A: Except for a few, almost all secreted proteins contain a signal sequence or signal peptide at the N-terminal, and its length is generally 15-35 amino acid residues. The middle part of signal peptide mostly contains hydrophobic amino acid fragments (about 14 ~ 20 amino acid residues), so it has no strict specificity.

Intracellular signal recognition protein (SRP) can recognize the ribosome of the polypeptide being synthesized and will pass through the endoplasmic reticulum membrane. SRP binds to the signal peptide of this kind of ribosome synthetic peptides, and forms the SRP- signal peptide-ribosome complex, which temporarily stops the synthesis of polypeptide chain. Then SRP leads the complex from cytoplasm to endoplasmic reticulum membrane. There is a receptor docking protein (DP) of SRP on the membrane. When DP binds to the complex, SRP is released, which can be used for the transport of another ribosome. At this time, through a GTP-dependent process, the endoplasmic reticulum membrane was opened, and the signal peptide passed through the membrane and combined with another receptor SSR in the membrane, and the polypeptide chain temporarily stopped on the ribosome resumed synthesis and extension. In this way, the new peptide chain follows the signal peptide and extends across the membrane into the endoplasmic reticulum. The signal peptide entering the endoplasmic reticulum was excised by the signal peptidase in the endoplasmic reticulum before the synthesis of protein peptide chain was completed.