THE STRUCTURE OF PORE-FORMING PROTEINS IN PROKARYOTES AND EUKARYOTES

Kornelius Zeth¹, Joachim Diez², Max Dolder³, Volker Adams⁴, Kay Diederichs², Wolfram Welte², Theo Wallimann³, Harald Engelhardt¹

¹Max-Planck-Institut für Biochemie, Abt. Molekulare Strukturbiologie, Am Klopferspitz 18a, D-82152 Martinsried/München, Germany.
²Fakultät für Biologie, Universität Konstanz, Universitätsstraße 10, D-78457 Konstanz, Germany.
³Institut für Zellbiologie, ETH Zürich, ETH-Hönggerberg, CH-8093 Zürich, Switzerland.
⁴Herzzentrum Leipzig GmbH, Universitätsklinik, Russenstraße 19, D-04289 Leipzig, Germany.

Keywords: Porin, VDAC, integral membrane protein

Pore-forming proteins are found in prokaryotes and eukaryotes. They are integral membrane proteins containing water-filled pores which were in the case of prokaryotic outer membrane proteins originally called porins. Eukaryotic cells contain similar ion channels called voltage-dependent anion channels (VDACs). because of their voltage dependency and their tendency toward anion selectivity as determined for several eukaryotic VDAC channels in artifical lipid bilayers under low transmembrane potentials.

Porins are responsible for the exchange of small hydrophilic molecules like ions, small sugars or organic acids between gram-negative bacteria and their environment. VDAC occurs in the outer membrane of mitochondria and is thought to be the main permeability pathway for the exchange of ATP and ADP, ions and other metabolic substances between mitochondria and cytosol therefore playing an important role for the exchange of energetically important compounds. It is although known that VDAC is the site of attachment of two cytoplasmic kinases (hexokinase and glycerol kinase) and kinases binding from the opposite site (i.e. creatine kinase), the mitochondrial matrix space. The cytosolic kinases are involved in several metabolic pathways getting preferetial access to ATP upon binding to VDAC.

The anion-selective porin from Comamonas acidovorans and VDAC isoform one from human (HVDAC1) were compared and it was found that the size of both pore forming proteins is very similar as well as their high content of antiparallel b-sheet which gives the membrane spanning part of both channels their barrel (or pore) form. The orientation and assoziation of single pores into intact channels seems to be different as in bacteria each porin channel is similar oriented in the homotrimer revealed using high resolution X-ray crystallography of single crystals whereas the orientation of the two VDAC channels is antiparallel (or in a head-to-tail fashion) as analysed by electron microscopy using highly ordered ice embedded 2D protein arrays.