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Cell fusion experiments reveal distinctly different association characteristics of cell-surface receptors

Péter Nagy^1,2,3,*, László Mátyus^1,*, Attila Jenei^3,*, György Panyi¹, Sándor Varga⁴, János Matkó¹, János Szöllsi¹, Rezs Gáspár¹, Thomas M. Jovin³ and Sándor Damjanovich^1,

¹ Department of Biophysics and Cell Biology, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary
² Cell-biophysical Workgroup of the Hungarian Academy of Sciences, Nagyerdei krt. 98, PO Box 39, H-4012 Debrecen, Hungary
³ Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, Am Faßberg 11, D-37077 Göttingen, Germany
⁴ Central Research Service Laboratory, Medical and Health Science Center, University of Debrecen, H-4012 Debrecen, Hungary
^* These authors contributed equally to the work

Author for correspondence (e-mail: dami{at}jaguar.dote.hu)

Accepted July 26, 2001

The existence of small- and large-scale membrane protein clusters, containing dimers, oligomers and hundreds of proteins, respectively, has become widely accepted. However, it is largely unknown whether the internal structure of these formations is dynamic or static. Cell fusion was used to perturb the distribution of existing membrane protein clusters, and to investigate their mobility and associations. Scanning near-field optical microscopy, confocal and electron microscopy were applied to detect the exchange of proteins between large-scale protein clusters, whereas photobleaching fluorescence energy transfer was used to image the redistribution of existing small-scale membrane protein clusters. Large-scale clusters of major histocompatibility complex (MHC)-I exchanged proteins with each other and with MHC-II clusters. Similarly to MHC-I, large-scale MHC-II clusters were also dynamic. Exchange of components between small-scale protein clusters was not universal: intermixing did not take place in the case of MHC-II homoclusters; however, it was observed for homoclusters of MHC-I and for heteroclusters of MHC-I and MHC-II. These processes required a fluid state of the plasma membrane, and did not depend on endocytosis-mediated recycling of proteins. The redistribution of large-scale MHC-I clusters precedes the intermixing of small-scale clusters of MHC-I indicating a hierarchy in protein association. Investigation of a set of other proteins ( subunit of the interleukin 2 receptor, CD48 and transferrin receptor) suggested that a large-scale protein cluster usually exchanges components with the same type of clusters. These results offer new insight into processes requiring time-dependent changes in membrane protein interactions.

Key words: Fluorescence resonance energy transfer, FRET, Scanning near-field optical microscopy (SNOM), Lipid rafts, Protein association, Cell fusion

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