A classical secretory protein vary from a cytosolic protein by having a sequence about 13-35 amino acids long at its N-terminal end called as a signal peptide or signal sequence.  The  signal  peptides  of  various  secretory  proteins  vary  in amino  acid  sequence  but  there  are  some  regular  features,  for  instance  the center of the sequence commonly having of 10-15 hydrophobic  amino acids.   An signal hypothesis was proposed from early work in this area and predicted in which the signal  peptide  directs  the secretory  protein  to the RER  membrane  and  so goals  the  protein  to  cross  into  the  RER  lumen  and  be  exported.  The signal hypothesis has been explained to apply to protein secretion in plant, animal and bacterial cells.