Cytoplasmic tubulin from squid nerve fully retains C-terminal tyrosine

Cytoplasmic tubulin from squid nerve fully retains C-terminal tyrosine as a result of the almost zero activity of detyrosinating enzyme and low but definite activity of tubulin-tyrosine ligase in its cytoplasm, while the tyrosine content of cytoplasmic tubulin from mammalian brain is about 30 to 60% of that expected for the fully tyrosinated state because of much higher activity of detyrosinating enzyme in mammalian brain. On the other hand, membrane tubulin from both squid optic ganglion and mammalian brain retains only a small amount of C-terminal tyrosine. The significance of these differences is discussed.