PAR-4/LKB1 Mobilizes Nonmuscle Myosin through Anillin to Regulate C. elegans Embryonic Polarization and Cytokinesis

BACKGROUND The serine/threonine kinase LKB1 regulates cell growth and polarity in metazoans, and loss of LKB1 function is implicated in the development of some epithelial cancers. Despite its fundamental role, the mechanism by which LKB1 regulates polarity establishment and/or maintenance is unclear. In the present study, we use the nematode C. elegans to investigate the role of the LKB1 ortholog PAR-4 in actomyosin contractility, a cellular process essential for polarity establishment and cell division in the early embryo. RESULTS 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. CONCLUSION 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.