Parenting Between Comps

Can anyone tell me how to parent transform data between two different comps? Specifically, I have a 3D cube created and saved as a PSD. In AE, I have a comp with an animated camera. I would like to parent the parent controller in the PSD comp to the camera in the original AE comp. The end result I am looking for is to have the 3D cube rotate and translate in synch with the original camera such that it appears that the cube is part of the original scene. Thanks.