Common evolutionary origin of swapped-hairpin and double-psi beta barrels

The core of swapped-hairpin and double-psi beta barrels is formed by duplication of a conserved betaalphabeta element, suggesting a common evolutionary origin. The path connecting the two folds is unclear as the two barrels are not interconvertible by a simple topological modification, such as circular permutation. We have identified a protein family whose sequence properties are intermediate to the two folds. The structure of one of these proteins, Pyrococcus horikoshii PhS018, is also built by duplication of the conserved betaalphabeta element but shows yet a third topology, which we name the RIFT barrel. This topology is widespread in the structure database and spans three folds of the SCOP classification, including the middle domain of EF-Tu and the N domain of F1-ATPase. We propose that swapped-hairpin beta barrels arose from an ancestral RIFT barrel by strand invasion and double-psi beta barrels by a strand swap. We group the three barrel types into a metafold, the cradle-loop barrels.     

AbrB-like transcription factors assume a swapped hairpin fold that is evolutionarily related to double-psi beta barrels

AbrB is a key transition-state regulator of Bacillus subtilis. Based on the conservation of a betaalphabeta structural unit, we proposed a beta barrel fold for its DNA binding domain, similar to, but topologically distinct from, double-psi beta barrels. However, the NMR structure revealed a novel fold, the "looped-hinge helix." To understand this discrepancy, we undertook a bioinformatics study of AbrB and its homologs; these form a large superfamily, which includes SpoVT, PrlF, MraZ, addiction module antidotes (PemI, MazE), plasmid maintenance proteins (VagC, VapB), and archaeal PhoU homologs. MazE and MraZ form swapped-hairpin beta barrels. We therefore reexamined the fold of AbrB by NMR spectroscopy and found that it also forms a swapped-hairpin barrel. The conservation of the core betaalphabeta element supports a common evolutionary origin for swapped-hairpin and double-psi barrels, which we group into a higher-order class, the cradle-loop barrels, based on the peculiar shape of their ligand binding site.     

Barrel structures in proteins: automatic identification and classification including a sequence analysis of TIM barrels.

Automated methods for identifying and characterizing regular beta-barrels from coordinate data have been developed to analyze and classify various kinds of barrel structures based on geometric parameters such as the barrel strand number (n) and shear number (S). In total, we find 1,316 barrels in the January 1998 release of Protein Data Bank. Of 1,316 barrels, 1,277 barrels had an even shear number, corresponding to 50 nonhomologous families. The (beta alpha)8 triose phosphate isomerase (TIM) barrel (n = 8, S = 8) fold has the largest number of apparently nonhomologous entries, 16, although the trypsin like antiparallel (n = 6, S = 8) barrels (representing only three families) are the most common with 527 barrels. Of all the protein families that exhibit barrel structures, 68% are found to be various kinds of enzymes, the remainder being binding proteins and transport membrane proteins. In addition, the layers of side chains, which form the cores of barrels with S = n and S = 2n, are also analyzed. More sophisticated methods were developed for detecting TIM barrels specifically, including consideration of the amino acid propensities for the side chains that form the layers. We found that the residues on the outside of the eight stranded parallel beta-barrel, buried by the alpha-helices, are much more hydrophobic than the residues inside the barrel.     

Energetic approach to the folding of alpha/beta barrels

The folding of a polypeptide into a parallel (alpha/beta)8 barrel (which is also called a circularly permuted beta 8 alpha 8 barrel) has been investigated in terms of energy minimization. According to the arrangement of hydrogen bonds between two neighboring beta-strands of the central barrel therein, such an alpha/beta barrel structure can be folded into six different types: (1) left-tilted, left-handed crossover; (2) left-tilted, right-handed crossover; (3) nontilted, left-handed crossover; (4) nontilted, right-handed crossover; (5) right-tilted, left-handed crossover; and (6) right-tilted, right-handed crossover. Here "tilt" refers to the orientational relation of the beta-strands to the axis of the central beta-barrel, and "crossover" to the beta alpha beta folding connection feature of the parallel beta-barrel. It has been found that the right-tilted, right-handed crossover alpha/beta barrel possesses much lower energy than the other five types of alpha/beta barrels, elucidating why the observed alpha/beta barrels in proteins always assume the form of right tilt and right-handed crossover connection. As observed, the beta-strands in the energy-minimized right-tilted, right-handed crossover (alpha/beta)8-barrel are of strong right-handed twist. The value of root-mean-square fits also indicates that the central barrel contained in the lowest energy (alpha/beta)8 structure thus found coincides very well with the observed 8-stranded parallel beta-barrel in triose phosphate isomerase (TIM). Furthermore, an energetic analysis has been made demonstrating why the right-tilt, right-handed crossover barrel is the most stable structure. Our calculations and analysis support the principle that it is possible to account for the main features of frequently occurring folding patterns in proteins by means of conformational energy calculations even for very complicated structures such as (alpha/beta)8 barrels.     

