= Mouse-human heterokaryon membrane-mixing experiment
In 1970 <Larry Frye> and <Michael Edidin> fused mouse C11D and human VA-2 <cell lines> with <Sendai virus>, forming <heterokaryons>. Suspensions were maintained in a shaking water bath at $37\,{}^\circ\mathrm{C}$. Removed aliquots were stained by indirect <immunofluorescence>: <antibodies> against parental surface <antigens>, then differently fluorescent secondary <antibodies>. A Leitz Ortholux <fluorescence microscope> used a mercury lamp and color-selecting filters. Green mouse and red human surface patterns became intermixed.<The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons by Larry Frye and Michael Edidin>[Frye and Edidin, The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons, pp. 319–325: Summary, Cell fusion, Fluorescent staining, Fluorescence microscopy, Table 1, time-course results]{ref}
Same-species fusions tested <antibody specificity>; stable hybrid lines supplied a two-color control. Inhibitors of <biological translation> did not prevent mixing; lowering <temperature> inhibited it. These observations supported lateral redistribution over rapid synthesis. The authors could not conclusively exclude passage through <cytoplasm> and return. The readout compared populations, not trajectories of a continuously observed individual <biological cell>, and did not establish unrestricted motion of every <membrane protein>.<The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons by Larry Frye and Michael Edidin>[pp. 324–331: specificity controls, inhibitor studies, temperature dependence and Discussion of mechanisms iii and iv]{ref}
\Image[/membrane-protein-mobility-experiment.png]
{title=Separate time-point samples reveal changing surface distributions}
{description=Original qualitative schematic of the <mouse-human heterokaryon membrane-mixing experiment>. Dot counts and shapes are illustrative; separate samples show distributions rather than individual trajectories or measured mixing curves.}
The decisive design idea is to begin with spatially distinguishable material. A uniformly labeled surface can look unchanged while its components move. Bringing together differently identifiable parental surfaces creates an initial pattern whose loss reveals redistribution; <antibody specificity> makes that loss interpretable. <Membrane topology> asks which side a part faces, whereas this experiment asks where it can move along that side.
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