Fig. 6: Importins protect positively charged ribonucleic acid binding domains.
From: Co-translational binding of importins to nascent proteins
a Visualization of the Gene Ontology (GO)-enrichment for molecular functions. Co-translationally targeted cargo shows strong enrichment for DNA-, RNA-, chromatin-, protein-binding properties, and structural proteins of the ribosome. Only significantly enriched GO-terms are shown (P-value < 0.1, not adjusted, Fisher Exact Test (two-sided) relative to all proteins quantified). b Onsets are frequently observed at ribonucleic acid binding sites. Apparent onsets were mapped as described in Methods and classified according to their annotated function. c Importins capture preferentially positively charged nascent cargoes. While the pl-values of nuclear proteins are distributed bimodally, nascent cargoes are shifted towards high pI-values. Violin plot shows median and quartiles. Figure shows n = 859 (nuclear proteome), n = 71 (all hits), n = 27 (Srp1-Kap95), n = 9 (Kap121) and n = 30 (Kap123) individual proteins. ****P = 3.53 × 10−6 (nuclear proteome, all hits); ****P = 5.72 × 10−9 (nuclear proteome, Kap123); **P = 0.0080 (Srp1/Kap95, Kap123). Non-parametric Mann–Whitney U test (two-sided). Source data are provided as a Source Data file. d Amino acid enrichment in apparent onsets for nascent Srp1-Kap95 and Kap123 cargoes as compared to the full-length proteins (Fisher Exact Test; two-sided). *P = 0.0101 (Srp1-Kap95; proline); *P = 0.0152 (Srp1-Kap95; histidine); ***P = 0.0015 (Srp1-Kap95; asparagine); ****P = 6.16 × 10−9 (Srp1-Kap95; lysine); ****P = 1.32 × 10−4 (Srp1-Kap95; arginine); ****P = 3.93 × 10−4 (Kap123; valine). *P = 0.0489 (Kap123; glycine). ****P = 2.42 × 10−4 (Kap123; glutamate). e, f Metagene analysis of the ribosome occupancy at apparent onset sites for nascent Srp1-Kap95 (e) and Kap123 (f) cargoes. Prior to the onset, an increased occupancy is observed, implying a decrease in elongation. ***P = 0.0028 (Srp1: before, after onset); *P = 0.0113 (Kap95: before, after onset); ns = 0.8642 (Kap123: before and after onset); two-sided t-test across entire distribution. Solid lines represent averaged profiles; shades reflect a 95% confidence interval. For (c–f), ns P > 0.05, *P < 0.05, **P < 0.01, ***P < 0.005, ****P < 0.001. g Conceptual model of nascent cargo complex formation. As the nascent chain emerges from the ribosome, it may be bound by ubiquitous chaperones (e.g. Ssb1) that temporarily chaperone structured patches. Once released, importins nascently form cargo complexes and shield-charged patches. GO gene ontology, HAT histone acetyltransferase, CTD C-terminal domain, pI isoelectric point.