beta-Trefoil fold. Patterns of structure and sequence in the Kunitz inhibitors interleukins-1 beta and 1 alpha and fibroblast growth factors.

Previous crystallographic analyses of the Kunitz inhibitors from soybean. Erythrina caffra and wheat, the interleukins-1 beta and 1 alpha and the acidic and basic fibroblast growth factors have shown that they contain a most unusual fold. It is formed by six two-stranded hairpins. Three of these form a barrel structure and the other three are in a triangular array that caps the barrel. The arrangement of the secondary structures gives the molecules a pseudo 3-fold axis. Although the different proteins have very similar structures, many of their sequences have no significant similarities overall. The structural determinants of this fold are described and discussed in this paper. The barrels in the different proteins have the same geometrical features: six strands tilted at 56 degrees to the barrel axis; a barrel diameter of 16 A, and the beta-sheet hydrogen bonded so that it is staggered with a shear number of 12. These features fit McLachlan's equations for ideal barrels formed by beta-sheets. The wide diameter of the barrels is filled by layers of residues that, while not identical in the different proteins, are, in almost all cases, large. The structure of the triangular array of hairpins is determined by the coiling of the strands and the packing of hairpin residues against each other and against residues from the interior of the barrel. The major sequence requirements of this fold are large or medium hydrophobic residues at 18 buried sites. In the different structures the total volume of these residues is 3000 (+/- 120) A. The polyhedron model of protein architecture is used to demonstrate that the main, and in particular the symmetrical, features of this fold arise from the ideal and equal packing of six hairpins, modified only slightly to form hydrogen bonds between the hairpins.     

In vivo imaging of neural circuit formation in the neonatal mouse barrel cortex

Higher brain function in mammals primarily relies on complex yet sophisticated neuronal circuits in the neocortex. In early developmental stages, neocortical circuits are coarse. Mostly postnatally, the circuits are reorganized to establish mature precise connectivity, in an activity-dependent manner. These connections underlie adult brain function. The rodent somatosensory cortex (barrel cortex) contains a barrel map in layer 4 (L4) and has been considered an ideal model for the study of postnatal neuronal circuit formation since the first report of barrels in 1970. Recently, two-photon microscopy has been used for analyses of neuronal circuit formation in the mammalian brain during early postnatal development. These studies have further highlighted the mouse barrel cortex as an ideal model. In particular, the unique dendritic projection pattern of barrel cortex L4 spiny stellate neurons (barrel neurons) is key for the precise one-to-one functional relationship between whiskers and barrels and thus an important target of studies. In this article, I will review the morphological aspects of postnatal development of neocortical circuits revealed by recent two-photon in vivo imaging studies of the mouse barrel cortex and other related works. The focus of this review will be on barrel neuron dendritic refinement during neonatal development.     

The equilibrium unfolding pathway of a (beta/alpha)8 barrel

The (beta/alpha)(8) barrel is the most commonly occurring fold among enzymes. A key step towards rationally engineering (beta/alpha)(8) barrel proteins is to understand their underlying structural organization and folding energetics. Using misincorporation proton-alkyl exchange (MPAX), a new tool for solution structural studies of large proteins, we have performed a native-state exchange analysis of the prototypical (beta/alpha)(8) barrel triosephosphate isomerase. Three cooperatively unfolding subdomains within the structure are identified, as well as two partially unfolded forms of the protein. The C-terminal domain coincides with domains reported to exist in four other (beta/alpha)(8) barrels, but the two N-terminal domains have not been observed previously. These partially unfolded forms may represent sequential intermediates on the folding pathway of triosephosphate isomerase. The methods reported here should be applicable to a variety of other biological problems involving protein conformational changes.     

