Border cells are an in vivo model for collective cell migration. Here, we identify the gene cactin as essential for border cell cluster organization, delamination, and migration. In Cactin-depleted cells, the apical proteins aPKC and Crumbs (Crb) become abnormally concentrated, and overall cluster polarity is lost. Apically tethering excess aPKC is sufficient to cause delamination defects, and relocalizing apical aPKC partially rescues delamination. Cactin is conserved from yeast to humans and has been implicated in diverse processes. In border cells, Cactin’s evolutionarily conserved spliceosome function is required. Whole transcriptome analysis revealed alterations in isoform expression in Cactin-depleted cells. Mutations in two affected genes, Sec23 and Sec24CD, which traffic Crb to the apical cell surface, partially rescue border cell cluster organization and migration. Overexpression of Rab5 or Rab11, which promote Crb and aPKC recycling, similarly rescues. Thus, a general splicing factor is specifically required for coordination of cluster polarity and migration, and migrating border cells are particularly sensitive to splicing and cell polarity disruptions.

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