Coffee cake recipe calls for a tube pan. Is this interchangeable with a bundt pan?

Make sure you grease it well as a bundt pan has more surface area from the fluting.

Excellent point! I usually make the transition the other way, from bundt to tube, so it's not an issue.

Except a bundt pan usually doesn't have the same capacity as a standard tube pan.

They are not the same -- a Bundt pan is decoratively fluted -- but I don't see why you couldn't substitute. I certainly use them as functional equivalents in my kitchen.