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Type-IV Pilus deformation can explain retraction behavior.

Ghosh R, Kumar A, Vaziri A - PLoS ONE (2014)

Bottom Line: This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly.The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load.Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States of America.

ABSTRACT
Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100 pN during their retraction before stalling, powering surface translocation(twitching). Single TFP level experiments have shown remarkable nonlinearity in the retraction behavior influenced by the external load as well as levels of PilT molecular motor protein. This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly. In order to explain this behavior, we analyze the coupling of TFP elasticity and interfacial behavior with PilT kinetics. We model retraction as reaction controlled and elongation as transport controlled process. The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load. Elongation is controlled by monomer transport which suffer entrapment due to excess PilT in the cell periplasm. Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.

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Related in: MedlinePlus

Model simplification of TFP biological apparatus and their consequences.(a) A simplified reduction of the TFP processing bio-system into an axi-symmetric structure with a sliced view of TFP-protein/periplasm interfaces. The cylindrical retraction apparatus(RA) sits below the TFP base on the cytoplasmic part of the cell and the shallow cylindrical polar complex at the end of the TFP(shown in dotted red lines) is an electrostatic complex which is essential for recruiting pilins for elongation [18], [31] (b) the top part of the RA (only PilT shown) forming the RA-plane is responsible for the binding regime of the retraction process and is assumed to be very closely packed with PilT units sitting close to the base of the TFP. Note that the empty space surrounding the TFP and above the RA plane in this figure is actually filled by PilQ, enclosing periplasm and embedded minor proteins. (c) binding energy at zero deformation as a function of size of the RA-plane indicating three distinct zones and a strongly saturating characteristic assuming a van-derWalls type binding. The x axis is RA radius normalized by the pilus radius and y-axis is current binding energy normalized by that of an infinite plane. (Inserts: White circle indicates the size of RA plane and black the TFP cross section). (d)Normalized force-radius characteristic of TFP. The numbers on the loading curve (green) represent  (Insert: Free body diagram of loaded TFP,  is binding force due to RA).
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pone-0114613-g002: Model simplification of TFP biological apparatus and their consequences.(a) A simplified reduction of the TFP processing bio-system into an axi-symmetric structure with a sliced view of TFP-protein/periplasm interfaces. The cylindrical retraction apparatus(RA) sits below the TFP base on the cytoplasmic part of the cell and the shallow cylindrical polar complex at the end of the TFP(shown in dotted red lines) is an electrostatic complex which is essential for recruiting pilins for elongation [18], [31] (b) the top part of the RA (only PilT shown) forming the RA-plane is responsible for the binding regime of the retraction process and is assumed to be very closely packed with PilT units sitting close to the base of the TFP. Note that the empty space surrounding the TFP and above the RA plane in this figure is actually filled by PilQ, enclosing periplasm and embedded minor proteins. (c) binding energy at zero deformation as a function of size of the RA-plane indicating three distinct zones and a strongly saturating characteristic assuming a van-derWalls type binding. The x axis is RA radius normalized by the pilus radius and y-axis is current binding energy normalized by that of an infinite plane. (Inserts: White circle indicates the size of RA plane and black the TFP cross section). (d)Normalized force-radius characteristic of TFP. The numbers on the loading curve (green) represent (Insert: Free body diagram of loaded TFP, is binding force due to RA).

Mentions: We first simplify the cell wall portion of TFP bio-system illustrated in Fig. 1(b) into an equivalent homogenized axially loaded axi-symmetric cylindrical structure, Fig. 2 (a). The TFP is surrounded by a large protein PilQ spanning about half of periplasm, minor proteins as well as the periplasmic material itself [17]. These minor proteins include for instance in N. gonorrhoeae, PilD which is a preplin peptidase [18] without which the bacterial will not be able to process the incipient pre-pilin into pilin subunits [19], PilG which is another crucial inner membrane protein closely related to PilD and also aids in pilus biogenesis [20], PilF which is an assembly ATPase without which the bacteria would not be able to assemble the mature pilin subunit [19] and PilC which acts as a tip-located adhesin for end attachment of TFP useful for instance in DNA uptake [21], [22]. The morphology of PilQ protein found widely in various gram negative species [18] is most well characterized in Neisseria meningitidis[18] where a four-fold symmetric cage like structure emerges through cryo-electron microscopy (EM) reconstruction [23]. A side view resembles a cylindrical hollow frustum with a tapering cavity which narrows down somewhat towards the bottom [23]. Absence of this elaborate pore would leave no place for the assembled TFP to emanate from the cell [24]. Interestingly, the binding capabilities of this protein for long helical DNA strands for both N. meningitidis and N. gonorrhoeae have been well known [25], [26] and the similarity of the machinery with TFP processing has been already theorized [3]. This suggests that the inner surface TFP-PilQ interaction is dominated by a radial adhesive traction field.


