This work deals with the role of myosin phosphorylation in anaphase chromosome
movement. Y27632 and ML7 block two different pathways for phosphorylation of the myosin
regulatory light chain (MRLC). Both stopped or slowed chromosome movement when added to
anaphase crane-fly spermatocytes. To confirm that the effects of the pharmacological agents
were on the presumed targets, we studied cells stained with antibodies against mono- or biphosphorylated myosin. For all chromosomes whose movements were affected by a drug, the corresponding spindle fibres of the affected chromosomes had reduced levels of 1P- and 2Pmyosin. Thus the drugs acted on the presumed target and myosin phosphorylation is involved in anaphase force production.
Calyculin A, an inhibitor of MRLC dephosphorylation, reversed and accelerated the altered movements caused by Y27632 and ML-7, suggesting that another phosphorylation pathway is involved in phosphorylation of spindle myosin. Staurosporine, a more general phosphorylation inhibitor, also reduced the levels of MRLC phosphorylation and caused anaphase chromosomes to stop or slow. The effects of staurosporine on chromosome movements were not reversed by Calyculin A, confirming that another phosphorylation pathway is involved in phosphorylation of spindle myosin.