La proteins couple use of sequence-specific and non-specific binding modes to engage RNA substrates.

La shuttles between the nucleus and cytoplasm where it binds nascent RNA polymerase III (pol III) transcripts and mRNAs, respectively. La protects the 3' end of pol III transcribed RNA precursors, such as pre-tRNAs, through the use of a well-characterized UUU-3'OH binding mode. La proteins are also RNA chaperones, and La-dependent RNA chaperone activity is hypothesized to promote pre-tRNA maturation and translation at cellular and viral internal ribosome entry sites via binding sites distinct from those used for UUU-3'OH recognition. Since the publication of La-UUU-3'OH co-crystal structures, biochemical and genetic experiments have expanded our understanding of how La proteins use UUU-3'OH-independent binding modes to make sequence-independent contacts that can increase affinity for ligands and promote RNA remodeling. Other recent work has also expanded our understanding of how La binds mRNAs through contacts to the poly(A) tail. In this review, we focus on advances in the study of La protein-RNA complex surfaces beyond the description of the La-UUU-3'OH binding mode. We highlight recent advances in the functions of expected canonical nucleic acid interaction surfaces, a heightened appreciation of disordered C-terminal regions, and the nature of sequence-independent RNA determinants in La-RNA target binding. We further discuss how these RNA binding modes may have relevance to the function of the La-related proteins.

FMRP recruitment of ß-catenin to the translation pre-initiation complex represses translation.

Canonical Wnt/ß-catenin signaling is an essential regulator of various cellular functions throughout development and adulthood. Aberrant Wnt/ß-catenin signaling also contributes to various pathologies including cancer, necessitating an understanding of cell context-dependent mechanisms regulating this pathway. Since protein-protein interactions underpin ß-catenin function and localization, we sought to identify novel ß-catenin interacting partners by affinity purification coupled with tandem mass spectrometry in vascular smooth muscle cells (VSMCs), where ß-catenin is involved in both physiological and pathological control of cell proliferation. Here, we report novel components of the VSMC ß-catenin interactome. Bioinformatic analysis of the protein networks implies potentially novel functions for ß-catenin, particularly in mRNA translation, and we confirm a direct interaction between ß-catenin and the fragile X mental retardation protein (FMRP). Biochemical studies reveal a basal recruitment of ß-catenin to the messenger ribonucleoprotein and translational pre-initiation complex, fulfilling a translational repressor function. Wnt stimulation antagonizes this function, in part, by sequestering ß-catenin away from the pre-initiation complex. In conclusion, we present evidence that ß-catenin fulfills a previously unrecognized function in translational repression.

Human La binds mRNAs through contacts to the poly(A) tail.

In addition to a role in the processing of nascent RNA polymerase III transcripts, La proteins are also associated with promoting cap-independent translation from the internal ribosome entry sites of numerous cellular and viral coding RNAs. La binding to RNA polymerase III transcripts via their common UUU-3'OH motif is well characterized, but the mechanism of La binding to coding RNAs is poorly understood. Using electromobility shift assays and cross-linking immunoprecipitation, we show that in addition to a sequence specific UUU-3'OH binding mode, human La exhibits a sequence specific and length dependent poly(A) binding mode. We demonstrate that this poly(A) binding mode uses the canonical nucleic acid interaction winged helix face of the eponymous La motif, previously shown to be vacant during uridylate binding. We also show that cytoplasmic, but not nuclear La, engages poly(A) RNA in human cells, that La entry into polysomes utilizes the poly(A) binding mode, and that La promotion of translation from the cyclin D1 internal ribosome entry site occurs in competition with cytoplasmic poly(A) binding protein (PABP). Our data are consistent with human La functioning in translation through contacts to the poly(A) tail.