Transmembrane scaffolds

Abdulaev, N. G., Strassmaier, T. T., Ngo, T., Chen, R., Luecke, H., Oprian, D. D., and Ridge, K. D. (2002). Grafting segments from the extracellular surface of CCR5 onto a bacteriorhodopstn transmembrane scaffold confers HfV-l coreceptor activity. Structure (Camb) 10, 515—525. 

Fig. 1. Potential complexity of signaling involving adrenergic receptor dimers, G proteins, and scaffolding/adaptor proteins. As an illustration, dimers of two (32-ARs are shown in a contact dimer formation (62) with some of the large variety of proteins suggested to bind to their intracellular surfaces, illustrating the potential complexity of intracellular signaling that may be initiated by receptor activation. Transmembrane helices are indicated by the barrel-like structures and intracellular and extracellular domains as lines. Interacting proteins are shown as various shapes in the approximate localization where they are thought to bind. Abbreviations are defined in the text. MEK, MAPK/ERK kinase.

For antibodies to penetrate inside fixed cells, the membranes must be opened with detergents. Membranes are lipid bilayers that have a hydrophilic or water-soluble side facing the cytoplasm and the extracellular space (Fig. 5.4a). The hydrophobic or water-insoluble sides face each other at the center of the membrane. Also, there are transmembrane proteins that interact with the lipids and are held in the membrane. Membranes are barriers because they do not allow water or hydrated molecules to pass. For immunocytochemistry, the membrane must be parhally dissolved to allow antibodies to cross. It is also important that the transmembrane proteins remain cross-linked to other proteins so that they are not washed away (Fig. 5.4b). Detergent will dissolve the membranes but not the transmembrane proteins, which are cross-linked by the fixative to other proteins (e.g., scaffold proteins). For tissue sections, antibodies must penetrate through many cell layers into the center of the section. Achieving this depth of penetration requires removing most of the cell membranes but leaving the proteins so that they can bind antibodies when needed. 

Fig. 5.4 Membrane permeabilization. Membranes are lipid bilayers, with the lipid hydrophobic tail toward other lipid tails and the hydrophilic head toward the cytoplasm or extracellular space, (a) Transmembrane proteins pass through the membrane and are held in place by cytoskeletal and scaffold proteins, (b) Following fixation, detergent removal of the lipids wiU leave transmembrane proteins cross-linked to cytoplasmic proteins...

---