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<span style="font-size:160%"><b>Structural basis for regulation of rhizobial nodulation

protein NolR.</b></span>

<span style="font-size:100%">

[https://doi.org/10.1073/pnas.1402243111] (2014).

[https://doi.org/10.1073/pnas.1402243111]

<StructureSection size='[250,500]' side='right' caption='' scene='83/834714/Filament/7'>

===Background===

Previously, pili of ''Geobacter sulfurreducens'' were thought to be composed of PilA-N, a 61-amino acid protein<ref name="blindmen">PMID: 33070100</ref><ref name="homolmod">PMID: 25736881 </ref><ref name="lovleyreview">PMID: 31608018 </ref>. 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<ref>PMID:22408162 </ref><ref>PMID:28348867</ref>. Gene fission of pilins is widely distributed in ''Desulfuromonadales'' including ''Geobacteracae''<ref>PMID: 28066394</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><ref name="omcz">PMID: 32807967</ref>.

===Pilus Structure===

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Our electron cryomicroscopic structure of ''Geobacter sulfurreducens'' pili (<scene name='83/834714/Filament/7'>restore initial scene</scene>), [[6vk9]], reveals them to be <scene name='83/834714/Filament/9'>composed of a core</scene> of '''<font color='#e87000'>PilA-N</font>''' (61 amino acids) coated with an outer surface layer of '''<font color='00a0a0'>PilA-C</font>''' (104 amino acids). Here is a <scene name='83/834714/Filament/10'>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 '''PilA-N''' subunits have extensive hydrophobic contacts with each other, stabilizing the hydrophobic core of the filament. <scene name='83/834714/Filament/11'>View PilA-N with PilA-C hidden</scene>. 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 (<scene name='83/834714/Filament/12'>view PilA-C with front half and PilA-N hidden</scene>) have little contact with each other: 14 atoms, which are mostly hydrogen bonded, with one salt bridge (not shown).

===Heterodimers===

The pilus filament is assembled from <scene name='83/834714/Dimer/5'>heterodimers</scene>. Dimer <scene name='83/834714/Dimer/6'>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/7'>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]] and [[Flaps Morph for HIV Protease]].) <scene name='83/834714/Flaps/8'>Four glycines</scene> (<font color="red">'''red: 10, 11, 31, 37'''</font>) 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 20-fold less electrically conductive than the nanowires composed of OmcS cytochromes<ref name="nw1" /><ref name="nw2" />, and 20,000-fold less conductive than OmcZ nanowires<ref name="omcz" />. 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.

* [[Malvankar]]: A list of all interactive 3D complements for publications from the Malvankar group, including:

** Structure of the OmcS conductive nanowire: [[Malvankar/2|2019, Cell: Structure of Microbial Nanowires Reveals Stacked Hemes that Transport Electrons over Micrometers.]]

==Notes & References==

<references />