Mitochondria to the er, Suppose a protein of interest is

Question: Suppose a protein of interest is known to bind the mitochondria to the ER. Suppose these proteins are small enough to fit into the vesicles and transfer to the Goldgi apparatus. A putative KDEL sequence similar to the one on Bip proteins has been identified.  How could one determine whether the sequence is necessary for our ‘protein of interest’?  We do not have access to antibodies, but all lab equipment and necessary reagents are available at our disposal.

I understand how to make antibodies, and the gist of running subcellular fractionation and gels.  Some assistance in figuring this out would be really appreciated.