PAR proteins and the cytoskeleton: a marriage of equals.

The PAR proteins are a group of widely conserved regulators of polarity, many of which are asymmetrically localized in polarized cells. Recent work shows that distinct modes of actomyosin- and microtubule-based transport contribute to the establishment of PAR asymmetries in different cell types. Cross-regulatory interactions among PAR proteins and with other conserved polarity complexes stabilize asymmetries once they form, and shape the evolution of PAR protein distributions in response to cytoskeletal transport or other polarizing inputs. The PAR proteins in turn modulate the actomyosin and microtubule cytoskeletons. In some cases, this is a form of feedback control, central to the establishment and maintenance of PAR asymmetries. In others, it underlies the elaboration of functional cell polarity.