STRUCTURE-FUNCTION RELATIONSHIPS IN THE MEMBRANE CHANNEL PORIN. Georg E. Schulz, Institut fur Organische Chemie und Biochemie Albertstr. 21, 79104 Freiburg im Breisgau, Germany

Porins form channels in the protective outer membrane of Gram-negative bacteria that are permeable for polar molecules, but discriminate against nonpolar ones. The first crystal structure had been reported in 1990 for Rhodobacter capsulatus [1]. All structurally known porins have subunits with 16- or 18-stranded [[beta]]-barrels surrounding a pore with a diameter of about 1 nm. Three barrels associate along their axes to form a trimer [2-6]. All porins contain two girdles of aromatic residues facing the membrane at its two polar-nonpolar borderlines, which are likely to fulfill a shielding function. Moreover, all general pores are lined by ionogenic groups that segregate into negatively and positively charged rims. It is suggested that they constitute an electric separator testing solute polarity [6-8]. In two porins which had been classified as unspecific, we detected ligand binding sites.

Large amounts of the porin from R.blastica were expressed in inclusion bodies in E. cold and recovered to form crystals identical to those of the wild-type. This opens the possibility of protein engineering for changing channel properties [9]. The report includes the porins from R. capsulatus and R. blastica, as well as the maltoporin from Salmonella typhimurium. Purification for crystallization of these porins and other membrane proteins will be described.

References:

1. M.S.Weiss et al., FEBS-Lett. 267,268-272(1990).

2. M.S.Weiss h G.E.Schulz, J.Mol.Biol. 227,493-509(1992).

3. M.S.Weiss & G.E.Schulz, J.Mol.Biol. 231,817-824(1993).

4. A.Kreusch et al., Protein Sci.3,58-63(1994).

5. A.Kreusch & G.E.Schulz, J.Mol.Biol. 243,891-905(1994).

6. G.E.Schulz, Curr.Opin.Cell Biol. 5,701-707(1994).

7. G.E.Schulz, in Membrane Proteins, Structures, Interactions and Models(A.Pullman et al.,eds.) 403-412(1992), Kluwer, Dordrecht.

8. G.E.Schulz, in New Comprehensive Biochemistry(J.-M. Guysen et al., eds.), vol.27,343-352 (1994), Elsevier, Amsterdam.

9. B.Schmid, M.Kromer & G.E.Schulz, FEBS-Lett., in press (1996)