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A) Wt, pseudo-wt and crystallized PI4K IIα. Binding sites for ubiquitin ligase Itch and AP3 are shown within the proline rich of the wt enzyme (left). The N-terminus is deleted in the pseudo-wt construct (lower left) used for biochemical assays. In the crystallized construct (right) the last 12 residues are deleted as well and the palmitoylated CCPCC motif is replaced with T4 lysozyme.
B) The overall fold of PI4K IIα. On the left, view oriented to show the ATP binding pocket localized between the N- and C-lobes. On the right, view rotated 180° and tilted to place T4 lysozyme in the back. The kinase has 11 α-helices (numbered H1 – H11), H1 to H4 are located in the N- lobe, where they surround two pairs of antiparallel β-sheets (S1-S4). A pair of β-sheets (S5, S6) in the C-lobe is closely surrounded by helices H5, H6 and H7, while the helices H8, H9 and H10 form a "roof". A second ATP is bound in a lateral hydrophobic pocket of the C-lobe that is partially supported by helix H11. The palmitic acid residues would occupy the loop that is replaced by the T4 lysozyme. The N-lobe is depicted in orange, the C-lobe in cyan, the T4 lysozyme in grey.

(Baumlova A, et al. 2014 EMBO Reports)

 

Eighteen conformations of the ESCRT-I/ESCRT-II supercomplex, with ESCRT-I in green and ESCRT-II in magenta. Estimated populations obtained from the fitting weights and the maximum intra-atomic distance D max are indicated. Models were derived from SAXS, DEER and smFRET data.

(Boura E, Rozycki B, et al. 2012 Structure)
 

The entire process of ESCRT-mediated MVB biogenesis, from start to finish, is considered with respect to crescent-shaped ESCRT-I-II supercomplexes geometry.

(A) ESCRT-0 complexes begin to cluster ubiquitinated (yellow dots) cargo (orange shapes below the translucent membrane, per their ability to cluster membranetethered ubiquitin in vitro ( Wollert and Hurley, 2010 ) and to form flat clathrin coats in cells ( Raiborg et al., 2001; Sachse et al., 2002 ).

(B) ESCRT-0 recruits ESCRT-I ( Bache et al., 2003; Bilodeau et al., 2003; Katzmann et al., 2003; Lu et al., 2003; Pornillos et al., 2003 ), with a stoichiometry that is probably 1:1 in humans ( Im et al., 2010 ) and 1:1 or 1:2 in yeast ( Ren and Hurley, 2011 ).

(C) ESCRT-I recruits ESCRT-II with 1:1 stoichiometry ( Gill et al., 2007 ).

(D and E) The ESCRT-I-II supercomplex is the principal entity that stabilizes the initial membrane bud ( Wollert and Hurley, 2010 ).

(F) The same UEV domain of ESCRT-I binds to both ESCRT-0 and ubiquitin, suggesting the ubiquitin tags need only travel a short distance in the plane of the membrane to be handed off to ESCRT-I.

(G) The observation that the ESCRT-I-II supercomplex is in an equilibrium between open and closed conformations suggests a mechanism for ubiquitinated cargo transfer from the limiting membrane into the bud neck via an open-to-closed conformational change in the supercomplex.

(H) ESCRT-II initiates the assembly of ESCRT-III by binding to and activating Vps20 monomers. Two Vps20 monomers bind to each ESCRT-II complex ( Im et al., 2009; Teo et al., 2004a ), and both binding events are essential for function ( Hierro et al., 2004; Teis et al., 2010 ).

(I) ESCRT-III recruits deubiquitinating enzymes (not shown; McCullough et al., 2004; Richter et al., 2007 ), which replenish the pool of cytosolic ubiquitin monomers and may also help disengage cargo from ESCRT-I and -II.

(J) ESCRT-III filaments associate laterally with one another ( Hanson et al., 2008; Lata et al., 2008 ).

(K and L) Vps4 is not essential for membrane scission in a minimal in vitro system but has been shown by live-cell imaging of cytokinesis ( Elia et al., 2011 ) and HIV-1 budding ( Baumga¨ rtel et al., 2011; Jouvenet et al., 2011 ) to engage with ESCRT-III prior to membrane scission. Multiple Vps4 dodecamers appears to be recruited, and (L) their assembly onto the ESCRT-III lattice has been modeled in terms of a two-dimensional hexagonal lattice ( Yang and Hurley, 2010 ).

(M–O) ESCRT-III filament self-association, perhaps promoted by the polyvalent binding of Vps4 and its ESCRT-III binding cofactor, Vta1, deform the membrane into a dome-shaped scission intermediate ( Fabrikant et al., 2009 ).

(P and Q) ESCRT-I and -II dissociate at least partially in advance of ATP hydrolysis by Vps4, whereas the dissociation and recycling of ESCRT-III depends strictly on ATP hydrolysis.

(Boura E, Rozycki B, et al. 2012 Structure)
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Structure and membrane-binding mechanism of the MABP domain. (A) Electron density (2Fo - Fc) synthesis highlighting the tip of the ß2 – ß3 loop of subdomain I. (B) Overall fold of the MABP domain colored by subdomain. (C) Model for membrane binding colored according to electrostatic potential,with blue electropositive and red electronegative. Hydrophobic residues at the tip of the ß2 – ß3 loop are highlighted and their potential role in membrane insertion shown.

(Boura et Hurley, 2012 PNAS)

 

Subcellular targeting by the MABP domain. mCherry-MVB12B-MABP is predominantly cytosolic with some punctate localization. The tandem construct mCherry-MVB 12 B-MABP 2 localizes predominantly to the plasma membrane and punctate structures.

(Boura et Hurley, 2012 PNAS)

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Model of FOXO DNA binding domain in complex with DNA derived from fluorescence data.

(Boura E. et al., 2007 JBC)

 

A fluorescence image of a GUV (giant unilamellar vesicle) imobilized on an EM grid. (taken by LSM780, Bethesda, 2012)

 
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