Yeast myosin light chain, Mlc1p, interacts with both IQGAP and Class II myosin to effect cytokinesis

MLC1 (myosin light chain) acts as a dosage suppressor of a temperature sens itive mutation in the gene encoding the S, cerevisiae IQGAP protein. Both p roteins localize to the bud neck in mitosis although Mlc1p localisation pre cedes Iqg1p, Mlc1p is also found at the incipient bud site in G(1) and the growing bud tip during S and G(2) phases of the cell cycle. A dominant nega tive CST-Mlc1p fusion protein specifically blocks cytokinesis and prevents Iqg1p localisation to the bud neck, as does depletion of Mlc1p, These data support a direct interaction between the two proteins and immunoprecipitati on experiments confirm this prediction. Mlc1p is also shown to interact wit h the class II conventional myosin (Myo1p), All three proteins form a compl ex, however, the interaction between Mlc1p and Iqg1p can be separated from the ML1p/Myo1p interaction. Mlc1p localisation and maintenance at the bud n eck is independent of actin, Myo1p and Iqg1p, It is proposed that Mlc1p the refore functions to recruit Iqg1p and in turn actin to the actomyosin ring and that it is also required for Myo1p function during ring contraction.