Balakrishnan Kannan

Molecular diffusion measurement in lipid bilayers over wide concentration ranges: a comparative study.

Molecular diffusion in biological membranes is a determining factor in cell signaling and cell function. In the past few decades, three main fluorescence spectroscopy techniques have emerged that are capable of measuring molecular diffusion in artificial and biological membranes at very different concentration ranges and spatial resolutions. The widely used methods of fluorescence recovery after photobleaching (FRAP) and single-particle tracking (SPT) can determine absolute diffusion coefficients at high (>100 microm(-2)) and very low surface concentrations (single-molecule level), respectively. Fluorescence correlation spectroscopy (FCS), on the other hand, is well-suited for the intermediate concentration range of about 0.1-100 microm(-2). However, FCS in general requires calibration with a standard dye of known diffusion coefficient, and yields only relative measurements with respect to the calibration. A variant of FCS, z-scan FCS, is calibration-free for membrane measurements, but requires several experiments at different well-controlled focusing positions. A recently established FCS method, electron-multiplying charge-coupled-device-based total internal reflection FCS (TIR-FCS), referred to here as imaging TIR-FCS (ITIR-FCS), is also independent of calibration standards, but to our knowledge no direct comparison between these different methods has been made. Herein, we seek to establish a comparison between FRAP, SPT, FCS, and ITIR-FCS by measuring the lateral diffusion coefficients in two model systems, namely, supported lipid bilayers and giant unilamellar vesicles.

Ca2+ binding by domain 2 plays a critical role in the activation and stabilization of gelsolin

Gelsolin consists of six homologous domains (G1–G6), each containing a conserved Ca-binding site. Occupation of a subset of these sites enables gelsolin to sever and cap actin filaments in a Ca-dependent manner. Here, we present the structures of Ca-free human gelsolin and of Ca-bound human G1–G3 in a complex with actin. These structures closely resemble those determined previously for equine gelsolin. However, the G2 Ca-binding site is occupied in the human G1–G3/actin structure, whereas it is vacant in the equine version. In-depth comparison of the Ca-free and Ca-activated, actin-bound human gelsolin structures suggests G2 and G6 to be cooperative in binding Ca2+ and responsible for opening the G2–G6 latch to expose the F-actin-binding site on G2. Mutational analysis of the G2 and G6 Ca-binding sites demonstrates their interdependence in maintaining the compact structure in the absence of calcium. Examination of Ca binding by G2 in human G1–G3/actin reveals that the Ca2+ locks the G2–G3 interface. Thermal denaturation studies of G2–G3 indicate that Ca binding stabilizes this fragment, driving it into the active conformation. The G2 Ca-binding site is mutated in gelsolin from familial amyloidosis (Finnish-type) patients. This disease initially proceeds through protease cleavage of G2, ultimately to produce a fragment that forms amyloid fibrils. The data presented here support a mechanism whereby the loss of Ca binding by G2 prolongs the lifetime of partially activated, intermediate conformations in which the protease cleavage site is exposed.

Structural characterization of a capping protein interaction motif defines a family of actin filament regulators

Capping protein (CP) regulates actin dynamics by binding the barbed ends of actin filaments. Removal of CP may be one means to harness actin polymerization for processes such as cell movement and endocytosis. Here we structurally and biochemically investigated a CP interaction (CPI) motif present in the otherwise unrelated proteins CARMIL and CD2AP. The CPI motif wraps around the stalk of the mushroom-shaped CP at a site distant from the actin-binding interface, which lies on the top of the mushroom cap. We propose that the CPI motif may act as an allosteric modulator, restricting CP to a low-affinity, filament-binding conformation. Structure-based sequence alignments extend the CPI motif–containing family to include CIN85, CKIP-1, CapZIP and a relatively uncharacterized protein, WASHCAP (FAM21). Peptides comprising these CPI motifs are able to inhibit CP and to uncap CP-bound actin filaments.

Characterization of ultra-wideband channels for outdoor office environment

We present a statistical model for the characterization of ultra-wideband (UWB) channels for the outdoor office environment. The bandwidth is from 3-6 GHz and measurements are done for the line-of-sight (LOS) and non-line-of-sight (NLOS) cases. Small scale effects are modeled by the Saleh-Valenzuela (S-V) model with modifications on the ray arrival times and amplitude statistics to fit the empirical data. The ray arrival times are described by a mixture of Poisson processes while the amplitude statistics are follow a Nakagami distribution with the m parameter following a lognormal distribution. The parameters for the cluster and ray power-decay time constants, cluster and ray arrival times, mean excess delay and RMS excess delay are also extracted to define the channel model completely.

