SP loop is probably what opens the channel. Growing protein chain passes through channel as chain is synthesized. Protein can go all the way through the membrane and end up as a soluble protein in the lumen or protein can go part way through and end up as a transmembrane protein. How proteins get locked into the membrane. Start and stop-transfer sequences are probably equivalent.
Role depends on where in protein they occur.
Soluble protein in lumen . Happens if protein passes all the way through the membrane and SP is removed. Carboxyl end on outside. SP not removed. Multipass transmembrane protein.