Possible Roles of Specific Amino Acids in β-Tubulin Isotypes in the Growth and Maintenance of Neurons: Novel Insights From Cephalopod Mollusks

Abstract

Microtubules, are formed of the protein tubulin, which is a heterodimer of α- and β-tubulin subunits. Both α- and β-tubulin exist as numerous isotypes, differing in amino acid sequence and tissue distribution. Among the vertebrate β isotypes, βIII has a very narrow distribution, being found primarily in neurons and in advanced cancers. The places in the amino acid sequence where βIII differs from the other β isotypes are highly conserved in evolution. βIII appears to be highly resistant to reactive oxygen species and it forms highly dynamic microtubules. The first property would be very useful in neurons, which have high concentrations of free radicals, and the high dynamicity would aid neurite outgrowth. The same properties make βIII useful in cancers. Examination of the amino acid sequences indicates a cysteine cluster at positions 124–129 in βIII (CXXCXC). This occurs in all βIII isotypes but not in βI, βII, or βIV. βIII also lacks the easily oxidized C239. Both features could play roles in free radical resistance. Many aggressive tumors over-express βIII. However, a recent study of breast cancer patients showed that many of them mutated their βI, βII, and βIV at particular places to change the residues to those found at the corresponding sites in βIII; these are all sites that are highly conserved in vertebrate βIII. It is possible that these residues are important, not only in the resistance to free radicals, but also in the high dynamicity of βIII. The cephalopod mollusks are well known to be highly intelligent and can remodel their own brains. Interestingly, several cephalopods contain the cysteine cluster as well as up to 7 of the 17 residues that are highly conserved in vertebrate βIII, but are not found in βI, βII, or βIV. In short, it is possible that we are looking at a case of convergent evolution, that a βIII-like isotype may be required for neuronal growth and function and that a structure-function study of the particular residues conserved between vertebrate βIII and cephalopod tubulin isotypes could greatly increase our understanding of the role of the various tubulin isotypes in neuronal growth and function and could aid in the development of novel anti-tumor drugs.

Phylogenetic and functional characterization of water bears (Tardigrada) tubulins

“…4, Supplementary Data 4). While the Tub2A isoform from Echiniscus testudo (Doyère, 1840) is distant from other tardigrade β-tubulin isoforms, the others share a relatively high degree of similarity, including the heterotardigrade AAs at these positions are conserved in many βIII-tubulins in vertebrates and some invertebrates 42,43 . However, other conserved vertebrate βIII-tubulin AAs are absent in Tub2C (Supplementary Data 4).…”

“…An interesting isoform is the Tub2C, which might be a homolog of vertebrate βIII-tubulins expressed primarily in neurons and in certain cancer cells 42 . We base our hypothesis on the presence of a hydrophobic AA at the relative position 371 and an alanine 295 in our alignment (designated as AA351 and AA275, respectively, in the literature on vertebrate tubulins 42 ).…”

“…An interesting isoform is the Tub2C, which might be a homolog of vertebrate βIII-tubulins expressed primarily in neurons and in certain cancer cells 42 . We base our hypothesis on the presence of a hydrophobic AA at the relative position 371 and an alanine 295 in our alignment (designated as AA351 and AA275, respectively, in the literature on vertebrate tubulins 42 ). The change from a polar threonine to a non-polar isoleucine or valine at the position equivalent to the vertebrate AA351 is conserved in many βIII-tubulins in vertebrates and even invertebrates 42,43 .…”

“…We base our hypothesis on the presence of a hydrophobic AA at the relative position 371 and an alanine 295 in our alignment (designated as AA351 and AA275, respectively, in the literature on vertebrate tubulins 42 ). The change from a polar threonine to a non-polar isoleucine or valine at the position equivalent to the vertebrate AA351 is conserved in many βIII-tubulins in vertebrates and even invertebrates 42,43 . Interestingly, the highly extremotolerant Ramazzottius varieornatus Bertolani & Kinchin, 1993 50 lacks the Tub2C isoform and the ε-tubulin as well 51 .…”

Ciliate Microtubule Diversities: Insights from the EFBTU3 Tubulin in the Antarctic Ciliate Euplotes focardii

“…The post-translational modifications affect the overall dynamic properties of microtubules during interphase as well as in mitosis [14,15], and often involve stable subpopulations of microtubules. Tubulin isotypes themselves modulate microtubule dynamics and organization [16]; they are responsible for changes in cell sensitivity to anti-cancer drugs [17][18][19][20] and for growth and maintenance of neurons and neurological disorders in humans [21][22][23].…”