MECHANISM AND SIZE CUTOFF BAR STERIC EXCLUSION FROM ACTIN-RICH CYTOPLASMIC DOMAINS

Subdomains of the cytoplasmic volume in tissue culture cells exclude l arge tracer particles relative to small, Evidence suggests that exclus ion of the large particles is due to molecular sieving by the dense me shwork of microfilaments found in these compartments, but exclusion as a result of the close apposition of the dorsal and ventral plasma mem brane of the cell in these regions has not been ruled out conclusively . In principle, these two mechanisms can be distinguished by the depen dence of exclusion on tracer particle size, By fluorescence ratio imag ing we have measured the partition coefficient (P/P-o) into excluding compartments for tracer particles ranging in radius from 1 to 41 nm. T he decay of P/P-o as a function of particle radius is better fitted by three molecular sieving models than by a slit pore model. The sieving models predict a percolation cutoff radius of the order of 50 nm for partitioning into excluding compartments.