Polyamine Sharing between Tubulin Dimers Favours Microtubule Nucleation and Elongation via Facilitated Diffusion

We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends. Facilitated diffusion can promote microtubule assembly, because, upon encountering a growing nucleus or the microtubule wall, random GTP-tubulin sliding on their surfaces will increase the probability of association to the target sites (nucleation sites or MT ends). This is an original explanation for understanding the apparent discrepancy between the high rate of microtubule elongation and the low rate of tubulin association at the microtubule ends in the viscous cytoplasm. The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions. Natural polyamines (putrescine, spermidine, and spermine) are present in all living cells and are potent agents to trigger tubulin self-attraction. By using an analytical model, we analyze the implication of facilitated diffusion mediated by polyamines on nucleation and elongation of microtubules. In vitro experiments using pure tubulin indicate that the promotion of microtubule assembly by polyamines is typical of facilitated diffusion. The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics

School Smoking Policy Characteristics and Individual Perceptions of the School Tobacco Context: Are They Linked to Students’ Smoking Status?

The purpose of this study was to explore individual- and school-level policy characteristics on student smoking behavior using an ecological perspective. Participants were 24,213 (51% female) Grade 10–11 students from 81 schools in five Canadian provinces. Data were collected using student self-report surveys, written policies collected from schools, interviews with school administrators, and school property observations to assess multiple dimensions of the school tobacco policy. The multi-level modeling results revealed that the school a student attended was associated with his/her smoking behavior. Individual-level variables that were associated with student smoking included lower school connectedness, a greater number of family and friends who smoked, higher perceptions of student smoking prevalence, lower perceptions of student smoking frequency, and stronger perceptions of the school tobacco context. School-level variables associated with student smoking included weaker policy intention indicating prohibition and assistance to overcome tobacco addiction, weaker policy implementation involving strategies for enforcement, and a higher number of students smoking on school property. These findings suggest that the school environment is important to tobacco control strategies, and that various policy dimensions have unique relationships to student smoking. School tobacco policies should be part of a comprehensive approach to adolescent tobacco use

Structural model for the interaction of a designed Ankyrin Repeat Protein with the human epidermal growth factor receptor 2

Designed Ankyrin Repeat Proteins are a class of novel binding proteins that can be selected and evolved to bind to targets with high affinity and specificity. We are interested in the DARPin H10-2-G3, which has been evolved to bind with very high affinity to the human epidermal growth factor receptor 2 (HER2). HER2 is found to be over-expressed in 30% of breast cancers, and is the target for the FDA-approved therapeutic monoclonal antibodies trastuzumab and pertuzumab and small molecule tyrosine kinase inhibitors. Here, we use computational macromolecular docking, coupled with several interface metrics such as shape complementarity, interaction energy, and electrostatic complementarity, to model the structure of the complex between the DARPin H10-2-G3 and HER2. We analyzed the interface between the two proteins and then validated the structural model by showing that selected HER2 point mutations at the putative interface with H10-2-G3 reduce the affinity of binding up to 100-fold without affecting the binding of trastuzumab. Comparisons made with a subsequently solved X-ray crystal structure of the complex yielded a backbone atom root mean square deviation of 0.84-1.14 Ångstroms. The study presented here demonstrates the capability of the computational techniques of structural bioinformatics in generating useful structural models of protein-protein interactions