A novel patch assembly domain in Num1 mediates dynein anchoring at the cortex during spindle positioning

During mitosis in budding yeast, cortically anchored dynein generates pulling forces on astral microtubules to position the mitotic spindle across the mother-bud neck. The attachment molecule Num1 is required for dynein anchoring at the cell membrane, but how Num1 assembles into stationary cortical patches and interacts with dynein is unknown. We show that an N-terminal Bin/Amphiphysin/Rvs (BAR)-like domain in Num1 mediates the assembly of morphologically distinct patches and its interaction with dynein for spindle translocation into the bud. We name this domain patch assembly domain (PA; residues 1-303), as it was both necessary and sufficient for the formation of functional dynein-anchoring patches when it was attached to a pleckstrin homology domain or a CAAX motif. Distinct point mutations targeting the predicted BAR-like PA domain differentially disrupted patch assembly, dynein anchoring, and mitochondrial attachment functions of Num1. We also show that the PA domain is an elongated dimer and discuss the mechanism by which it drives patch assembly.