Biomedical subjects

Michelle A Digman

Publications and source records attributed to Michelle A Digman.

8 recordsLinked to original sources

PubMed2005

Phosphoinositide specificity of and mechanism of lipid domain formation by annexin A2-p11 heterotetramer.

Annexin A2 is a phospholipid-binding protein that forms a heterotetramer (annexin II-p11 heterotetramer; A2t) with p11 (S100A10). It has been reported that annexin A2 is involved in binding to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and in inducing membrane microdomain formation. To understand the mechanisms underlying these findings, we determined the membrane binding properties of annexin A2 wild type and mutants both as monomer and as A2t. Our results from surface plasmon resonance analysis showed that A2t and annexin A2 has modest selectivity for PtdIns(4,5)P2 over other phosphoinositides, which is conferred by conserved basic residues, including Lys279 and Lys281, on the convex surface of annexin A2. Fluorescence microscopy measurements using giant unilamellar vesicles showed that A2t of wild type, but not (K279A)2-(p11)2 or (K281A)2-(p11)2, specifically induced the formation of 1-microm-sized PtdIns(4,5)P2 clusters, which were stabilized by cholesterol. Collectively, these studies elucidate the structural determinant of the PtdIns(4,5)P2 selectivity of A2t and suggest that A2t may be involved in the regulation of PtdIns(4,5)P2 clustering in the cell.

Amino Acid Sequence↗

PubMed2005

Measuring fast dynamics in solutions and cells with a laser scanning microscope.

Single-point fluorescence correlation spectroscopy (FCS) allows measurements of fast diffusion and dynamic processes in the microsecond-to-millisecond time range. For measurements on living cells, image correlation spectroscopy (ICS) and temporal ICS extend the FCS approach to diffusion times as long as seconds to minutes and simultaneously provide spatially resolved dynamic information. However, ICS is limited to very slow dynamics due to the frame acquisition rate. Here we develop novel extensions to ICS that probe spatial correlations in previously inaccessible temporal windows. We show that using standard laser confocal imaging techniques (raster-scan mode) not only can we reach the temporal scales of single-point FCS, but also have the advantages of ICS in providing spatial information. This novel method, called raster image correlation spectroscopy (RICS), rapidly measures during the scan many focal points within the cell providing the same concentration and dynamic information of FCS as well as information on the spatial correlation between points along the scanning path. Longer time dynamics are recovered from the information in successive lines and frames. We exploit the hidden time structure of the scan method in which adjacent pixels are a few microseconds apart thereby accurately measuring dynamic processes such as molecular diffusion in the microseconds-to-seconds timescale. In conjunction with simulated data, we show that a wide range of diffusion coefficients and concentrations can be measured by RICS. We used RICS to determine for the first time spatially resolved diffusions of paxillin-EGFP stably expressed in CHOK1 cells. This new type of data analysis has a broad application in biology and it provides a powerful tool for measuring fast as well as slower dynamic processes in cellular systems using any standard laser confocal microscope.

Algorithms↗

PubMed2005

Fluctuation correlation spectroscopy with a laser-scanning microscope: exploiting the hidden time structure.

Images obtained with a laser-scanning microscope contain a time structure that can be exploited to measure fast dynamics of molecules in solution and in cells. The spatial correlation approach provides a simple algorithm to extract this information. We describe the analysis used to process laser-scanning images of solutions and cells to obtain molecular diffusion constant in the microsecond to second timescale.

Algorithms↗

PubMed2005

Diacylglycerol-induced membrane targeting and activation of protein kinase Cepsilon: mechanistic differences between protein kinases Cdelta and Cepsilon.

