Sandbox
Structural basis for regulation of rhizobial nodulation and symbiosis gene expression by the regulatory protein NolR.
Soon Goo Lee, Hari B.Krishnan and Joseph M.Jez
Structure Tour
BackgroundPreviously, pili of Geobacter sulfurreducens were thought to be composed of PilA-N, a 61-amino acid protein[1][2][3]. Immediately downstream from the pilA-N gene is pilA-C, coding for a 104 amino acid protein suspected to be the missing C-terminal globular domain of PilA-N[4][5]. Gene fission of pilins is widely distributed in Desulfuromonadales including Geobacteracae[6]. In addition to pili, electrically conductive nanowires composed of linear polymers of cytochromes OmcS and OmcZ have been reported[7][8][9]. Pilus Structure
Our electron cryomicroscopic structure of Geobacter sulfurreducens pili (restore initial scene), 6vk9, reveals them to be composed of a core of PilA-N (61 amino acids) coated with an outer surface layer of PilA-C (104 amino acids). Here is a cutaway view (front half hidden). The C-termini of PilA-N protrude into sockets in PilA-C. The PilA-N subunits have extensive hydrophobic contacts with each other, stabilizing the hydrophobic core of the filament. View PilA-N with PilA-C hidden. Each PilA-N chain contacts 75 carbon atoms from 11 adjacent PilA-N chains, and also has 4 hydrogen bonds and 4 salt bridges with adjacent PilA-N chains (not shown). In contrast, PilA-C subunits (view PilA-C with front half and PilA-N hidden) have little contact with each other: 14 atoms, which are mostly hydrogen bonded, with one salt bridge (not shown). HeterodimersThe pilus filament is assembled from heterodimers. Dimer secondary structure: PilA-N consists of two alpha helices, while PilA-C includes a 3-stranded beta sheet. The C-terminal protrusion of PilA-N is held between two flaps (darker) of PilA-C. The flaps have almost no contact with each other. They are held in place by apolar contacts and hydrogen bonds with the C-terminal protrusion of PilA-N. These flaps might be open before PilA-N arrives to form a dimer, reminiscent of the flaps of HIV protease[10]. (See, for example, 1hxw and Flaps Morph for HIV Protease.) Four glycines (red: 10, 11, 31, 37) provide flexibility that could enable opening of the flaps. Other Findings and ConclusionsAs detailed in the journal publication, the PilA-N-C pili studied here are 20-fold less electrically conductive than the nanowires composed of OmcS cytochromes[7][8], and 20,000-fold less conductive than OmcZ nanowires[9]. These PilA-N-C pili lack the structural hallmarks of type 4 pili, but share structural characteristics with pseudopili. PilA-N and PilA-C remain in the inner membrane, unless the gene for OmcS (or OmcZ) is deleted, in which case they form the pili extending outside the cell studied here. When the pilA-N gene is deleted, OmcS nanowires fail to be produced. It is proposed in the journal publication that PilA-N-C is part of a secretion system required for production of OmcS/OmcZ nanowires.
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See Also
- 6vk9, the structure described here.
- 1hxw: A list of all interactive 3D complements for publications from the Malvankar group, including:
- Structure of the OmcS conductive nanowire: Flaps Morph for HIV Protease
