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Paper text:
antimicrobial cationic peptides are host defense molecules produced by the innate immune system of organisms all across the evolutionary spectrum .
indolicidin , encoded by a member of cathelicidin gene family , a cationic antimicrobial tridecapeptide amide ( h - ile - leu - pro - trp - lys - trp - pro - trp - trp - pro - trp - arg - arg - nh2 ) , was isolated from cytoplasmic granules of bovine neutrophils .
it is one of the shortest known natural - occurring antimicrobial peptide , toxic to both prokaryotes and eukaryotes [ 3 , 4 ] .
unlike several other antimicrobial peptides , the structure of indolicidin upon membrane interaction is not a well - defined helix or a -turn and does not display their characteristic amphipathic nature [ 47 ] .
it has been reported that indolicidin expresses its antimicrobial activity by creating pores through cell membranes .
other studies showed that indolicidin treatment resulted total disintegration of membrane structures   or that it did not cause cell lysis even at high concentration .
compared to -helical antibiotic peptides , indolicidin is less able to dissipate the bacterial inner membrane potential and forms smaller pores , yet it kills bacteria rapidly . it was reported that an interfacial membrane location was preferred by indolicidin [ 6 , 11 ] .
these results point to a mechanism of action that is different from well - defined channel formation .
lipid bilayer on polyelectrolyte films can be used as a useful experimental approach to study basic problems of biological membrane structures [ 1214 ] .
bacteriorhodopsin is a light - driven proton pump in the plasma membrane of halobacterium salinarum .
this integral membrane protein is tightly packed in two - dimensional crystalline from termed purple membrane with high ( 75% w / w ) bacteriorhodopsin content , with no other protein .
microscopic ( afm ) experiments could provide detailed information about these systems .   here ,
we present an afm study on the interaction of indolicidin with an artificial and a natural membrane .
freshly cleaved mica ( spi - chem mica sheets , structure probe , inc . ,
west chester , pa , usa ) surface was covered with poly(l - lysine)-poly(l - glutamic acid)-dipalmitoyl phosphatidylcholine ( pll - pga - dppc ) layers [ 14 , 17 ] , or with purified purple membrane of halobacterium salinarum [ 1820 ] on the following ways : experiments were performed in tris(hydroxymethyl)aminomethane ( tris - hcl , 10  mm ) , and sodium chloride ( nacl , 0.15  m ) buffer at ph = 7.4 .
the polyelectrolytes pll ( mr = 32600  g / mol ) and pga ( mr = 17000  g / mol ) have been both dissolved in the above mentioned buffer , at a concentration of 1  mg / ml . dppc was dissolved in chloroform : methanol ( 2  :  1 ) at final concentration of 20  mg / ml . afterward the solvent was evaporated by n2 flow .
buffer was added to the dried dppc ( 250  g / ml ) , and liposomes were formed by sonication .
no special care was taken to have unilamellar liposomes .   to form polyelectrolyte layers , pll was adsorbed first to the freshly cleaved mica
, then it was washed with buffer , and pga was adsorbed on pll layer , and it was washed again .
these steps have been followed by covering the treated mica with dppc layer ; it was heated for 1 hour at 46c , and let to cool slowly .
purple membrane of halobacterium salinarum was prepared according to the method of oesterhelt and stoeckenius , and it was adsorbed from a buffer containing 10  mm tris and 150  mm nacl at ph = 8.0 to the freshly cleaved mica surface treated with the same buffer containing 10  mm cacl2 .
indolicidin was added just before the measurements to the same buffer in which the membrane surfaces were prepared at 0.52 , 2.6 , 5.2 , 7.9 , and 15.7  m concentrations ( 1 , 5 , 10 , 15 , and 30  g / ml , resp . ) [ 10 , 22 ] .  
all chemicals were purchased from sigma - aldrich ( st . louis , mo , usa ) .
afm measurements were carried out with an asylum mfp-3d head and controller ( asylum research , santa barbara , ca , usa ) in tapping / noncontact ( ac ) mode .
the driver program mfp-3d xop was written in igor pro software ( version 5.05a , wavemetrics , lake oswego , or , usa ) .
silicon nitride ( biolever mini bl - ac40ts ) cantilevers ( olympus optical co. , ltd . , tokyo , japan ) were used for the experiments ( resonance frequency was 25  khz in water with 0.09  n / m spring constant ) .
typically 512  512 points were taken at 1  line / s scan rate in ac mode under buffer solution .
in order to clarify which model may be the more likely explanation of indolicidin 's antimicrobial activity ( i.e. , pore formation that disturbs the metabolism / homeostasis / ion balance of a cell , or the cell membrane destruction ) , we developed a modified supported membrane model : a polyelectrolyte film composed of pll and pga attached to flat mica surface and covered with a lipid bilayer of dppc .
