John G. McCaffrey

Rab11-FIP3 is critical for the structural integrity of the endosomal recycling compartment.

Rab11‐FIP3 is an endosomal recycling compartment (ERC) protein that is implicated in the process of membrane delivery from the ERC to sites of membrane insertion during cell division. Here we report that Rab11‐FIP3 is critical for the structural integrity of the ERC during interphase. We demonstrate that knockdown of Rab11‐FIP3 and expression of a mutant of Rab11‐FIP3 that is Rab11‐binding deficient cause loss of all ERC‐marker protein staining from the pericentrosomal region of A431 cells. Furthermore, we find that fluorophore‐labelled transferrin cannot access the pericentrosomal region of cells in which Rab11‐FIP3 function has been perturbed. We find that this Rab11‐FIP3 function appears to be specific because expression of the equivalent Rab11‐binding deficient mutant of Rab‐coupling protein does not perturb ERC morphology. In addition, we find that other organelles such as sorting and late endosomes are unaffected by loss of Rab11‐FIP3 function. Finally, we demonstrate the presence of an extensive coiled‐coil region between residues 463 and 692 of Rab11‐FIP3, which exists as a dimer in solution and is critical to support its function on the ERC. Together, these data indicate that Rab11‐FIP3 is necessary for the structural integrity of the pericentrosomal ERC.

A spectroscopic characterization of the ã 3Π0−, A 3Π0+, and E 3∑+ states of the MgAr van der Waals molecule

The metastable a(3Π0−) and A(3Π0+) states of the Mg(3s3p)Ar molecule were synthesized in a free‐jet expansion of metastable Mg(3PJ) atoms, produced by the laser vaporization of a magnesium rod, in pure Ar. The a(3Π0−), A(3Π0+), and E(3∑+) states were characterized spectroscopically by laser‐induced fluorescence. The 3Π(v=0) states were found to be very near the Hund’s case (a) limit, with only 0.31 cm−1 separating the 3Π0+ and 3Π0− levels. The internuclear separations, re, were found to be 3.63 A and the dissociation energies 316 and 296 cm−1, respectively. The E(3∑+) state is quite deeply bound (1128 cm−1) with a much smaller re, 2.83 A, and it is postulated that the Ar atom has substantially penetrated the outer lobe of the 4s Rydberg orbital of the Mg(3s4s 3S1) atomic state to which this state correlates. Despite large populations of both Mg(3P1) and Mg(3P2), neither the 3Π1 nor 3Π2 states of MgAr were observed, apparently because the molecular 3Π1,2 states are collisionally quenched to the lower‐ene...

Spectroscopy and photodissociation of chlorine monomers and clusters in argon matrices

Steady‐state and time‐resolved emission spectroscopy of the A–X system of Cl2 is used to distinguish molecular chlorine isolated as monomers and clusters in solid argon samples. The lifetime of the metastable A’ 3Π2u state is measured to be an order of magnitude less, when chlorine is present as clusters, than when it is truly isolated. Photodissociation of molecular chlorine clusters was found to be insignificant as monitored by the emission of Ar2Cl at 260 nm. Measurement of the dissociation threshold of molecular chlorine in the 9 eV region as a function of temperature showed little variation. Using spectroscopic data it is concluded that dissociation is occurring by an impulsive mechanism involving curve‐crossing from the initially populated Ar+(Cl2)− charge transfer state to repulsive potentials correlating with ground state atomic chlorine and not via a harpooning mechanism. A simple microscopic model, drawn from experimental data and pairwise addition of ArCl potential terms, is constructed to desc...

Spectroscopy and photodissociation of molecular chlorine in argon matrices

A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10−18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge‐transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the ...

Photodissociation of molecular chlorine in xenon matrices

The photodissociation of Cl2 in crystalline Xe is examined using synchrotron radiation for all state of Cl2 from the C 1Π1u state to the region of Xe exciton absorption. Isolation of atomic chlorine in two dominant trapping sites is observed following photodissociation throughout the spectral region studied. The production efficiency of the thermally stable trapping site was found to increase significantly with temperature and was most pronounced in the region of the onset of the Xe2Cl excitation feature. Correlation between these two processes is explained in terms of a charge‐transfer‐induced dissociation mechanism involving the formation of a Xe+(Cl2)− intermediate. In contrast to Ar and Kr matrices, photodissociation of Cl2 in Xe appears to occur without a pronounced cage effect. This observation is rationalized in terms of the larger lattice parameters of Xe which allows isolation of Cl2 and Cl at single substitutional and octahedral interstitial sites, respectively. From this structural information,...

