The avidin protein family : properties of family members and engineering of novel biotin-binding protein tools

Abstract

Eliöiden perimä sisältää kullekin lajille tyypillisen geneettisen tiedon ja määrittelee niiden ominaisuuksia. Lajien perimän yksityiskohtainen tarkastelu ja vertailu selvittää lajien ominaisuuksien perustaa. Suuri osa perimän sisältämistä geeneistä koodaa proteiineja, jotka ovat keskeisiä solujen rakenteille ja toiminnoille.Vesa Hytönen tarkasteli väitöskirjassaan kanan avidiini-proteiinin ja samaan geeniperheeseen kuuluvien avidiinin kaltaisten geenien (AVR) koodaamien AVR-proteiinien tuottamista ja ominaisuuksia. Avidiinin tehtävä on sitoa vesiliukoista H-vitamiinia, jota kutsutaan myös biotiiniksi. Avidiinia esiintyy erityisesti kananmunan valkuaisessa, jossa sen arvellaan toimivan puolustusproteiinina suojaamassa kehittyvää alkiota esimerkiksi mikrobien toiminnalta. Hytönen selvitti AVR-geenien koodaamien proteiinien ominaisuuksia ja hankki tietoa AVR-geenien tehtävästä. Lisäksi hän kehitti menetelmän, jolla avidiinia ja sen kaltaisia proteiineja voidaan tuottaa taloudellisesti ja nopeasti bakteerisoluissa.

Chicken avidin family consists of several proteins which bind a small vitamin, D-biotin. One of these proteins, avidin, is a widely used biological tool in the life sciences. The other members of the family are avidin-related proteins (AVRs). Avidin is thought to be an antimicrobial protein whereas the biological role of AVRs is not known. AVRs have been characterised only as recombinant proteins.The aim of this study was to characterise the structural and functional features of AVRs, especially the identical proteins coded by avidin related genes 4 and 5 (AVR4/5). Another objective was to develop new tools for avidin-biotin technology.In order to enable bacterial avidin production, the original avidin signal peptide was replaced with a secretion signal from a bacterial protein. The avidin produced in Escherichia coli was isolated and purified using affinity chromatography. In its properties, the produced avidin resembled avidin extracted from chicken egg-white, but it lacked glycosylation. Successful E. coli expression provided the possibility to economically produce avidin and avidin-like proteins for research and applications.Mutagenesis of a C-terminal cysteine residue of AVR4/5 to serine changed its oligomerization properties without affecting those of biotin-binding or thermostability. We were able to substitute part of the avidin sequence with a sequence stretch from AVR4/5. The chimeric protein thus obtained showed increased thermostability when compared to that of avidin. The high-resolution structure of AVR4/5 was determined by X-ray crystallography. The higher thermal stability of AVR4/5 is the result of only slight changes in the structural elements compared to avidin.The novel proteins obtained in this study may offer tools for avidin-biotin technology and also improve the understanding of biotin-binding process.