Calreticulin is also found on the cell surface, nucleus and cytosol so while it works fine as an ER maker for in tact cells (due to the higher conc of CRT in the ER than anywhere else), it could be problematic for cellular fractions. I think if you're trying to distinguish ER from MAM on a blot you will have to use two proteins. One that should be enriched in MAM and one that should be primarily ER.
As per the ER specific proteins are concernrd. Most of the ER membrane proteins shuttle between ER, Golgi, nucleus and plasma membrane. Though following is the list of proteins reported only in ER so far.
DOLK, ELOVL4, SGPL1, BSCL2, SARAF, DERL3, MBOAT4,
Following are the proteins those shuttle between major membrane components-