A laser spectroscopic study of the X 1Σ+0−C 1Π1 transition of MgAr: Evidence for Λ‐type doubling

The X 1Σ+0 and the C 1Π1 states of MgAr have been characterized spectroscopically. The MgAr van der Waals molecules, created in a new laser‐vaporization, pulsed supersonic jet apparatus, were studied using laser‐induced fluorescence. High‐resolution spectra revealed lambda‐type doubling consistent with the presence of the nearby higher‐lying repulsive MgAr (D 1Σ+0) electronic state.

Rydberg series of charge‐transfer excitations: Cl and H in rare gas crystals

Rydberg series observed in the excitation spectra of Cl and H atoms isolated in the rare gas (Rg) crystals Kr and Xe are associated with the charge transfer species Cl−Rg+ and H−Rg+. The progressions originate from hole states of Rg+ which converge to the top of the rare gas valence band in an analogous manner to the convergence of conventional exciton states to the bottom of the conduction band. A model based on the effective mass approximation and a quantum defect concept predicted such progressions in Xe and Kr crystals and it is supported by the present results.

Laser excitation spectroscopy of the A and B states of jet‐cooled copper dimer: Evidence for large electronic isotope shifts

Fluorescence excitation spectra recorded for the A–X system of jet‐cooled Cu2 show conclusive evidence of a ΔΛ=0 transition, and the A state is thereby definitively assigned as 1Σ+u. A previous assignment of the B state as 1Σ+u is confirmed, but the vibrational levels of this state are complicated by the presence of a perturbation at v’=0. The perturbing state does not, however, appear to be either of the two optically accessible electronic states in this spectral region. Anomalously large electronic isotope shifts are observed for the A and B states, and this behavior is discussed in terms of the correspondingly large ‘‘specific mass shifts’’ observed in the optical spectra of atomic copper for transitions that couple states differing in the number of d electrons. Due to the large spin‐orbit coupling constants in the ‘‘d‐hole’’ configurations, it is proposed that the low‐energy‐excited molecular states of Cu2 derived from these configurations should be described by Hund’s case (c) coupling. Dynamical eff...

Luminescence spectroscopy of 3P1 and 3P0 state atomic mercury isolated in solid Ar, Kr, and Xe

Multicomponent emission bands are recorded for the 3P1→1S0 transition of atomic mercury isolated at single sites in solid Ar, Kr, and Xe matrices. A blueshift observed at elevated temperatures on the 273 nm emission of Hg/Xe is identified in line shape analysis as arising from decreasing intensity of the central component in the band profile. The origin of the multiple components in the emission bands is ascribed to the existence of several vibronic modes which lead to excited state stabilization in the Hg(3P1)/RG matrix systems. A detailed description of these modes and their energetics is presented in the paper directly following. Photoexcitation of the 3P1 state also yields small amounts of 3P0 state emission. Hg atom 3P1 to 3P0 state intramultiplet relaxation (IMR) is most efficient in Hg/Xe where the ratio of this relaxation channel to 3P1 state radiative decay is 1/200 as established in time-integrated emission spectra. Despite the weakness of IMR, pulsed laser excitation combined with photon counti...

Simulation of atomic zinc luminescence in rare gas solids: A localized pair potentials approach

The luminescence spectroscopy of atomic zinc isolated in the solid rare gases (Zn/RG) is compared with theoretical predictions obtained from the sum of diatomic Zn⋅RG and RG⋅RG pair potentials. In particular the existence of pairs of emission bands, both of which are assigned to the same gas phase electronic transition, is examined with the use of diatomic pair potentials to simulate the potential energy surfaces of the Jahn–Teller active vibrational modes of Zn in the solid rare gases Ar, Kr, and Xe. Simulations of the solid state Zn/RG luminescence are developed from a consideration of the excited state Zn(1P1)⋅RGn van der Waals cluster species in the gas phase. The maximum binding energy of the Zn(1P1)⋅RGn clusters is found in the Zn⋅RG4 cluster having a square planar structure at the energy minimum. Based on the results of the cluster calculations, lattice distortions which led to a dominant interaction between the Zn atom and four of its host atoms were sought to simulate the solid state luminescence...