Dictyostelium myosin IK is involved in the maintenance of cortical tensionand affects motility and phagocytosis

Dictyostelium discoideum myosin Ik (MyoK) is a novel type of myosin disting uished by a remarkable architecture. MyoK is related to class I myosins but lacks a cargo-binding tail domain and carries an insertion in a surface lo op suggested to modulate motor velocity. This insertion shows similarity to a secondary actin-binding site present in the tail of some class I myosins , and indeed a GST-loop construct binds actin, Probably as a consequence, b inding of MyoK to actin was not only ATP- but also salt-dependent. Moreover , as both binding sites reside within its motor domain and carry potential sites of regulation, MyoK might represent a new form of actin crosslinker. MyoK was distributed in the cytoplasm with a significant enrichment in dyna mic regions of the cortex. Absence of MyoK resulted in a drop of cortical t ension whereas overexpression led to significantly increased tension. Absen ce and overexpression of MyoK dramatically affected the cortical actin cyto skeleton and resulted in reduced initial rates of phagocytosis. Cells lacki ng MyoK showed excessive ruffling, mostly in the form of large lamellipodia , accompanied by a thicker basal actin cortex. At early stages of developme nt, aggregation of myoK null cells was slowed due to reduced motility. Alto gether, the data indicate a distinctive role for MyoK in the maintenance an d dynamics of the cell cortex.