Two novel protein kinases C (PKC), PKCdelta and PKCepsilon, have been reported to have opposing functions in some mammalian cells. To understand the basis of their distinct cellular functions and regulation, we investigated the mechanism of in vitro and cellular sn-1,2-diacylglycerol (DAG)-mediated membrane binding of PKCepsilon and compared it with that of PKCdelta. The regulatory domains of novel PKC contain a C2 domain and a tandem repeat of C1 domains (C1A and C1B), which have been identified as the interaction site for DAG and phorbol ester. Isothermal titration calorimetry and surface plasmon resonance measurements showed that isolated C1A and C1B domains of PKCepsilon have comparably high affinities for DAG and phorbol ester. Furthermore, in vitro activity and membrane binding analyses of PKCepsilon mutants showed that both the C1A and C1B domains play a role in the DAG-induced membrane binding and activation of PKCepsilon. The C1 domains of PKCepsilon are not conformationally restricted and readily accessible for DAG binding unlike those of PKCdelta. Consequently, phosphatidylserine-dependent unleashing of C1 domains seen with PKCdelta was not necessary for PKCepsilon. Cell studies with fluorescent protein-tagged PKCs showed that, due to the lack of lipid headgroup selectivity, PKCepsilon translocated to both the plasma membrane and the nuclear membrane, whereas PKCdelta migrates specifically to the plasma membrane under the conditions in which DAG is evenly distributed among intracellular membranes of HEK293 cells. Also, PKCepsilon translocated much faster than PKCdelta due to conformational flexibility of its C1 domains. Collectively, these results provide new insight into the differential activation mechanisms of PKCdelta and PKCepsilon based on different structural and functional properties of their C1 domains.

Amino Acid Sequence↗

PubMed2004

Partitioning of NaPi cotransporter in cholesterol-, sphingomyelin-, and glycosphingolipid-enriched membrane domains modulates NaPi protein diffusion, clustering, and activity.

In dietary potassium deficiency there is a decrease in the transport activity of the type IIa sodium/phosphate cotransporter protein (NaPi) despite an increase in its apical membrane abundance. This novel posttranslational regulation of NaPi activity is mediated by the increased glycosphingolipid content of the potassium-deficient apical membrane. However, the mechanisms by which these lipids modulate NaPi activity have not been determined. We determined if in potassium deficiency NaPi is increasingly partitioned in cholesterol-, sphingomyelin-, and glycosphingolipid-enriched microdomains of the apical membrane and if the increased presence of NaPi in these microdomains modulates its activity. By using a detergent-free density gradient flotation technique, we found that 80% of the apical membrane NaPi partitions into the low density cholesterol-, sphingomyelin-, and GM1-enriched fractions characterized as "lipid raft" fractions. In potassium deficiency, a higher proportion of NaPi was localized in the lipid raft fractions. By combining fluorescence correlation spectroscopy and photon counting histogram methods for control and potassium-deficient apical membranes reconstituted into giant unilamellar vesicles, we showed a 2-fold decrease in lateral diffusion of NaPi protein and a greater than 2-fold increase in size of protein aggregates/clusters in potassium deficiency. Our results indicate that NaPi protein is localized in membrane microdomains, that in potassium deficiency a larger proportion of NaPi protein is present in these microdomains, and that NaPi lateral diffusion is slowed down and NaPi aggregation/clustering is increased in potassium deficiency, both of which could be associated with the decreased Na/Pi cotransport activity in potassium deficiency.

Animals↗

PubMed2004

Mechanism of diacylglycerol-induced membrane targeting and activation of protein kinase Cdelta.

The regulatory domains of novel protein kinases C (PKC) contain two C1 domains (C1A and C1B), which have been identified as the interaction site for sn-1,2-diacylglycerol (DAG) and phorbol ester, and a C2 domain that may be involved in interaction with lipids and/or proteins. Although recent reports have indicated that C1A and C1B domains of conventional PKCs play different roles in their DAG-mediated membrane binding and activation, the individual roles of C1A and C1B domains in the DAG-mediated activation of novel PKCs have not been fully understood. In this study, we determined the roles of C1A and C1B domains of PKCdelta by means of in vitro lipid binding analyses and cellular protein translocation measurements. Isothermal titration calorimetry and surface plasmon resonance measurements showed that isolated C1A and C1B domains of PKCdelta have opposite affinities for DAG and phorbol ester; i.e. the C1A domain with high affinity for DAG and the C1B domain with high affinity for phorbol ester. Furthermore, in vitro activity and membrane binding analyses of PKCdelta mutants showed that the C1A domain is critical for the DAG-induced membrane binding and activation of PKCdelta. The studies also indicated that an anionic residue, Glu(177), in the C1A domain plays a key role in controlling the DAG accessibility of the conformationally restricted C1A domain in a phosphatidylserine-dependent manner. Cell studies with enhanced green fluorescent protein-tagged PKCdelta and mutants showed that because of its phosphatidylserine specificity PKCdelta preferentially translocated to the plasma membrane under the conditions in which DAG is randomly distributed among intracellular membranes of HEK293 cells. Collectively, these results provide new insight into the differential roles of C1 domains in the DAG-induced membrane activation of PKCdelta and the origin of its specific subcellular localization in response to DAG.