the reason why we chose this membrane model is that it contains a layer between the hard surface ( mica ) and the lipid bilayer through which small peptides are able to penetrate . in other words ,
if indolicidin creates a pore through a membrane , this structure may ensure space for indolicidin on the other side of the membrane , and it can access the membrane from the side that is close to mica , too . in models
used previously for studying the action mechanism of indolicidin [ 8 , 23 , 24 ] , lipid bilayers were directly attached to mica , leaving no space between the membrane and the hard surface , which may be important for full pore formation . to test the effect of indolicidin on our membrane model , various concentrations of peptide solutions
were used ( in a range of 0.52 to 15.7  m ) [ 10 , 21 ] . on one hand ,
when we used 0.52  m of indolicidin , no membrane alterations could be detected with afm ( figure 1(a ) ) . if the concentration was increased above 2.6  m , instead of creating pores , indolicidin induced the appearance of aggregates , but the membranes were still intact ( figures 1(b ) ( 2.6  m ) , 1(c ) ( 5.2  m ) ) .
we hypothesize that these aggregates are formed from excess amount of indolicidin that is present in the system .
however , the aggregation process was too fast , and it did not allow us to follow the formation of these aggregates during afm imaging ( taking one image required approximately 10 minutes of scanning ) .   when we further increased the concentration of the antimicrobial peptide ( 7.9  m ( data not shown ) or 15.7  m ) , after about 2 hours , the membrane bilayer structure was destroyed ; this phenomenon was not observed at lower concentrations ( figure 1(d ) ) .   in summary , in our model membrane , we could not find a concentration where pore formation occurred .
this suggests that the mechanism through which indolicidin expresses its antimicrobial activity is more likely via disintegrating membranes . to make sure that smaller concentrations have a different effect relative to higher concentrations , and for better understanding of the membrane destruction process , the time dependence of indolicidin treatment was also measured ( figure 2 ) .
15.7  m of indolicidin started to induce detectable changes in the membrane structure after the first 4050  min ( figures 2(b ) and 2(d ) ) .
the collapse of the membrane required 140  min at room temperature ( figures 2(e ) and 2(f ) ) .
based upon the above observations , that is , ( 1 ) indolicidin needs a minimal concentration to affect the bilayer structure ( below which there is no detectable impact , and above which there is a major / significant impact ) .
these results have a good agreement with the proposed mechanism reported by melo et al . .
indolicidin may behave as a surfactant - like material in a sense that it breaks down the continuity of membranes .
the purple membrane disks of halobacterium salinarum have been checked also to test the importance of pll / pga layer .
these membrane disks were deposited directly on the ca covered mica surface , without any polyelectrolyte film . in this case , the interaction was slower and more specific .
as it can be seen , indolicidin binds to the membrane , but the edges were preferred ( figures 3(b ) ( 7.9  m ) , 3(d ) ( 15.7  m ) ) , since the lipids were more accessible in those regions .
also the roughness of the membrane increased , probably caused by the aggregation of the indolicidin on the surface . even at higher concentration ( 15.7  m ) , indolicidin attached to the membrane surface and especially on the border of membrane disk only but did not break the membrane integrity .
this transient layer has about half height ( 3 - 4  nm ) of the purple membrane , in agreement with the results of shaw et al .   and mecke et al .
based on the height of the layer , it is probably a monolayer of lipids stabilized with indolicidin .
the subject of our present publication is an in situ atomic force microscopy study of interaction of indolicidin with supported planar bilayer membranes of dppc , and purple membrane of halobacterium salinarum .
the effect of the peptide on membranes was concentration dependent for dppc and it resulted in the destruction of intact membranes . among the samples examined , the purple membrane was less sensitive to indolicidin treatment , most likely due to the high membrane protein content .
for these membranes , indolicidin tended to bind to the border of membrane disks , where the lipids are easier to interact with . in this paper
, we demonstrated that the association of indolicidin with supported planar bilayers causes membrane thinning similar to the work of shaw et al .
[ 23 , 24 ] and mecke et al .   or solubilization of supported membrane on polyelectrolyte film rather than initiating pore formation .
these results are in good correlation with molecular dynamics simulations [ 27 , 28 ] .
indolicidin , a cationic antimicrobial tridecapeptide amide , is rich in proline and tryptophan residues . 
 its biological activity is intensively studied , but the details how indolicidin interacts with membranes are not fully understood yet . 
 we report here an in situ atomic force microscopic study describing the effect of indolicidin on an artificial supported planar bilayer membrane of dipalmitoyl phosphatidylcholine ( dppc ) and on purple membrane of halobacterium salinarum . 
 concentration dependent interaction of the peptide and membranes was found in case of dppc resulting the destruction of the membrane . 
 purple membrane was much more resistant against indolicidin , probably due to its high protein content . 
 indolicidin preferred the border of membrane disks , where the lipids are more accessible . 
 these data suggest that the atomic force microscope is a powerful tool in the study of indolicidin - membrane interaction .