Fig. 10. Model of SER patch formation. (A) In HeLa cells not expressing GFP-b(5) tail, ribosome-free and ribosome-covered areas are interspersed along the same tubules. A protein or protein complex (red squares) is responsible for maintaining relatively straight tubules and for regulating the branching frequency. (B) GFP-b(5)tail expression induces an increase in ER surface area without a concomitant increase in the expression of ER membrane proteins. This results in dilution of the protein complex that regulates tubule geometry. (C) Minor perturbations induced by PDMP cause the regulatory protein(s) to cluster with ribosomes, leaving large areas of ER free to collapse into a random tubular network. See text for further explanation.