LSG1-VAP binding promotes 60S Large Ribosomal Subunit Assembly

Ribosome biogenesis requires coordinated assembly of ribosomal subunits in the nucleus and cytoplasm. The GTPase LSG1 facilitates cytoplasmic maturation of the large 60S subunit by releasing the adaptor NMD3, enabling its nuclear recycling. Here we identify LSG1 as an interactor of the endoplasmic reticulum membrane proteins VAPA and VAPB through two functional FFAT motifs. Mutation of conserved aromatic residues within these motifs abolishes VAP binding and disrupts LSG1 localization to the ER. Split-proximity labeling reveals that LSG1-VAP sites are enriched for NMD3, the upstream GTPase GTPBP4, and ribosomal proteins. Importantly, loss of VAP binding impairs NMD3 nuclear recycling and causes accumulation of free ribosomal subunits, indicating defective 80S assembly. Preliminary evidence suggests phosphorylation of Thr320 within the second FFAT motif enhances VAP binding, potentially linking LSG1 activity to cellular metabolic states. These findings establish a critical role for ER-localized LSG1 in coordinating late-stage ribosome maturation.