Staufen1 localizes to the mitotic spindle and controls the localization of RNA populations to the spindle

Staufen1 (STAU1) is an RNA-binding protein involved in the post-transcriptional regulation of mRNAs. We report that a large fraction of STAU1 localizes to the mitotic spindle in colorectal cancer HCT116 cells and in non-transformed hTERT-RPE1 cells. Spindle-associated STAU1 partly co-localizes with ribosomes and active sites of translation. We mapped the molecular determinant required for STAU1-spindle association within the first 88 N-terminal amino acids, a domain that is not required for RNA binding. Interestingly, transcriptomic analysis of purified mitotic spindles revealed that 1054 mRNAs and the precursor ribosomal RNA (pre-rRNA), as well as the long non-coding RNAs and small nucleolar RNAs involved in ribonucleoprotein assembly and processing, are enriched on spindles compared with cell extracts. STAU1 knockout causes displacement of the pre-rRNA and of 154 mRNAs coding for proteins involved in actin cytoskeleton organization and cell growth, highlighting a role for STAU1 in mRNA trafficking to spindle. These data demonstrate that STAU1 controls the localization of subpopulations of RNAs during mitosis and suggests a novel role of STAU1 in pre-rRNA maintenance during mitosis, ribogenesis and/or nucleoli reassembly.