抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

基礎講座 電気泳動(Electrophoresis)

Motivated by experiments in which a polynucleotide is driven through a proteinaceous pore by an electric field, we study the diffusive motion of a polymer threaded through a narrow channel with which it may have strong interactions. We show that there is a range of polymer lengths in which the system is approximately translationally invariant, and we develop a coarse-grained description of this regime. From this description, general features of the distribution of times for the polymer to pass through the pore may be deduced. We also introduce a more microscopic model. This model provides a physically reasonable scenario in which, as in experiments, the polymer's speed depends sensitively on its chemical composition, and even on its orientation in the channel. Finally, we point out that the experimental distribution of times for the polymer to pass through the pore is much broader than expected from simple estimates, and speculate on why this might be.

Driven Polymer Translocation Through a Narrow Pore

In recent years polymer translocation, i.e., transport of polymeric molecules through nanometer-sized pores and channels embedded in membranes, has witnessed strong advances. It is now possible to observe single-molecule polymer dynamics during the motion through channels with unprecedented spatial and temporal resolution. These striking experimental studies have stimulated many theoretical developments. In this short theory-experiment review, we discuss recent progress in this field with a strong focus on non-equilibrium aspects of polymer dynamics during the translocation process.

Through the Eye of the Needle: Recent Advances in Understanding Biopolymer Translocation

Afin de faciliter le sequencage des genomes, une troisieme generation de systemes de sequencage est necessaire. La translocation de biomolecules est le phenomene cle mis en jeu dans l'optique d'un sequencage par nanopore. Nous nous sommes interesse a ce phenomene avec une approche a la fois theorique et numerique. Dans le cadre de nos travaux, nous avons tout d'abord elabore un modele dit gros grain de polymere structure (presentant une structure proche de celle de l'ADN) adapte a une etude par dynamique moleculaire. Apres avoir verifie la pertinence de notre modele avec les lois de la physique statistique des polymeres, nous nous sommes concentres sur la translocation. Nous avons revisite le cas standard d'un nanopore au sein d'une membrane fixe et propose un modele theorique dans le cas de la traction du polymere. L'arrivee des membranes fines, cles du succes d'un eventuel sequencage non destructif par nanopore entraine de nouvelles interactions avec la membrane qui n'ont pas encore ete etudiees. Nous presentons la premiere etude numerique de grande ampleur sur ses interactions. Nos resultats permettent d'etudier l'influence des vibrations, de la deformabilite et de la flexibilite de la membrane.

Etude théorique de la translocation de biomolécules à travers une membrane fine

We investigate the dynamics of polymer translocation through a nanopore under an externally applied field using the two-dimensional fluctuating bond model with single-segment Monte Carlo moves. We concentrate on the influence of the field strength E, length of the chain N, and length of the pore L on forced translocation. As our main result, we find a crossover scaling for the translocation time tau with the chain length from tau approximately N2nu for relatively short polymers to tau approximately N1+nu for longer chains, where nu is the Flory exponent. We demonstrate that this crossover is due to the change in the dependence of the translocation velocity v on the chain length. For relatively short chains v approximately N-nu, which crosses over to v approximately N(-1) for long polymers. The reason for this is that with increasing N there is a high density of segments near the exit of the pore, which slows down the translocation process due to slow relaxation of the chain. For the case of a long nanopore for which R parallel, the radius of gyration Rg along the pore, is smaller than the pore length, we find no clear scaling of the translocation time with the chain length. For large N, however, the asymptotic scaling tau approximately N1+nu is recovered. In this regime, tau is almost independent of L. We have previously found that for a polymer, which is initially placed in the middle of the pore, there is a minimum in the escape time for R parallel approximately L. We show here that this minimum persists for weak fields E such that EL is less than some critical value, but vanishes for large values of EL.

https://absimage.aps.org/image/MAR13/MWS_MAR13-2012-020023.pdf

Kaifu Luo

Papers

Dynamics of polymer translocation through a nanopore under an applied external field