Using high-resolution time-lapse imaging of GFP-tagged nonmuscle myosin II (NMY-2), we found that par-4 mutations reduce actomyosin contractility during polarity establishment, leading to the mispositioning of anterior PAR proteins and to defects in contractile ring ingression during cytokinesis. Fluorescence recovery after photobleaching analysis revealed that the mobility of a cortical population of NMY-2 was reduced in par-4 mutants. Interestingly, the contractility defects of par-4 mutants depend on the reciprocal activity of ANI-1 and ANI-2, two C. elegans homologs of the actin cytoskeletal scaffold protein anillin.
Because loss of PAR-4 promoted inappropriate accumulation of ANI-2 at the cell cortex, we propose that PAR-4 controls C. elegans embryonic polarity by regulating the activity of anillin family scaffold proteins, thus enabling turnover of cortical myosin and efficient actomyosin contractility. This work provides the first description of a cellular mechanism by which PAR-4/LKB1 mediates cell polarization.