, we describe our development
and application of
chemoproteomic strategies for the study of RNA-modification-associated
proteins. We present two orthogonal methods based on nucleoside and
oligonucleotide chemical probes: 1) RNA-mediated activity-based protein
profiling (RNABPP), a metabolic labeling strategy based on reactive
modified nucleoside probes to profile RNA-modifying enzymes in cells
and 2) photo-cross-linkable diazirine-containing synthetic oligonucleotide
probes for identifying RNA-modification reader proteins.
We
use RNABPP with C5-modified cytidine and uridine nucleosides
to capture diverse RNA-pyrimidine-modifying enzymes including methyltransferases,
dihydrouridine synthases, and RNA dioxygenase enzymes. Metabolic labeling
facilitates the mechanism-based cross-linking of RNA-modifying enzymes
with their native RNA substrates in cells. Covalent RNA–protein
complexes are then isolated by denaturing oligo(dT) pulldown, and
cross-linked proteins are identified by quantitative proteomics. Once
suitable modified nucleosides have been identified as mechanism-based
proteomic probes, they can be further deployed in transcriptome-wide
sequencing experiments to profile the substrates of RNA-modifying
enzymes at nucleotide resolution. Using 5-fluorouridine-mediated RNA–protein
cross-linking and sequencing, we analyzed the substrates of human
dihydrouridine synthase DUS3L. 5-Ethynylcytidine-mediated cross-linking
enabled the investigation of ALKBH1 substrates. We also characterized
the functions of these RNA-modifying enzymes in human cells by using
genetic knockouts and protein translation reporters.
We profiled
RNA readers for N
6-methyladenosine
(m6A) and N
1-methyladenosine
(m1A) using a comparative proteomic workflow based on diazirine-containing
modified oligonucleotide probes. Our approach enables quantitative
proteome-wide analysis of the preference of RNA-binding proteins for
modified nucleotides across a range of...