Space-time coded MIMO-OFDM for high capacity and high data-rate wireless communication over frequency selective fading channels

This paper presents a space-time block coded multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) scheme over frequency selective fading channels. The system provides spectrally efficient transmissions to increase system capacity or system throughput for individual users. The proposed technique utilizes OFDM to transform frequency selective fading channels into multiple flat fading sub channels on which space-time block coding is applied. We consider a multi-user environment with K synchronous cochannel users, each equipped with n transmit antennas. The receiver is equipped with m(/spl ges/K) receive antennas. At the receiver, signals from K users are decoded using minimum mean squared error (MMSE) interference cancellation followed by maximum likelihood (ML) decoding. It is shown that the proposed scheme, with two cochannel terminals, doubles the data rate while maintaining the frame error rate (FER) performance as a single user scheme with some additional decoding complexity.

Performance of DS-UWB multiple-access systems with diversity reception in dense multipath environments

Ultrawideband (UWB) technology is characterized by transmitting extremely short duration radio impulses. To improve its multiple-access capability, the UWB technology can be combined with traditional spread-spectrum techniques. This paper demonstrates the influence of spatial and temporal diversities on the performance of direct-sequence (DS) UWB multiple-access systems in dense multipath environments. Numerical results show that the bit error rate performance of the DS-UWB system can be improved significantly by increasing the number of antenna array elements and/or by adding more multipaths coherently at the receiver. Furthermore, this paper studies the impact of array geometry on system performance and shows that a rectangular array can capture more energy and thus can offer better performance than a uniform linear array

The crystal structure of the C-terminus of adseverin reveals the actin-binding interface

Adseverin is a member of the calcium-regulated gelsolin superfamily of actin severing and capping proteins. Adseverin comprises 6 homologous domains (A1–A6), which share 60% identity with the 6 domains from gelsolin (G1–G6). Adseverin is truncated in comparison to gelsolin, lacking the C-terminal extension that masks the F-actin binding site in calcium-free gelsolin. Biochemical assays have indicated differences in the interaction of the C-terminal halves of adseverin and gelsolin with actin. Gelsolin contacts actin through a major site on G4 and a minor site on G6, whereas adseverin uses a site on A5. Here, we present the X-ray structure of the activated C-terminal half of adseverin (A4–A6). This structure is highly similar to that of the activated form of the C-terminal half of gelsolin (G4–G6), both in arrangement of domains and in the 3 bound calcium ions. Comparative analysis of the actin-binding surfaces observed in the G4–G6/actin structure suggests that adseverin in this conformation will also be able to interact with actin through A4 and A6, whereas the A5 surface is obscured. A single residue mutation in A4–A6 located at the predicted A4/actin interface completely abrogates actin sequestration. A model of calcium-free adseverin, constructed from the structure of gelsolin, predicts that in the absence of a gelsolin-like C-terminal extension the interaction between A2 and A6 provides the steric inhibition to prevent interaction with F-actin. We propose that calcium binding to the N terminus of adseverin dominates the activation process to expose the F-actin binding site on A2.

Modeling of multiple access interference and BER derivation for TH and DS UWB multiple access systems

A novel analytical method is introduced for exact statistical modeling of multiple access interference (MAI), in time hopping pulse position modulation and pulse amplitude modulation (TH-PPM and TH-PAM) ultra wideband (UWB) systems operating in additive white Gaussian noise (AWGN) channels. Based on this method, exact bit error rates (BER) are expressed in simple formulas. In a similar fashion, the exact BER of direct sequence (DS) UWB is also derived for PAM modulation. The proposed modeling of MAI considers complete asynchronism in user access, and is also suitable for accurately modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables. We further extend this method to derive general expressions for the BER performance in log-normal fading multi-path channels. In the course of these derivations, we also introduce a more accurate numerical approach to evaluate the characteristic function (CF) of a lognormal random variable

Performance of UWB multiple-access impulse radio systems with antenna array in dense multipath environments

In this letter, the influence of temporal and spatial diversities on the performance of ultra-wideband time-hopping pulse-position modulated multiple-access impulse radio (IR) systems is analyzed. We investigate how an antenna array can be used at the receiver to improve the bit-error rate (BER) performance and can cope with the effects of multiple-access interference of IR system in dense multipath environments. Analytical and simulation results show that the BER performance of the IR systems can be improved when the number of array elements is increased. The performance can be further improved by coherently adding more multipaths at the receiver

Performance of UWB multiple access impulse radio systems in multipath environment with antenna array

Impulse radio (IR) is based on the ultra-wideband (UWB) time-hopping spread spectrum technique in which subnanosecond pulses are modulated to convey information by precisely shifting the relative time position of the pulses. The influence of temporal and spatial diversities on the performance of ultra-wideband impulse radio systems is analyzed. We investigate how antenna diversity (uniform linear array and rectangular array) can be used to improve the bit error rate performance and to mitigate multiple user interference (MUI) of an IR system in multipath fading channels. Numerical results show that the bit error rate performance and multiple access capability of the IR system can be improved when the number of array elements and selected multipaths are increased.