User:Nikhil Malvankar/Geobacter pilus structure and function: Difference between revisions

[[Interactive_3D_Complement_in_Proteopedia|Interactive 3D Complement in Proteopedia]]<br><br>

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<span style="font-size:160%"><b>Structure of novel pili evolved for extracellular translocation of microbial nanowires.</b></span>

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and Nikhil S. '''[https://malvankarlab.yale.edu/pi Malvankar]'''.

(journal article link here) (2020).

(DOI link here)

<StructureSection size='[250,500]' side='right' caption='' scene='83/834714/Filament/1'>

Previously, pili of ''Geobacter sulfurreducens'' were thought to be composed of PilA-N, a 61-amino acid protein<ref>PMID: 25736881 </ref><ref>PMID: 31608018 </ref>. In addition to pili, electrically conductive nanowires composed of linear polymers of cytochromes OmcS and OmcZ have been reported<ref name="nw1">PMID: 30951668</ref><ref name="nw2">PMID: 31925024</ref>.

Our electron cryomicroscopic structure of ''Geobacter sulfurreducens'' pili, [[6vk9]], reveals them to be composed of a core of '''<font color='#e87000'>PilA-N</font>''' coated with an outer surface layer of '''<font color='00a0a0'>PilA-C</font>''' (104 amino acids; <scene name='83/834714/Filament/1'>restore initial scene)</scene>. Here is a <scene name='83/834714/Filament/2'>cutaway view</scene> (front half hidden). The C-termini of '''<font color='#e87000'>PilA-N</font>''' <scene name='83/834714/Filament/3'>protrude into sockets</scene> in '''<font color='00a0a0'>PilA-C</font>'''.

The '''<font color='#e87000'>PilA-N</font>''' subunits have extensive hydrophobic contacts with each other, stabilizing the hydrophobic core of the filament. Each '''<font color='#e87000'>PilA-N</font>''' chain contacts 75 carbon atoms from adjacent '''<font color='#e87000'>PilA-N</font>''' chains, and also has 4 hydrogen bonds and 4 salt bridges with adjacent  '''<font color='#e87000'>PilA-N</font>''' chains. In contrast, '''<font color='00a0a0'>PilA-C</font>''' subunits have little contact with each other: 14 atoms, which are mostly hydrogen bonds with one salt bridge. <font color="red">[Scenes not yet constructed.]</font>

The filament is assembled from <scene name='83/834714/Dimer/3'>heterodimers</scene>. Dimer <scene name='83/834714/Dimer/2'>secondary structure</scene>: '''<font color='#e87000'>PilA-N</font>''' consists of two alpha helices, while '''<font color='00a0a0'>PilA-C</font>''' includes a 3-stranded beta sheet. The C-terminal protrusion of '''<font color='#e87000'>PilA-N</font>''' is <scene name='83/834714/Flaps/4'>held between two flaps</scene> (darker) of '''<font color='00a0a0'>PilA-C</font>'''. 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 '''<font color='#e87000'>PilA-N</font>'''. These flaps might be open before '''<font color='#e87000'>PilA-N</font>''' arrives to form a dimer, reminiscent of the flaps of HIV protease<ref>PMID: 16418268</ref>. (See, for example, [[1hxw]].)

As detailed in the journal publication, the PilA-N-C pili studied here are 50-fold less conductive than the nanowires composed of cytochromes<ref name="nw1" /><ref name="nw2" />. 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.

* (to be added)