What determines the substrate specificity of the multi-drug-resistance pump?

Multi-drug-resistance protein (P-glycoprotein) turns out to be an ATP-hydrolysing transmembrane pump that increases the resistance of cells in which it is expressed by actively extruding toxic chemicals. The baffling question is how does the pump know which chemicals to extrude? Common features among its substrates are still elusive. The question raised here concerns the relationship between this pump and that known for many years as capable of extruding glutathionyl and cysteinyl S-conjugates of xenobiotics. Are excreted drugs conjugated before excretion? Does the multi-drug-resistance pump recognize a simple chemical tag put on xenobiotics by a family of transferase enzymes?