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Abstract Serine/arginine‐rich splicing factor 1 (SRSF1) is key in the mRNA lifecycle including transcription, splicing, nonsense‐mediated decay, and nuclear export. Consequently, its dysfunction is linked to cancers, viral evasion, and developmental disorders. The functionality of SRSF1 relies on its interactions with other proteins and RNA molecules. These processes are regulated by phosphorylation of its unstructured arginine/serine‐rich tail (RS). Here, we characterize how phosphorylation affects SRSF1's protein and RNA interaction and phase separation. Using NMR paramagnetic relaxation enhancement and chemical shift perturbation, we find that when unphosphorylated, SRSF1's RS interacts with its first RNA‐recognition motif (RRM1). Phosphorylation of RS decreases its interactions with the protein‐binding site of RRM1 and increases its interactions with the RNA‐binding site of RRM1. This change in SRSF1's intramolecular interactions increases the availability of protein‐interacting sites on RRM1 and weakens RNA binding of SRSF1. Phosphorylation alters the phase separation of SRSF1 by diminishing the role of arginine in intermolecular interactions. These findings provide an unprecedented view of how SRSF1 influences the early‐stage spliceosome assembly. 

ABSTRACT During ϕX174 morphogenesis, 240 copies of the external scaffolding protein D organize 12 pentameric assembly intermediates into procapsids, a reaction reconstituted in vitro . In previous studies, ϕX174 strains resistant to exogenously expressed dominant lethal D genes were experimentally evolved. Resistance was achieved by the stepwise acquisition of coat protein mutations. Once resistance was established, a stimulatory D protein mutation that greatly increased strain fitness arose. In this study, in vitro biophysical and biochemical methods were utilized to elucidate the mechanistic details and evolutionary trade-offs created by the resistance mutations. The kinetics of procapsid formation was analyzed in vitro using wild-type, inhibitory, and experimentally evolved coat and scaffolding proteins. Our data suggest that viral fitness is correlated with in vitro assembly kinetics and demonstrate that in vivo experimental evolution can be analyzed within an in vitro biophysical context. IMPORTANCE Experimental evolution is an extremely valuable tool. Comparisons between ancestral and evolved genotypes suggest hypotheses regarding adaptive mechanisms. However, it is not always possible to rigorously test these hypotheses in vivo . We applied in vitro biophysical and biochemical methods to elucidate the mechanistic details that allowed an experimentally evolved virus to become resistant to an antiviral protein and then evolve a productive use for that protein. Moreover, our results indicate that the respective roles of scaffolding and coat proteins may have been redistributed during the evolution of a two-scaffolding-protein system. In one-scaffolding-protein virus assembly systems, coat proteins promiscuously interact to form heterogeneous aberrant structures in the absence of scaffolding proteins. Thus, the scaffolding protein controls fidelity. During ϕX174 assembly, the external scaffolding protein acts like a coat protein, self-associating into large aberrant spherical structures in the absence of coat protein, whereas the coat protein appears to control fidelity.