Leaving the cell body, the vesicle contains both chains of amino acids and proteases which cleave the peptide chain, creating a vesicle filled with both active neurotransmitter and excess amino acid chains. Fast anterograde transport is the means by which materials produced in the cell body are carried, enclosed by vesicles, down the axon to the terminals of the nerve cell. This process is dependent upon a system of microtubules running lengthwise along the interior of the axon. These microtubules provide a track by which the direction of vesicle movement is guided. The actual movement of vesicles down the axon is facilitated by a second protein called kinesin. Kinesin is a rod-shaped ATP-ase which forms cross-bridges between individual vesicles and microtubules. At one end of the kinesin molecule are two globular heads which bind to the microtubule. The other end is characterized by a fan-shaped tail which binds to proteins on the vesicular membrane. A hinge along the middle of the rod allows the kinesin molecule to bend and pass the vesicle on to other kinesin molecules further down the axon. The vesicle thus slides along the length of the axon in a stop-and-go (saltatory) fashion, from kinesin to kinesin, until it reaches the terminal.
The majority of neuropeptide-processing takes place within the vesicle as it travels down the axon. This is where PENK, for example, may be cleaved to produce four copies of Met-enkephalin (ME), a single copy of Leu-enkephalin (LE), and single copies of the extended MEs, ME-Arg-Gly-Leu and ME-Arg-Phe. PENK may also be cleaved into larger ME- and LE-containing fragments to produce Peptides E, F, and B. ME and LE share similar structures. Both are relatively simple peptides made up of five amino acids (pentapeptides). The chemical structure of LE is H-tyrosine-glycine-glycine-phenylalanine-leucine-OH, while the structure for ME is the same, except for the last amino acid: H-tyrosine-glycine-glycine-phenylalanine-methionine-OH. The larger LE- or ME-containing peptides derived from PENK contain within their structure one of these two base chains in addition to any number of adjacent amino acids that make up the propeptide chain.