ABSTRACT
The assumption has been made that the topology of polytopic proteins is determined by the linear succession of SSTs, USPs, and STs. Gene fusions have also been used to distinguish between periplasmic and cytoplasmic domains of E. coli membrane proteins. Most yeast transmembrane proteins transit through the secretory pathway, which means that they are first translocated across the endoplasmic reticulum membrane. Transport of newly synthesized proteins to the cell surface has been analyzed by labeling with and treatment with trinitrobenzene sulfonic acid at 0°C. In other integral membrane proteins, the polypeptide chain crosses the membrane several times; they are called polytopic. The yeast STE6 encodes a protein very similar to the mammalian multidrug resistance P-glycoprotein. Class III integral membrane proteins have SST with one or more negatively charged residues on the NH2-terminal side of an apolar anchor segment.