Animals↗

PubMed2004

Polarized fluorescence correlation spectroscopy of DNA-DAPI complexes.

We discuss the use of fluorescence correlation spectroscopy for the measurement of relatively slow rotations of large macromolecules in solution or attached to other macromolecular structures. We present simulations and experimental results to illustrate the range of rotational correlation times and diffusion times that the technique can analyze. In particular, we examine various methods to analyze the polarization fluctuation data. We have found that by first constructing the polarization function and then calculating the autocorrelation function, we can obtain the rotational motion of the molecule with very little interference from the lateral diffusion of the macromolecule, as long as the rotational diffusion is significantly faster than the lateral diffusion. Surprisingly, for common fluorophores the autocorrelation of the polarization function is relatively unaffected by the photon statistics. In our instrument, two-photon excitation is used to define a small volume of illumination where a few molecules are present at any instant of time. The measurements of long DNA molecules labeled with the fluorescent probe DAPI show local rotational motions of the polymers in addition to translation motions of the entire polymer. For smaller molecules such as EGFP, the viscosity of the solution must be increased to bring the relaxation due to rotational motion into the measurable range. Overall, our results show that polarized fluorescence correlation spectroscopy can be used to detect fast and slow rotational motion in the time scale from microsecond to second, a range that cannot be easily reached by conventional fluorescence anisotropy decay methods.

Anisotropy↗

PubMed2003

Activation mechanisms of conventional protein kinase C isoforms are determined by the ligand affinity and conformational flexibility of their C1 domains.

The regulatory domains of conventional and novel protein kinases C (PKC) have two C1 domains (C1A and C1B) that have been identified as the interaction site for diacylglycerol (DAG) and phorbol ester. It has been reported that C1A and C1B domains of individual PKC isoforms play different roles in their membrane binding and activation; however, DAG affinity of individual C1 domains has not been quantitatively determined. In this study, we measured the affinity of isolated C1A and C1B domains of two conventional PKCs, PKCalpha and PKCgamma, for soluble and membrane-incorporated DAG and phorbol ester by isothermal calorimetry and surface plasmon resonance. The C1A and C1B domains of PKCalpha have opposite affinities for DAG and phorbol ester; i.e. the C1A domain with high affinity for DAG and the C1B domain with high affinity for phorbol ester. In contrast, the C1A and C1b domains of PKCgamma have comparably high affinities for both DAG and phorbol ester. Consistent with these results, mutational studies of full-length proteins showed that the C1A domain is critical for the DAG-induced activation of PKCalpha, whereas both C1A and C1B domains are involved in the DAG-induced activation of PKCgamma. Further mutational studies in conjunction with in vitro activity assay and monolayer penetration analysis indicated that, unlike the C1A domain of PKCalpha, neither the C1A nor the C1B domain of PKCgamma is conformationally restricted. Cell studies with enhanced green fluorescent protein-tagged PKCs showed that PKCalpha did not translocate to the plasma membrane in response to DAG at a basal intracellular calcium concentration due to the inaccessibility of its C1A domain, whereas PKCgamma rapidly translocated to the plasma membrane under the same conditions. These data suggest that differential activation mechanisms of PKC isoforms are determined by the DAG affinity and conformational flexibility of their C1 domains.

Animals↗