We han, e shown that spedfic binding of divalent cations to a polysaechafide polyelectro]ym, leading firm cohesion between the chains, can cause characteristic effects ~n ~e c~rcutar diehroism spectrum which are understandabb in terms of modem theo~, [ l ]. For atginate, tiffs binding is a co-operative p,~o-tess that predominantly in~oNes consecuti~'e ~luro-nale residues. Some other systems ha~e now been inves,~igated in an attempt to formulate a general inSetpretafion of bio]ogics] phenomena of this type. T~e known streng~ and specificity of comp~exafion are explained m termg of an "'egg-box model" which ~s ,derN,ed from our measmemems, the known coordination geometries in mode] compounds, and the requiremen~s roar ~ooperafivity.In pectin, • .the methyl este~ of poly(ga~acturonie acid), the interactions with ca~on.~ is relatively weak because ~he chains are unch~r~ed. In contrasl to the beha~onr of alginates, in which the amplitude diminishes w~th gelatben I ] ], the broad, positive, n ~ ~* Cations are absent, however, and their cha~acte~stic in-quence on the ~ orbha~s is therefore no~ seen. After saponification, the pe!ysacchafide binds Ca 2+ wiih ge~ formation and with a large decrease ~n amplitude of the n -~ .~* band ( fig. 1) which shows a gaussian difference spectrum centered on 208 ~[rn. This is therefore [ l ] explained as a specific bind,~ng of most of the umnam residues m Ca 2+ wNch tends to reverse the sign of the n ~ ~* band. Derivatives that are or,.ly partly saponified, as in "%w me~hoxy pectins" used industr~ally and p~esent in MomeNtal systems, show similar specUoseopic changes during gela~on wi~.~ Ca 2÷, except that an additional peak remains with ~he position (210 nm) and ampfitude expected fo,~ methyl galactaronate Nsidues. Being uncha~ged ~Lhe~,c groups wosld noi pa{~¢ipaie directly in ion bin~!k~ng, and the ~esiduai peak therefore confirms our v~ew that the extreme spectra," changes arise horn ~pecific pert~rbatio~ of bound ~e~idues by proximity of the ionic cha~ge: *~buIed between a nnmbez Of rotations] states about .... Po]ymarin~ionate as well as po]yg~]mmnate an~ ,~(5)--C(5) which, due to ~hc d~rent ofienta~ion~ of polyga~acturona~e sequences can be perturbed spotthe ~emboxy] ehromo~no~e Lq the ~yn~met~Je erl.~i~on-m:enI of ~he ~aga~, ring, ha~e ~ ->~* &rods. of ~arying .

No abstract

1. A method was developed which allows the rapid preparation of pure plasma membranes in high yield from cultured fibroblasts. 2. Cells are lysed in hypo-osmotic borate/EDTA and, after differential centrifugation, the membranes collected by centrifugation on a sucrose barrier. 3. Electron microscopy of the isolated material shows large membrane vesicles essentially free from contaminating organelles. 4. There is no detectable activity of the endoplasmic-reticulum enzyme marker, NADH2—lipoamide oxidoreductase (EC 1.6.4.3), and that of succinate dehydrogenase (EC 1.3.99.1), a marker for mitochondria, is substantially decreased. Chemical compositions are in good agreement with previous observations. 5. This study confirms the usefulness of applied isotopic markers for isolating plasma membranes.

No abstract

Although cell adhesion to substrate must ultimately depend on 'sticking' by physical forces at the outer surface, we suggest that control is exercised through the 'grip' of the cytoskeleton. Our experiments indicate that grip can be relaxed by agents which release surface glycoproteins from some form of side-to-side organisation, and can be consolidated by suitable cross-linking to augment this organisation.