Actionscript 3 :: Change The Button Label Dynamically?

Is there any way to change Spark button label dynamically? When i click on it, i want the label to change. I bind the String to label and gives value for the first time, but even flashBuilder shows me that Data binding will not be able to detect assignments. Here is my button:

<s:Button name="button" label="{butt}" x="5" y="3" useHandCursor="true"
click="start()" buttonMode="true" cornerRadius="5"
skinClass="skins.CustomButtom"/>

And here is assigment:

public var butt:String = "Start";