Abstract
Proteins of the Omp85 family are conserved in all kingdoms of life. They mediate protein transport across or protein insertion into membranes and reside in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. Omp85 proteins contain a C-terminal transmembrane beta-barrel and a soluble N terminus with a varying number of polypeptide-transport-associated or POTRA domains. Here we investigate Omp85 from the cyanobacterium Anabaena sp. PCC 7120. The crystallographic three-dimensional structure of the N-terminal region shows three POTRA domains, here named P1 to P3 from the N terminus. Molecular dynamics simulations revealed a hinge between P1 and P2 but in contrast show that P2 and P3 are fixed in orientation. The P2-P3 arrangement is identical as seen for the POTRA domains from proteobacterial FhaC, suggesting this orientation is a conserved feature. Furthermore, we define interfaces for protein-protein interaction in P1 and P2. P3 possesses an extended loop unique to cyanobacteria and plantae, which influences pore properties as shown by deletion. It now becomes clear how variations in structure of individual POTRA domains, as well as the different number of POTRA domains with both rigid and flexible connections make the N termini of Omp85 proteins versatile adaptors for a plentitude of functions.
Highlights
Membrane proteins of the -barrel type are pore proteins made up from a varying number of -strands crossing the membrane
The class I PTB FhaC from B. pertussis is known by crystallographic three-dimensional structure determination to comprise two POTRA domains and a 16-stranded -barrel [24]
Initial attempts to solve the structures by molecular replacement using either full-length, truncated or polyalanine models of known POTRA domains as search models failed
Summary
In cyanobacteria like Anabaena, homo-oligomeric complexes exist in vivo [3], and oligomerization in vitro was found to be dependent on the POTRA region [20, 21]. A receptor function is described for Omp from Anabaena (anaOmp85) determined by in vitro pulldown [21], and for POTRA domains of BamA, probed for by NMR spectroscopy [16]. We report the structure of the Anabaena Omp POTRA domains, and derive common principles between proteobacterial and cyanobacterial POTRA domains. The similarities include interfaces on POTRA domains proposed in protein-protein interaction, and a hinge region between those domains. Differences must exist for pore gating as the long loop shown here to regulate pore activity is a unique structural feature seen in cyanobacteria and predicted for plantae
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