Structural analysis of the 2.8 A model of Xylose isomerase from Actinoplanes missouriensis

The structure of Xylose isomerase (X.I.) from Actinoplanes missouriensis has been solved to 2.8 Angstroms resolution. Phases were determined from a single Eu3+ derivative and from the noncrystallographic 222 symmetry of the tetrameric molecule. An atomic model was built and subjected to restrained crystallographic refinement. The resulting model is shown to be closely similar to the recently reported X.I.'s structures from three other bacterial sources. Each monomer is found to be composed of an eight-stranded alpha/beta "T.I.M." barrel forming an N-terminal domain of 328 residues followed by a large loop of 66 residues embracing an adjacent subunit. Analysis of intersubunit packing shows that the X.I. tetramer is an assembly of two tight dimers. The beta barrel fits a simple hyperboloid model as other T.I.M. barrels do. The active site, identified as the binding site for the inhibitor xylitol, is located at the carboxyl end of the beta strands in the barrel next to a pair of binding sites for Eu3+ ions, which are assumed to be sites for the divalent ions involved in catalysis. Active sites in the tetramer are oriented towards the interface between dimers. It is suggested that subunit interfaces might stabilize the active site region and this might explain the oligomeric nature of other alpha/beta barrel enzymes.     

An improved method for constructing and selectively silanizing double-barreled,neutral liquid-carrier,ion-selective microelectrodes

We describe an improved, efficient and reliable method for the vapour-phase silanization of multi-barreled, ion-selective microelectrodes of which the silanized barrel(s) are to be filled with neutral liquid ion-exchanger (LIX). The technique employs a metal manifold to exclusively and simultaneously deliver dimethyldichlorosilane to only the ion-selective barrels of several multi-barreled microelectrodes. Compared to previously published methods the technique requires fewer procedural steps, less handling of individual microelectrodes, improved reproducibility of silanization of the selected microelectrode barrels and employs standard borosilicate tubing rather than the less-conventional theta-type glass. The electrodes remain stable for up to 3 weeks after the silanization procedure. The efficacy of a double-barreled electrode containing a proton ionophore in the ion-selective barrel is demonstrated in situ in the leaf apoplasm of pea (Pisum) and sunflower (Helianthus). Individual leaves were penetrated to depth of ∼150 μm through the abaxial surface. Microelectrode readings remained stable after multiple impalements without the need for a stabilizing PVC matrix.     

The structure of yeast enolase at 2.25-A resolution. An 8-fold beta + alpha-barrel with a novel beta beta alpha alpha (beta alpha)6 topology

The three-dimensional structure of yeast enolase has been determined by the multiple isomorphous replacement method followed by the solvent flattening technique. A polypeptide model, corresponding with the known amino acid sequence, has been fitted to the electron density map. Crystallographic restrained least-squares refinement of the model without solvent gave R = 20.0% for 6-2.25-A resolution with good geometry. A model with 182 water molecules and 1 sulfate which is still being refined has presently R = 17.0%. The molecule is a dimer with subunits related by 2-fold crystallographic symmetry. The subunit has dimensions 60 X 55 X 45 A and is built from two domains. The smaller N-terminal domain has an alpha + beta structure based on a three-stranded antiparallel meander and four helices. The main domain is an 8-fold beta + alpha-barrel. The enolase barrel is, however, different from the triose phosphate isomerase barrel; its topology is beta beta alpha alpha (beta alpha)6 rather than (beta alpha)8 as found in triose phosphate isomerase. The inner beta-barrel is not entirely parallel, the second strand is antiparallel to the other strands, and the direction of the first helix is also reversed with respect to the other helices. This supports the hypothesis that some enzymes evolved independently producing the stable structure of beta alpha barrels with either enolase or triose phosphate isomerase topology. The active site of enolase is located at the carboxylic end of the barrel. A fragment of the N-terminal domain and two long loops protruding from the barrel domain form a wide crevice leading to the active site region. Asp246, Glu295, and Asp320 are the ligands of the conformational cation. Other residues in the active site region are Glu168, Asp321, Lys345, and Lys396.     

