Signal sequences. Just a few of the hundreds of known signal sequences are shown here. Hemagglutinin is a transmembrane protein on the surface of the influenza virus. Lipoproteins are components of plasma membranes and organelle membranes; they also transport fats in the bloodstream. Proteins destined for secretion include growth hormone, the bee venom toxin melittin (as its precursor promelittin), a Drosophila glue protein used in forming the pupa, and insulin (as its precursor, preproinsulin). Hydrophobic residues are shown in yellow; charged residues are in blue.