Signalling dependent regulation of the fission yeast actomyosin cytoskeleton

The actomyosin cytoskeleton is a highly dynamic complex of proteins which facilitate diverse processes within the cell. It is regulated by a network of signalling pathways to coordinate growth and division in response to changes in the extracellular environment. A wide range of actin binding proteins control the organisation and dynamics of the actin network within eukaryotic cells. This study explored the function of signalling dependent phosphorylation events on two S. pombe actin binding proteins, the actin capping protein, Acp1, and the class I myosin, Myo1.

Disruption of the TOR-signaling complex, TORC2, results in defects in the formation and constriction of the cytokinetic actomyosin ring (CAR). This study explores the localisation of TORC2 to the CAR, and the function of TORC2dependent phosphorylation of Acp1. Data shows that phosphorylation of Acp1 functions to regulate the stability and cellular organization of the actin cytoskeleton. This allows a cell to couple growth and division with changes in the environment.

Similarly, a conserved serine residue in the IQ motif of the neck region of Myo1 was shown to be TORC2-dependent. Fission yeast express two calmodulin-like light chains, Cam1 and Cam2, which are capable of associating with the IQ domains of Myo1. The location of this phosphorylation site, between the two IQ motifs, provides a potential mechanism of determining which light chain associates with Myo1. This study provides evidence that this TORC2-dependent phosphorylation event is cell cycle dependent, and may result in a switch between Cam1 and Cam2 binding.

These potentially conserved mechanisms of regulating the organization of the actomyosin cytoskeleton provide an important insight into the system of control networks that regulate cell growth and division in response to changes in the extracellular environment.