Energy Transfer

The  stochastic  quality  of  the  electron  track  as  it  deposits  energy  is  not adequately  described  by  the  usual  mathematical  formulations.  The Bethe-Bloch  stopping power equation  (which will be  discussed  in  detail  in Chapter  14)  describes  the  process  of energy  transfer  from  a  slowing down electron  as  a  continuous  process.  That  is  to  say,  the  energy  is  assumed  to be  deposited  in  a  continuous  stream  rather  than  in  the  discrete  and punctate  fashion  that  occurs  in  the  real  case.  This  has  come  to  be  known as  the  continuous slowing down  approximation.  The  Bethe-Bloch  approximation,  which  is  suitable  for  general  physics  questions,  proves  to  be inadequate  to  describe  the  events  that  ultimately  lead  to  molecular  alterations  in  the  medium.  For  example,  if  we  examine  the  tracks  of  two electrons  of  equal  starting  kinetic  energy  and  identical  starting  location, the  distribution  of  events  along  the  track will  be  different  and  so  will  be the  physical  locations  of  the  tracks.  Only  for  very  high  linear  energy transfer  (LET)  radiations  does  the  continuous  slowing  down  approximation approach  reality.

An Internalization Wave of Caveolae can be Stimulated by Virus

Studies on the entry of simian 40 (SV40) virus by Pelkmans and coworkers [52–54] have documented that caveolae can actually play a role in nonconstitutive endocytosis. Thus, after binding of SV40 virus to the cell surface via the MHC class I molecule, the virus particles move laterally in the plasma membrane to end up in caveolae. Although these caveolae are initially immobile, the virus initiates a complex signaling cascade leading to a profound disorganization of the cortical actin cytoskeleton and a transient recruitment of the GTP-binding protein dynamin known to be involved in membrane fission (see Section 4.8). Importantly, without SV40-stimulation, less than 10% of the caveolae were associated with dynamin.

These changes, in turn, resulted in a wave of incoming caveolar vesicles containing virus where reorganized actin filaments formed “tails” necessary for internalization of the SV40-containing caveolae [53]. Subsequently, the virus was delivered

to caveosomes (see Section 4.7) and transported further downstream to the endoplasmic reticulum (ER). However, a delay of several hours then occurs before caveolin returns from the caveosomes to the plasma membrane in vesicles now devoid of virus particles [52].

It is interesting to note that antibody-induced crosslinking of MHC class I moleules (the SV40 receptor) results in an accumulation of MHC class I clusters in small uncoated surface invaginations” identical to caveolae, as reported 25 years go by Huet and coworkers. No clusters were found in clathrincoated pits. From he caveolae-like invaginations the clusters were apparently internalized and delivred to lysosomes [55]. It is therefore tempting to speculate that it is the samen derlying mechanism that is responsible for caveolae-mediated uptake of SV40 virus particles after their binding to MHC class I and of MHC class I clusters

enerated by antibody crosslinking.

Source:: lipid raft and C. Fielding, J.C. Jhonwilley and Son

Evidence for Phase Separation in Model Membrane Systems

Liquid-Ordered and Liquid-Disordered Phases

Various model membrane systems have been used by physicists and chemists to study phase separation in lipid mixtures. They are either monolayers or bilayers. Monolayers are either assembled at an air-water interface with the packing density of the lipids being adjusted by applying lateral pressure, or on a supporting lipid monolayer that is fixed to a solid support. Bilayers are used in the supported version as described above, or in the form of vesicles. The most commonly used vesicles are large or giant unilamellar vesicles (LUV or GUV, respectively) composed of only a single bilayer, but also multilamellar vesicles (MLV) are used. The basic principles were first established in simple binary lipid mixtures, but recently

ternary mixtures which more closely mimic the composition of the cell plasma membrane have been used. The mixtures usually contain one lipid with a high melting temperature (Tm), one with a low Tm, and cholesterol. GUVs are probably the system closest to a cell membrane, because artifacts from a support are excluded. Still, cell membranes are asymmetric with different lipid compositions of the outer versus the inner leaflet, while the GUVs used so far were all symmetric.

Since maintaining an asymmetric lipid distribution is energy-consuming, perhaps by reconstituting lipid translocators into liposomes this drawback can be overcome in the future. Although model membrane systems produce very simplified pictures of cell membranes, there are many examples of a close correlation with experimental data obtained in living cells [14]. Ipsen et al. were the first to describe the formation of a liquid-ordered phase by cholesterol and saturated phospholipids [15,16]. This phase can coexist with other lipid phases, and its characteristics are described as follows: the translational order of lipid molecules within the liquid-ordered phase is similar to that in a fluid bilayer state, whereas the configurational order of the hydrocarbon chains compares more to that in a gel state. The formation of the liquid-ordered phase was attributed to the unique chemical nature of cholesterol (for a review, see [17]), but later it was shown that all natural sterols promote domain formation and that also

small amounts of ceramide (3%) can stabilize domains formed in vesicles [18]. Leventis and Silvius showed that the interaction of cholesterol with different lipid species is dependent on the nature of their hydrocarbon chains and, to a lesser extent, also on their headgroup. The interaction preference decreases with SM > PS > PC > PE and with increasing unsaturation of the acyl chains [19]. Whereas the kink in unsaturated hydrocarbon chains is likely to hinder tight packing with the flat sterol ring of cholesterol, the reason for the preferential interaction of cholesterol with SM is still a debated issue.

Source::

Fielding, J.Christtoper, lipid raft and caveolae . Jhown willey and Son Publishe