Curling receptors

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T I B S - A p r i l 1985 Science 107, 475 (abstract) 9 Rose, S. M. and Rose, F. C. (1952) Cancer Res. 12, 1-12 10 Mizell, M. (1960) Anat. Rec. 137, 282-283

(abstract) 11 Balls, M. (1962) Cancer Res. 22, 1142-1154 12 Goyotte,M., Petropoulos,C. J., Shank, P. R.

and Fausto, N. (1983) Science 219, 510-512 13 Slack, J.M.W. (1982) Z Embryol. Exp. Morph. 70, 241-260 14 Muller, R., Siamon, D. J., Tremblay,J. M., Cline, M. J. and Verma, I. M. (1982) Nature 299,640--644 15 Sorge, L. K., Levy, B. T. and Maness, P. F.

Curling receptors Several recent studies on receptor-mediated endocytosis have been directed toward elucidating the mechanism of sorting of receptors and their iigands. Many receptors involved in directing the delivery of exogenous ligands to the lysosomal compartment of a cell are now known to recycle. Studies suggest that the separation of receptors and ligands occurs intracellularly in an acidic compartment most commonly referred to as an endosome 1-2. Studies by Geuze et al. 3-5 using double-label immunoelectronmicroscopy have led to the characterization of a distinct morphological compartment which they have termed CURL (an acronym for Compartment of Uncoupling of Receptor and Ligand). Although technically an endosome, CURL is an appropriate name for this specialized compartment considering its proposed function and the curled morphology observed for many of these elements. Fig. 1 is a diagram of the structure, based on pub-. lished micrographs 3-5.

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A

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B

ation of published CURL structures, it occurred to me that the physical segregation of receptors into the membranous compartment might occur spontaneously. The shape of CURL allows one to define two distinct domains: a membrane rich domain (A) and a content or fluid-rich domain (B). Whereas nearly all of the aqueous fluid is in domain B, the total membrane is distributed asymmetrically between the two domains. Estimates by Geuze and Schwartz (personal communication) assign approximately 90% of the membrane to the membrane-rich domain and approximately 10% to the fluid-rich domain. I believe that this simple design will automatically sort receptors and ligands, the only assumptions being that the receptors and their ligands dissociate and that the receptors are free to migrate within the lipid bilayer. Once dissociated, the ligands will automatically be localized almost entirely in domain B. The receptors will rapidly distribute throughout the membrane and be found, on average, 90% enriched in domain A. Thus, without implicating any as yet undiscovered protein to induce accumulation of receptor in the

(1984) Cell 36, 249--257 PANAGIOTIS A. TSONIS

The Cancer Research Center, La Jolla Cancer Research Foundation, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA.

membrane domain, they will be enriched there purely by mass action. This self-sorting of receptors might appear to be somewhat inefficient; however, a continual influx of receptorligand containing vesicles fusing with CURL with a concomitant efflux of receptor-rich membrane vesicles could rapidly achieve exquisitely efficient sorting.

References 1 Helenius, A., Mellman, I., Wall, D. and Hubbard, A. (1983) Trends Biochem. Sci. 8, 245--250 2 Brown, M.S., Anderson, R. G.W. and Goldstein,J. L. (1983) Cell 32, 663--667 3 Geuze, H.J., Slot, J.W., Strous, G. J. A, M., Lodish, H.F. and Schwartz, A. L. (1983) Cell 32, 272-287 4 Geuze, H.J., Slot, J.W., Strous, G. J. A. M. and Schwartz, A. L. (1983) Eur. J. Cell Biol. 32, 38~4 5 Geuze, H.J., Slot, J.W., Strous, G. J. A. M., Peppard, J., von Figura, K., Hasilik, A. and Schwartz, A. L. (1984) Cell 37, 195-204 LEONARD H. ROME

Department of Biological Chemistry, UCLA School of Medicine, Los Angeles, CA 90024, USA.

On the affinity of dyes and 'colorless dyes' to bacterial membranes

In a recent scholarly review in T I B S 1, R . Scherrer sketched the 100-year-old hisThe presumption developed by tory of the Gram staining reaction. As Geuze et al., from numerous related one of those who followed up developstudies, involves the entry into CURL of ments in this field, I should like to draw a receptor-ligand complex. The ligands attention to another interdisciplinary dissociate into the lumenal space, pre- aspect of the Gram staining reaction that sumably because of the low pH inside, was not included in Scherrer's article. and the receptors are then thought to The Gram staining behavior of bacmigrate to the more membrane-rich teria is not an 'all or none' phenomenon. area. Following this segregation, it is Gram-negative bacteria are changed to a presumed that receptor-rich membrane Gram-positive state after treatment with vesicles bud from the tubular extensions reducing or alkaline agents, whereas of CURL, and then return to the plasma Gram-positive organisms become membrane. Gram-negative after exposure to oxidizAn issue still to be resolved is the ing and acidic agents2. This redox mechanism by which receptors are scheme of Gram-staining reversal can be moved into the membrane-rich tubular further specified by using Schiff's extensions of CURL. Upon examin- reagent (an indicator of reducing groups

and a reducing agent as well). As well as shifting the Gram staining behavior of Gram-negative organisms to the Grampositive state, Schiff's reagent only stains the cytoplasmic membrane3. Moreover, the time required to obtain the pink Schiff's base on the cytoplasmic membrane is longer for Gram-negative than for Gram-positive organisms because Gram-negative cells must be reduced before a reaction can take place with Schiff's reagent. In the light of this and other evidence, we have postulated that one of the factors involved in the conversion-reconversion mechanism is based on the breaking of S-S bonds to sulphhydryl in the cytoplasmic membrane of Gram-positive cells and the rebuilding of stable new linkages during