I have the following function, which updates the User Initiated Send definition to a new Data Extension.  I would like to know how to update the email name too for the User Initiated Send to target a new email dynamically.

    function runUIS(UISKey,DEName){
    			var deESD = Send.Definition.Init(UISKey);
    			var updatestatus = deESD.Update({"DataExtensionKey":"SendDE","XXXX":"EmailName"});

I have tried various object names (for XXXX): 
Email | EmailKey | Message | MessageKey | EmailSendKey | EmailSend | ESD | EmailMessage 
and none of them seem to work.

The wiki page doesn't tell what this value is either:
http://help.exacttarget.com/en/documentation/exacttarget/content/server_side_javascript/server_side_javascript_syntax_guide/send_server_side_javascript_functions/

Does anyone know what the object name might be, or how to find it?