Immunochemical analysis of rough and smooth microsomes from rat liver. Segregation of docking protein in rough membranes.

Docking protein (or signal recognition particle receptor) is an integral membrane protein essential for translocation of nascent polypeptides across the membrane of the endoplasmic reticulum. It serves as the receptor for the signal recognition particle, and represents the site of interaction between the translation and the translocation systems. Results presented here demonstrate that this protein is localized exclusively in rough microsomal membranes. Rough and smooth microsomes were prepared and their content of various marker proteins was determined by immunochemical techniques. Whereas a number of proteins, including cytochromes b5 and P-450 and their reductases were evenly distributed between rough and smooth microsomes, docking protein was found at 20-fold higher levels in the rough fraction. On the basis of these results it is concluded that docking protein is a functionally characterized integral protein specifically restricted to rough microsomal membranes.