User:Nikhil Malvankar/Geobacter pilus structure and function

Previously, pili of Geobacter sulfurreducens were thought to be composed of PilA-N, a 61-amino acid protein^[1][2]. In addition to pili, electrically conductive nanowires composed of linear polymers of cytochromes OmcS and OmcZ have been reported^[3][4].

Pilus Structure

Our electron cryomicroscopic structure of Geobacter sulfurreducens pili, 6vk9, reveals them to be composed of a core of PilA-N coated with an outer surface layer of PilA-C (104 amino acids; . Here is a (front half hidden). The C-termini of PilA-N in PilA-C.

The PilA-N subunits have extensive hydrophobic contacts with each other, stabilizing the hydrophobic core of the filament. Each PilA-N chain contacts 75 carbon atoms from adjacent PilA-N chains, and also has 4 hydrogen bonds and 4 salt bridges with adjacent PilA-N chains. In contrast, PilA-C subunits have little contact with each other: 14 atoms, which are mostly hydrogen bonds with one salt bridge. [Scenes not yet constructed.]

Heterodimers

The pilus filament is assembled from . Dimer : PilA-N consists of two alpha helices, while PilA-C includes a 3-stranded beta sheet. The C-terminal protrusion of PilA-N is (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^[5]. (See, for example, 1hxw.) (red: 10, 11, 31, 37) provide flexibility that could enable opening of the flaps.

Other Findings and Conclusions

As detailed in the journal publication, the PilA-N-C pili studied here are 50-fold less conductive than the nanowires composed of cytochromes^[3][4]. 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.