Question

The primary role of platelets is to control blood clotting. When they encounter the exposed basement membrane (collagen fibers) of a damaged blood vessel or a newly forming fibrin clot, they change their shape from round to spiky and stick to the damaged area. At the same time, they begin to secrete serotonin and ATP, which accelerate similar changes in newly arriving platelets, leading to the rapid formation of a clot. The platelet response is regulated by protein phosphorylation. Significantly, platelets contain high levels of two protein kinases: PKC, which initiates serotonin release, and myosin light-chain kinase, which mediates the change in shape.

When platelets are stimulated with thrombin, the light chain of myosin and an unknown protein of 40,000 daltons are phosphorylated. When platelets are treated with a calcium ionophore, which increases membrane permeability to Ca2+, only the myosin light chain is phrosphorylated: when they are treated with diacylglygercol, only the 40-kD proteins is phosphorylated. Experiments using a range of concentration of diacylglycerol in the presence or absence of calcium ionophore show that the extent of phosphorylation of the 40-kD proteins depends only on the concentration of diacylglycerol. Serotonin release, however, depends on diacylglycerol and the calcium ionophore.

a. Based on these experimental observations, decribe the normal sequence of molecular events that leads to phosphorylation of the myosin light chain and the 40-kD. Indicate how the calcium ionophore and diacylglyercol treatments interact with the normal sequence of events. Draw a diagram of the proposed signaling pathway to help you explain.

b. Why do you think serotonin release requires both calcium ionophore an diacylglycerol?

Answer 1

A.Myosin phosphorylation is required for change of shape of the platelets. So what happens is when calcium ionophore is given it increases the concentration of calcium in the cell which then binds to the inactive myosin light chain kinase and converts it to active myosin light chain kinase which then phosphorylates the myosin light chain

Whereas in the case of 40-kd protein it is mediated by diacylglycerol which is a lipid signal messenger. Basic function of it is to activate protein kinase C which then phosphorylates our protein 40-kd

B. As diacylglycerol is a lipid signal messenger it increases the membrane permeability which increases the calcium influx thus both are required for synergistic action helpful in serotonin release