Infrared dichroism of twisted beta-sheet barrels. The structure of E. coli outer membrane proteins

The infrared dichroic ratios of the amide bands from oriented beta-barrels yield an experimental value for the mean orientation, beta, of the beta-strands, relative to the barrel axis. For a barrel of n strands, this then gives the shear number, S, that characterizes the stagger of the beta-sheet. Combining values of beta and n specifies the barrel geometry by using the optimized model of Murzin, Lesk & Chothia for regular barrels. Application to published infrared data on the Escherichia coli outer membrane protein, OmpA yields S=9-10 (n=8), a barrel radius of 0.81(+/-0.01) nm, and an internal free volume of 0.031 nm(3) per residue, where the average twist of the beta-sheets is theta approximately 28 degrees, and their coiling angle is epsilon approximately 1 degrees. Hydrophobic matching of the 2.6 nm transmembrane stretch partly determines the shear number of the OmpA beta-barrel.     

The structure of a truncated phosphoribosylanthranilate isomerase suggests a unified model for evolution of the (βα)8 barrel fold

The (βα)₈ barrel is one of the most common protein folds, and enzymes with this architecture display a remarkable range of catalytic activities. Many of these functions are associated with ancient metabolic pathways, and phylogenetic reconstructions suggest that the (βα)₈ barrel was one of the very first protein folds to emerge. Consequently, there is considerable interest in understanding the evolutionary processes that gave rise to this fold. In particular, much attention has been focused on the plausibility of (βα)₈ barrel evolution from homodimers of half barrels. However, we previously isolated a three-quarter-barrel-sized fragment of a (βα)₈ barrel, termed truncated phosphoribosylanthranilate isomerase (trPRAI), that is soluble and almost as thermostable as full-length N-(5′-phosphoribosyl)anthranilate isomerase (PRAI). Here, we report the NMR-derived structure of trPRAI. The subdomain is monomeric, is well ordered and adopts a native-like structure in solution. Side chains from strands β₁ (Glu3 and Lys5), β₂ (Tyr25) and β₆ (Lys122) of trPRAI repack to shield the hydrophobic core from the solvent. This result demonstrates that three-quarter barrels were viable intermediates in the evolution of the (βα)₈ barrel fold. We propose a unified model for (βα)₈ barrel evolution that combines our data, previously published work and plausible scenarios for the emergence of (initially error-prone) genetic systems. In this model, the earliest proto-cells contained diverse pools of part-barrel subdomains. Combinatorial assembly of these subdomains gave rise to many distinct lineages of (βα)₈ barrel proteins, that is, our model excludes the possibility that there was a single (βα)₈ barrel from which all present examples are descended.     

Dipole moment in TIM alpha/beta fold proteins

TIM proteins of alpha/beta barrel fold from alpha/beta class as given in SCOP database were taken for dipole moment analysis. In all, 32 structures were analyzed for their dipole moment contributions. Representative structures from 20 super families in the alpha/beta fold, with different enzyme functions and 12 protein domains of TIM family in TIM super family were considered. The active sites of these proteins are located on the C-terminal side of the beta-strands. The molecules of same alpha/beta fold, but differing in their functionality also showed a common electrostatic field pattern along the barrel axis and had the dipole moment along the barrel axis and towards C-terminal end of the beta-strands. However, it is observed from our calculations that the dipole moment direction is possibly a consequence of the structural fold, with distribution of charges playing a modulatory role, and does not contribute to the location of active site. We show here that apart from the commonly held view as proposed by Hol et al [Hol W G L, van Duijnen PT and Berendsen H J C (1978) Nature (London), 273, 443-446] of the role of the alpha helical dipole moment, the beta-sheets in the barrel can also have a considerable dipole moment contribution. Taken together with our dipole moment analysis on integral membrane proteins [Vasanthi G and Krishnaswamy S (2002) Indian J Biochem Biophys 39, 93-100], this suggests the need to examine the role of dipole moment in the case of especially beta sheets forming barrels.     

