Isolation and preliminary characterization of that part of the plasma membrane which is apposed to the substratum.

We have developed a membrane isolation technique which allows us to isolate that part of the plasma membrane which is apposed to the solid substratum (LPM) to which cells in vitro attach free of the remainder of the plasma membrane (UPM). Our evidence suggests that the UPM is essentially free of contaminating cellular organelles while the LPM may be somewhat contaminated with endoplasmic reticulum fixed to it during the isolation procedure. Characterization of the peptides of the UPM and LPM suggests that some membrane components are nonrandomly distributed to one or the other membrane fraction while the bulk of the membrane peptides appear to be present in both membrane fractions; Membrane components labeled with 3H-fucose or 3H-glucosamine show a 3--5-fold higher specific activity in the LPM than in the UPM. A number of iodineateable surface components, in particular the LETS protein, show a preferential localization to the LPM. The development of this membrane isolation procedure should allow us to begin investigating the molecular basis of differences in cell-substratum adhesion and cell motility, two functions assigned operationally to the LPM.