Protein adaptation in polar environment: a comparative study of α-tubulin sequences in mesophilic and psychrophilic polar microalgae

Abstract

Although microtubules usually disassemble at low temperature below 4℃ by disturbing polymerization of tubulin dimer, organisms living in a cold environment seem to overcome this limit by amino acid substitution of tubulin. We aimed to investigate whether amino acid substitution on tubulin occur commonly in polar microalgae and is responsible for promoting growth at freezing temperature at which non-cold adapted algae seldom grow and eventually die. The full-length cDNAs of α-tubulin from eight genera with 14 microalgal strains (ArF04, ArF08, ArF13, ArF24, ArF25, ArF26, ArF27, ArF28, ArF29, ArF32, AnF48, AnM08, AnM30, and AnM45) collected from the Arctic and the Antarctic were obtained by RACE and compared them with that of α-tubulin from mesophilic alga, Chlamydomonas reinhardtii. We found that the several amino acid substitutions occurred in most tubulins of polar microalgae. These substitutions were found in overlapped sites reported in previous work, presumably due to selective pressure of cold environment. Of the substituted sequences, the A295V region was conspicuous, which has reported that play an important role in the producing protofilament due to the increasing hydrophobicity. 10 out of 14 microalgal strains (72%) showed a substitution of alanine (A) to valine (V) in the 295th residue demonstrating that the substitution of A295V region is a largely conserved feature among polar microalgae. The substitutions within tubulin sequences may increase the polymerization of tubulin dimer, consequently turning on a signal transduction pathway involved in microtubule-mediated cell polarization etc, which increase the survival in freezing environment such as polar region. Our data will provide the valuable information to dissect the cold-adaptive mechanism related to structural change and/or post-translational modification of tubulin.