Carboxy-terminal amino acid sequence of α-tubulin from porcine brain

Abstract

Tubulin is the main protein of microtubules, which are found in all eukaryotic cells. These structures participate in cell division, intracellular transport and secretion processes, ciliary and flagellar movement, morphogenesis and cell orientation. Tubulin is generally assumed to consist of two different chains, α and β, each of molecular weight about 55,000, which form a heterodimer in solution¹. Recently, however, heterogeneity within each type of chain has been claimed on the basis of immunological non-identity², peptide mapping³, isoelectric focusing and gel electrophoresis^4,5. Another type of heterogeneity has been studied in more detail: a specific ligase binds one tyrosine residue to the carboxy-terminal residue of only a fraction of the tubulin α-chain^5–7. The enzyme has been found in many tissues of various species^8,9, but the biological significance of the reaction is unknown. Elucidation of the primary structure should clarify whether a family of tubulins exists within one organism, and whether tubulin functions are regulated by post-translational modification. Furthermore, if a general principle underlies the various intracellular movements, one might expect sequence homologies between tubulin and muscle proteins. We have now established the sequence of the carboxy-terminal 78 amino acid residues in α-tubulin from porcine brain by Edman degradation of overlapping cyanogen bromide, tryptic and chymotryptic fragments. A terminal tyrosine was the only evidence for heterogeneity. The region is unusually acidic and not homologous to known proteins. Prediction of the secondary structure suggests a ‘jack-in-the-box-like’ structural change, dependent on the microenvironment.