keyboard shortcut for the expand/collapse button?

I've searched faq sites and forums all over, and can't find a keyboard shortcut for the expand/collapse button in the site panel in DWMX. Does anyone know of one? If there was a menu item for the function i could customise a shortcut, but I can't find a menu item for this either.

I would love a keyboard shortcut instead of having to fish around for this small button.