Twisted hyperboloid (strophoid) as a model of β-barrels in proteins

Using a least-squares fitting procedure, polypeptide backbones of one parallel and seven antiparallel β-barrels were approximated with various curved surfaces. Although the hyperboloid gave better approximations to all the β-barrel backbones than the ellipsoid, elliptical cylinder or catenoid, the best approximations were obtained with a novel surface, a twisted hyperboloid (strophoid). The root-mean-square errors between individual β-barrels and the fitted strophoid surfaces ranged from 0.75 Å to 1.64 Å. The parameters which determine the strophoid surface allow groups of β-barrel shapes to be defined according to their barrel twists (i.e. angles subtended by directions of the long axis of cross-section at the top and the bottom of the barrel), course of elliptical cross-sections (either monotonically increasing along the barrel axis, as in cones, or having a middle “waist”, as in hyperboloids), and types of backbone curvatures (either convex or concave). The curvatures at individual points of strophoid surface are local, variable quantities related to the local helicity (coil) of the polypeptide backbone, in contrast to values of β-sheet twist (i.e. dihedral angles subtended by adjacent β-strands) known to be virtually identical in all the β-sheets. The variability found in parameters such as barrel shapes and curvatures suggests that simple models (isotropically stressed surfaces, principle of minimal surface tension) proposed in the past to account for β-barrel shapes are not sufficient. Rather, the complex nature of best-fit theoretical surfaces points to an important role played by a local variability of the forces involved.     

The effect of destroying the whisker follicles in mice on the sensory nerve, the thalamocortical radiation and cortical barrel development.

Electrolytic destruction of whisker follicles in mice on the day of birth has been found to cause degeneration in the sensory nerve fibres supplying the follicles. The severity of the degeneration has been assessed in animals between 2 and 20 days old by counting the total number of myelinated fibres in the maxillary nerves on both normal and lesioned sides. The degeneration is apparent after 2 days and by 20 days the nerve on the lesioned side contains only 38% of the normal fibre content. This degeneration has also been shown to involve the trigeminal root, central to the ganglion. In addition, the lesioning procedure modifies the terminations of thalamocortical fibres in the barrel region of the sensory cortex. These terminations are normally in clusters, each corresponding to a barrel, but, after lesioning the follicles, the terminals appear to be evenly distributed in layer IV and cortical barrel structures no longer develop. In postnatal mice, electrolytic destruction of whisker follicles had less effect upon maxillary nerve fibres and cortical barrels. The number of myelinated axons surviving until day 20 increased progressively with later lesioning to reach nearly 80% of the control level when lesions were made on day 10. Cortical barrels became secure earlier than the maxillary nerve, for a normal number of cortical barrels was present at day 12 when follicles were destroyed on day 4. The implications of these results for the formation of cortical barrels is discussed.     

Homology among (betaalpha)(8) barrels: implications for the evolution of metabolic pathways

We provide statistically reliable sequence evidence indicating that at least 12 of 23 SCOP (betaalpha)(8) (TIM) barrel superfamilies share a common origin. This includes all but one of the known and predicted TIM barrels found in central metabolism. The statistical evidence is complemented by an examination of the details of protein structure, with certain structural locations favouring catalytic residues even though the nature of their molecular function may change. The combined analysis of sequence, structure and function also enables us to propose a phylogeny of TIM barrels. Based on these data, we are able to examine differing theories of pathway and enzyme evolution, by mapping known TIM barrel folds to the pathways of central metabolism. The results favour widespread recruitment of enzymes between pathways, rather than a "backwards evolution" model, and support the idea that modern proteins may have arisen from common ancestors that bound key metabolites.     

In vivo fragment complementation of a (beta/alpha)(8) barrel protein: generation of variability by recombination

The high representation of the TIM barrel as a scaffold for enzymatic proteins makes it an interesting model for protein engineering. Based on previous reports of folding mechanisms of TIM barrels that suggest an independent folding unit formed by six (beta/alpha) subunits, we interrupted the gene of phosphoribosylanthranilate isomerase (PRAI) from Escherichia coli at three different positions to yield fragments with different combinations of (beta/alpha) subunits. When these constructions were expressed as polycistrons in a TrpF-E. coli strain, complementation of the function only occurred with fragments beta1-alpha4 and beta5-alpha8, demonstrating that (beta/alpha)(4) subunits are stable enough to survive in vivo conditions and to assemble to yield a functional enzyme. The expression of these fragments in a separated plasmid/phagemid system to complement the function gave a slower complementation in the TrpF-E. coli strain; this was overcome by introducing extra secondary elements to the structure that reinforce their interaction.     

