Simulating the All-Order Hopping Expansion II: Wilson Fermions

We investigate the extension of the Prokof'ev-Svistunov worm algorithm to Wilson lattice fermions in an external scalar field. We effectively simulate by Monte Carlo the graphs contributing to the hopping expansion of the two-point function on a finite lattice to arbitrary order. Tests are conducted for a constant background field i. e. free fermions at some mass. For the method introduced here this is expected to be a representative case. Its advantage is that we know the exact answers and can thus make stringent tests on the numerics. The approach is formulated in both two and three space-time dimensions. In D=2 Wilson fermions enjoy special positivity properties and the simulation is similarly efficient as in the Ising model. In D=3 the method also works at sufficiently large mass, but there is a hard sign problem in the present formulation hindering us to take the continuum limit.Comment: 29 pages [12pt], 5 figure

Identification of targeting signals in human P2Y receptors in polarized MDCK(II) epithelial cells

P2Y receptors, which belong to the G-protein coupled receptor superfamily, play prominent roles in epithelial cell physiology, such as regulated ion transport and response to stress. Published studies utilizing indirect, pharmacology assays suggested a polarized distribution of P2Y receptors in a variety of epithelial cells. Therefore, we examined directly the distribution pattern of the entire P2Y receptor family in MDCK(II) epithelial cells by confocal microscopy as well as the localization of the Gq-coupled P2Y receptors (P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11) in epithelial cells from lung and colon. Our results showed that seven of the eight receptor subtypes are localized to either the apical or basolateral membrane surface of MDCK(II) cells. Moreover, a nearly identical pattern of distribution was observed in the other epithelial cell types (Wolff et al., 2005). The polarized targeting of cell-surface proteins is mediated by the protein-sorting machinery of the cell, which reads and interprets targeting signals contained within the primary sequence of polarized proteins and ensures delivery to the correct subcellular location. We postulated that P2Y receptors contain targeting signals that direct their polarized sorting in epithelial cells. To test this hypothesis, we analyzed a series of P2Y receptor mutants and chimeras, which allowed us to locate the targeting signals for all of the polarized P2Y receptor subtypes. Once the locations of the apical or basolateral targeting signals were determined, we fully characterized the basolateral targeting signal of the P2Y1 receptor and the apical targeting signal of the P2Y4 receptor, both of which are located in the C-terminal tail. The results of these studies demonstrated that the basolateral signal of the P2Y1 receptor is 25 amino acids in length and functions in a sequence-independent manner, with charged residues playing a key role in targeting, while the apical signal of the P2Y4 receptor is 23 amino acids long with no remarkable features or key amino acids identified as of yet. In this dissertation, we describe a series of experiments that completely characterized the apical and basolateral signals of these two purinergic receptors