Type-IV Pilus deformation can explain retraction behavior.

Ghosh R, Kumar A, Vaziri A - PLoS ONE (2014)

Model simplification of TFP biological apparatus and their consequences.(a) A simplified reduction of the TFP processing bio-system into an axi-symmetric structure with a sliced view of TFP-protein/periplasm interfaces. The cylindrical retraction apparatus(RA) sits below the TFP base on the cytoplasmic part of the cell and the shallow cylindrical polar complex at the end of the TFP(shown in dotted red lines) is an electrostatic complex which is essential for recruiting pilins for elongation [18], [31] (b) the top part of the RA (only PilT shown) forming the RA-plane is responsible for the binding regime of the retraction process and is assumed to be very closely packed with PilT units sitting close to the base of the TFP. Note that the empty space surrounding the TFP and above the RA plane in this figure is actually filled by PilQ, enclosing periplasm and embedded minor proteins. (c) binding energy at zero deformation as a function of size of the RA-plane indicating three distinct zones and a strongly saturating characteristic assuming a van-derWalls type binding. The x axis is RA radius normalized by the pilus radius and y-axis is current binding energy normalized by that of an infinite plane. (Inserts: White circle indicates the size of RA plane and black the TFP cross section). (d)Normalized force-radius characteristic of TFP. The numbers on the loading curve (green) represent  (Insert: Free body diagram of loaded TFP,  is binding force due to RA).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4264779&req=5

pone-0114613-g002: Model simplification of TFP biological apparatus and their consequences.(a) A simplified reduction of the TFP processing bio-system into an axi-symmetric structure with a sliced view of TFP-protein/periplasm interfaces. The cylindrical retraction apparatus(RA) sits below the TFP base on the cytoplasmic part of the cell and the shallow cylindrical polar complex at the end of the TFP(shown in dotted red lines) is an electrostatic complex which is essential for recruiting pilins for elongation [18], [31] (b) the top part of the RA (only PilT shown) forming the RA-plane is responsible for the binding regime of the retraction process and is assumed to be very closely packed with PilT units sitting close to the base of the TFP. Note that the empty space surrounding the TFP and above the RA plane in this figure is actually filled by PilQ, enclosing periplasm and embedded minor proteins. (c) binding energy at zero deformation as a function of size of the RA-plane indicating three distinct zones and a strongly saturating characteristic assuming a van-derWalls type binding. The x axis is RA radius normalized by the pilus radius and y-axis is current binding energy normalized by that of an infinite plane. (Inserts: White circle indicates the size of RA plane and black the TFP cross section). (d)Normalized force-radius characteristic of TFP. The numbers on the loading curve (green) represent (Insert: Free body diagram of loaded TFP, is binding force due to RA).
Mentions: We first simplify the cell wall portion of TFP bio-system illustrated in Fig. 1(b) into an equivalent homogenized axially loaded axi-symmetric cylindrical structure, Fig. 2 (a). The TFP is surrounded by a large protein PilQ spanning about half of periplasm, minor proteins as well as the periplasmic material itself [17]. These minor proteins include for instance in N. gonorrhoeae, PilD which is a preplin peptidase [18] without which the bacterial will not be able to process the incipient pre-pilin into pilin subunits [19], PilG which is another crucial inner membrane protein closely related to PilD and also aids in pilus biogenesis [20], PilF which is an assembly ATPase without which the bacteria would not be able to assemble the mature pilin subunit [19] and PilC which acts as a tip-located adhesin for end attachment of TFP useful for instance in DNA uptake [21], [22]. The morphology of PilQ protein found widely in various gram negative species [18] is most well characterized in Neisseria meningitidis[18] where a four-fold symmetric cage like structure emerges through cryo-electron microscopy (EM) reconstruction [23]. A side view resembles a cylindrical hollow frustum with a tapering cavity which narrows down somewhat towards the bottom [23]. Absence of this elaborate pore would leave no place for the assembled TFP to emanate from the cell [24]. Interestingly, the binding capabilities of this protein for long helical DNA strands for both N. meningitidis and N. gonorrhoeae have been well known [25], [26] and the similarity of the machinery with TFP processing has been already theorized [3]. This suggests that the inner surface TFP-PilQ interaction is dominated by a radial adhesive traction field.

Bottom Line: This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly.The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load.Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States of America.

ABSTRACT
Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100 pN during their retraction before stalling, powering surface translocation(twitching). Single TFP level experiments have shown remarkable nonlinearity in the retraction behavior influenced by the external load as well as levels of PilT molecular motor protein. This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly. In order to explain this behavior, we analyze the coupling of TFP elasticity and interfacial behavior with PilT kinetics. We model retraction as reaction controlled and elongation as transport controlled process. The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load. Elongation is controlled by monomer transport which suffer entrapment due to excess PilT in the cell periplasm. Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.

Show MeSH
Related in: MedlinePlus