SB2013 L04gr5: Difference between revisions

[http://en.wikipedia.org/wiki/Lyme_disease Lyme disease] is the most pervasive tick-borne disease in Europe, the United States,and parts of Asia.<ref name="Coutte" /> It is a multistage infection caused by the spirochete [http://en.wikipedia.org/wiki/Borrelia_burgdorferi ''Borrelia burgdorferi'']. Early symptoms include headaches, depression, rash, and fever. If Lyme disease is left untreated serious complications of the joints, heart and central nervous system can occur. Infected [http://en.wikipedia.org/wiki/Ixodes ''Ixodes'' ticks] in their nymph stage transmit Lyme disease by attaching to humans and other mammals<ref name="Lab Tutor" />.  In most cases, the tick must be attached for 36-48 hours before the bacteria can be transmitted.<ref name="CDC" />  

Vmp-like sequence Expressed protein (VlsE), is a [http://en.wikipedia.org/wiki/Lipoprotein lipoprotein] on the surface of ''Borrelia burgdorferi''. VlsE contributes to the immune evasion and persistence of Lyme disease.<ref name="Coutte" /> VlsE contains invaraible and varaible domains. It was found that the varaible domain is highly immunogenic and the target of the immune response; however, through [http://en.wikipedia.org/wiki/Antigenic_variation antigenic variation] the lipoprotein is able to evade the host immune system.<ref name="Liang" />

Within the varible domain there are invariable regions that remain unchanged during antigenic varaition, and therefore may be targets of an immune response.  IR6, the most conserved IR, has been found to be immnodominant. The antigencity of each of the 6 IRs have been studied using peptide-based enzyme linked immunosorbent essays,[http://en.wikipedia.org/wiki/ELISA ELISA].<ref name="Liang" />

The lipoprotein, VlsE, consists of two invariable domains at the amino and carboxyl termini and a variable domain (Figure 1).  When referring to the primary structure of the protein, the variable domain is a cassette region located between the two termini.  The variable domain can be further broken down into variable regions (<scene name='SB2013_L04gr5/Variable_regions/3'>VRs</scene>) and invariable regions (<scene name='SB2013_L04gr5/Invariable_regions/2'>IRs</scene>) (Liang et al. 1999).  When crystallized, VlsE forms a four molecule asymmetric unit (image of the unit) with each molecule having slight differences in their conformation.  Although each molecule in the unit is slightly different, a single molecule of the protein consists of eleven [http://en.wikipedia.org/wiki/Alpha_helices α-helices] and four short [http://en.wikipedia.org/wiki/Beta_strand β-strands].  Helices α1 (aa 306-341), α2 (aa 68-87), α3 (aa 114-139), and α11 (aa 306-341) all form the <scene name='SB2013_L04gr5/Membrane_proximal_region/1'>membrane proximal region</scene> of VlsE while helices α4 through α10 form the primary region of the <scene name='SB2013_L04gr5/Membrane_distal_region/2'>membrane distal region</scene> of the protein.  The four short β-strands each consist of 3 amino acids and can also be located in the membrane distal region.  Covering the membrane distal part of VlsE are connecting loop regions, which lack any secondary structure and have different conformations in each of the molecules. (Eicken et al. 2002).  The helices α3 through α10 form the invariable regions and are connected by the connecting loops which are classified as the variable regions.  Although VlsE crystallizes into an asymmetrical unit, it appears primarily as monomeric in solution.  Because the interface between VlsE molecules in the crystal structure buries approximately 13% of the accessible surface area of the monomers, Eicken et al.suggest that there is a possibility of VlsE existing as a [http://en.wikipedia.org/wiki/Protein_dimer dimer] when in its natural state (Figure 2). <scene name='SB2013_L04gr5/Reset_button/1'>Reset</scene>

Antigenic variation is the process through which an organism is able to evade its host’s immune system.  The antigenic variation that occurs in VlsE uses a complex genetic conversion mechanism that is seen to be concentrated in the VRs. The complex genetic system for VlsE is located near the right telomere on a 28-kb linear plasmid (lp28-1) in strains of ''B. burgdorferi''.  This [http://en.wikipedia.org/wiki/Locus_%28genetics%29 locus] has been shown to be a crucial element for the persistence and virulence of Lyme disease (Bankhead and Chaconas 2007).  The vls antigenic variation locus consists of a vls expression site (vlsE) and 15 silent vls cassettes that reside just upstream of the site.  The vlsE cassette region, which is the variable domain and doesn’t include the invariable amino or carboxyl termini, has approximately 92% DNA sequence identity with the silent vls cassettes.  However, the silent vls cassettes lack promoter sequences and are therefore not expressed.  Throughout the course of infection, the sequences for the flanking termini and the silent vls cassettes are conserved while the vlsE sequence is recombined.  This suggests that the genetic variation mechanism occurs by copying segments of the 15 silent vls cassettes and completely replacing corresponding segments of vlsE sequences.  The resulting differences are centralized in the highly variable regions of the vls cassettes.  This allows for the constant evolution of the VR structures and evades the antibodies of the host immune system (Zhang and Norris 1998).

The Variable regions are an important component of the variable domain of VlsE. These regions have relatively high surface exposure, which accounts for approximately 37% of the total surface area of the crystallized protein [http://en.wikipedia.org/wiki/Monomer monomer] (Eicken 2002). The strategic location of the variable regions has been implicated in protecting the highly conserved invariable regions from antibody binding. These regions, which are almost completely buried within the membrane distal region, are surrounded by the protective loops of the variable regions.  

The variable regions’ location also supports the potential role that the protein plays in evading host immune responses through [http://en.wikipedia.org/wiki/Gene_conversion gene conversion] (Zhang and Norris 1998). These regions are hypothesized to expose highly variable [http://en.wikipedia.org/wiki/Epitopes epitopes], which would prevent the immune system from recognizing the antigen and subsequently prevent the onset of Lyme disease (Eicken 2002). This feature starkly contrasts to the invariable regions, which does not undergo constant variation and whose genetic composition remains mostly conserved (Zhang and Norris 1998).