Some Like it Hot

by S. Mar­vin Fried­man

How can ther­mophilic bac­te­ria not only sur­vive, but ac­tu­ally pro­lif­er­ate, at el­e­vated tem­per­a­tures that would be lethal to all other forms of life? Af­ter ex­ten­sive re­search dur­ing the past five decades, this ques­tion has been an­swered in a gen­eral way, but the mol­e­c­u­lar ba­sis for this un­usual ca­pa­bil­ity has not been clearly re­solved. Ther­mophiles [I use this term to in­clude both ther­mophiles (op­ti­mal growth tem­per­a­tures of 50–70 °C) and hy­per­ther­mophiles (op­ti­mal growth tem­per­a­tures >80 °C)] syn­the­size in­trin­si­cally ther­mostable cel­lu­lar com­po­nents and/or ex­trin­sic sta­bi­liz­ing fac­tors (chap­er­onins and polyamines, for ex­am­ple).

Red col­oration on rocks near Naples, Italy, pro­duced by the hy­per­ther­mophile Sul­folobus sol­fa­tar­i­cus. Source

Most pro­teins iso­lated from ther­mophiles are ther­mostable and the mech­a­nisms un­der­ly­ing this prop­erty have been ex­ten­sively stud­ied. These in­ves­ti­ga­tions have their roots in the pi­o­neer­ing work on the heat sta­bil­ity of he­mo­glo­bin and ferre­doxin car­ried out by Pe­rutz in 1975. Some of the strate­gies re­ported to bring about pro­tein ther­mosta­bil­ity in­clude higher lev­els of charged amino acids on the pro­tein sur­face that pro­mote ionic in­ter­ac­tions, amino acid pref­er­ences, hy­dropho­bic cores, aliphatic side chains, disul­fide bridges, and solute ac­cu­mu­la­tion. It is ob­vi­ous from these stud­ies that a uni­ver­sal mech­a­nism to achieve pro­tein ther­mosta­bil­ity does not ex­ist. The com­plex­ity of this prob­lem is high­lighted by dif­fer­ences that ap­pear to be in play for mul­ti­meric pro­teins ver­sus sin­gle polypep­tide chains, for sol­u­ble pro­teins ver­sus mem­brane pro­teins, and for pro­teins from ther­mophiles ver­sus pro­teins from hy­per­ther­mophiles.

Map­ping of ly­sine methy­la­tion sites (in­di­cated in yel­low) on the crys­tal struc­ture of the RNA poly­merase from Sul­folobus shi­batae, a hy­per­ther­mophilic, aci­dophilic cre­nar­chaeon. Source

There is also ev­i­dence for the in­volve­ment of a post-trans­la­tional event. Bot­ting and cowork­ers ex­pand on re­ports sug­gest­ing that ly­sine methy­la­tion may play an im­por­tant role in the heat sta­bil­ity of pro­teins from hy­per­ther­mophilic Cre­nar­chaea. Us­ing mass spec­trom­e­try, they iden­ti­fied 21 singly-methy­lated lysines across 9 sub­units of the RNA poly­merase iso­lated from Sul­folobus sol­fa­tar­i­cus. The mod­i­fied lysines oc­curred pri­mar­ily in al­pha-he­lices and near hair­pin turns. In ad­di­tion, the methyl trans­ferase in­volved did not dis­play the se­quence speci­ficity char­ac­ter­is­tic of the SET do­main methy­lases that act on hi­s­tones in Eu­karya. A pro­teomic study of Ther­mo­pro­teus taxis re­vealed 52 methyl-lysines (6 of which were di­methy­lated) in 30 dif­fer­ent pro­teins. In con­trast, a sur­vey of the pro­teomes from var­i­ous Eu­r­yarchaea (e.g., the halophile Haloferax vol­canii, the hy­per­ther­mophiles Ther­mo­coc­cus gam­ma­tol­er­ans and Py­ro­coc­cus fu­rio­sus, and the mesophile Methanococ­cus mari­paludis), did not re­veal ly­sine methy­la­tion.

Methy­la­tion of ly­sine in­creases the pK_a of the side chain, thus al­low­ing a stronger ionic in­ter­ac­tion to take place, and also changes the hy­dropho­bic­ity and sol­u­bil­ity of the pro­tein. There are many ex­am­ples of ly­sine methy­la­tion im­prov­ing the ther­mosta­bil­ity of pro­teins, such as in the com­mer­cially-pro­duced methy­lated bovine trypsin used for pro­tein di­ges­tion at el­e­vated tem­per­a­tures. More data will be re­quired to firmly es­tab­lish whether methy­la­tion of ly­sine is an adap­ta­tion to the hy­per­ther­mophilic lifestyle spe­cific to the Cre­nar­chaea.

De­spite such tan­ta­liz­ing hints, the ba­sic ques­tions re­gard­ing ex­treme ther­mal sta­bil­ity of pro­teins re­main elu­sive. Keep in mind the claim that life is pos­si­ble at 121°C. Is this a dif­fer­ent kind of bio­chem­istry, one that re­quires pon­der­ing about un­ex­pected and per­haps as­ton­ish­ing prop­er­ties of macro­mol­e­cules? Or will "reg­u­lar" bio­chem­istry suf­fice, pro­vided we ob­tain more data?

 

Ref­er­ence

Bot­ting CH, Tal­bot P, Pay­tubi S, White MF (2010). Ex­ten­sive ly­sine methy­la­tion in hy­per­ther­mophilic cre­nar­chaea: po­ten­tial im­pli­ca­tions for pro­tein sta­bil­ity and re­com­bi­nant en­zymes. Ar­chaea (Van­cou­ver, B.C.). PMID 20811616

 

S. Mar­vin Fried­man is Pro­fes­sor Emer­i­tus, De­part­ment of Bi­o­log­i­cal Sci­ences, Hunter Col­lege of CUNY, New York City.