Mechanism varies with the type of protein. Signal sequence on amino terminus enters first and continues to elongate. Signal is cleaved by protease inside the lumen. These proteins have rather long hydrophobic regions that will be anchored in the membrane. Type II proteins are threaded into the lumen with the C terminus leading.
Protein continues to be inserted until it reaches the hydrophobic stop signal sequence.
Same as Type II, only the N terminus leads into the lumen. The topogenic role of the hydrophobic signal domain. Tested charged amino acids and length of hydrophobic signal sequence. So, whichever end has the least positive charges near the signal anchor patch would go into the ER lumen. It shows the insertion of a double pass transmembrane protein with the loop inside the rough endoplasmic reticulum.