BDNF and trkB mRNA Expression in Neurons of the Neonatal Mouse Barrel Field Cortex: Normal Development and Plasticity after Cauterizing Facial Vibrissae

Development of the central somatosensory system is profoundly modulated by the sensory periphery. Cauterization of facial whiskers alters the segregation pattern of barrels in rodents only during a few days just after birth (critical period). Although a molecular basis of the segregation of barrel neurons and the critical period for the anatomical plasticity observed in layer IV barrel neuron is not clear yet, the accumulating evidence suggests that neurotrophins modulate synaptic connections including central nervous system. In this study, we showed by in situ hybridization that mouse barrel side neurons express brain-derived neurotrophic factor (BDNF) mRNA and both catalytic and non-catalytic forms of trkB mRNA. Cautery of row C vibrissae on the right side of the face within 24 h after birth (post natal day 0, PND0) reduced the expression of BDNF and trkB mRNA from the division region between the contralateral row C barrels at PND7. The vibrissae in row A, C, and E were cauterized at PND0 followed by quantitative RT-PCR for BDNF and trkB mRNA with total RNA isolated from the barrel region at PND7. The result showed that BDNF, but not trkB, mRNA was increased several-fold in the contralateral barrel region. These data suggest that the expression of BDNF mRNA is differentially regulated between injured barrels and actively innervated barrels. The differential expression of the mRNA encoding neurotrophins and their receptors may be important in regulating the injury-dependent re-segregation of barrels.     

The stability of cylindrin β‐barrel amyloid oligomer models—A molecular dynamics study

Small‐soluble amyloid oligomers are believed to play a significant role in the pathology of amyloid diseases. Recently, the atomic structure of a toxic oligomer formed by an 11 residue and its tandem repeat was found to have an out‐off register antiparallel β‐strands in the shape of a β‐barrel. In the present article we investigate the effect of mutations in the hydrophobic cores on the structure and dynamic of the β‐barrels using all atom multiple molecular dynamics simulations with an explicit solvent. Extending previous experiments with molecular dynamics simulations we systematically test how stability and formation of cylindrin depends on the interplay between hydrophobicity and steric effects of the core residues. We find that strong hydrophobic interactions between geometrically fitting residues keep the strands (both in register and out‐off‐register interface) in close proximity, which in turn stabilizes the side‐chain and main‐chain hydrogen bonds, and the salt bridges on the outer surface along the weak out‐of‐register interface. Our simulations also indicate presence of water molecules in the hydrophobic interior of the cylindrin β‐barrel.Proteins 2013. © 2013 Wiley Periodicals, Inc.     

Structural predictions of AgfA, the insoluble fimbrial subunit of Salmonella thin aggregative fimbriae.

The unusually stable and multifunctional, thin aggregative fimbriae common to all Salmonella spp. are principally polymers of the fimbrin subunit, AgfA. AgfA of Salmonella enteritidis consists of two domains: a protease-sensitive, 22 amino acid residue N-terminal region and a protease-resistant, 109 residue C-terminal core. The unusual amino acid sequence of the AgfA core region comprises two-, five- and tenfold internal sequence homology patterns reflected in five conserved, 18-residue tandem repeats. These repeats have the consensus sequence, Sx5QxGx2NxAx3Q and are linked together by four or five residues, (x)xAx2. The predicted secondary structure for this unusual arrangement of tandem repeats in AgfA indicates mainly extended conformation with the beta strands linked by four to six residues. Candidate proteins of known structure with motifs of alternating beta strands and short loops were selected from folds described in SCOP as a source of coordinates for AgfA model construction. Three all-beta class motifs selected from the Serratia marcescens metalloprotease, myelin P2 protein or vitelline membrane outer protein I were used for initial AgfA homology build-up procedures ultimately resulting in three structural models; beta barrel, beta prism and parallel beta helix. The beta barrel model is a compact, albeit irregular structure, with the beta strands arranged in two antiparallel beta sheet faces. The beta prism model does not reflect the 5 or 10-fold symmetry of the AgfA primary sequence. However, the favored, parallel beta helix model is a compact coil of ten helically arranged beta strands forming two parallel beta sheet faces. This arrangement predicts a regular, potentially stable, C-terminal core region consistent with the observed tandem repeat sequences, protease-resistance and strong tendency of this fimbrin to oligomerize and aggregate. Positional conservation of amino acid residues in AgfA and the Escherichia coli AgfA homologue, CsgA, provides strong support for this model. The parallel beta helix model of AgfA offers an interesting solution to a multifunctional fimbrin molecular surface having solvent exposed areas, regions for major and minor subunit interactions as well as fiber-fiber interactions common to many bacterial fimbriae.