Molecular sieve of cytoplasmic ER and PgR.

Cytosol from immature rabbit uteri was classified into six groups according to range of molecular weights. The molecular sieve can be achieved by AMICON ultrafiltration using membranes of different pore sizes. The results revealed that almost 50% of the total activity of ER and PgR was confined to the largest fraction with a molecular weight greater than 300,000. A detailed study by electrophoretic characterization in later experiments demonstrated that the cytoplasmic ER and PgR exist mainly in high polymer forms.