An actin filament matrix in hand-isolated nuclei of X. laevis oocytes.

The nuclear gel of Xenopus oocytes contains a meshwork of randomly oriented microfilaments which have been identified as F-actin by decoration with rabbit skeletal muscle myosin subfragment-1 (S-1). Nuclear gel preparations treated with S-1 differ in several respects from control preparations incubated in either aqueous medium alone, or medium containing BSA. Actin filaments in control preparations appear less well preserved than those in S-1 treated preparations of the nuclear gel. The nucleoli of control preparations are extremely dense, while those of S-1 treated preparations have a more open, granular appearance. Large granular aggregates, which are a prominent feature of the controls, are seen much less frequently in S-1-treated preparations of the nuclear gel. These morphological differences appear to be correlated with the binding of protein to F-actin, since nuclear gel preparations incubated in tropomyosin, which also binds to actin filaments, appear similar to those treated with S-1. Approximately 63% of the total nuclear actin exists in a globular state, while 37% is filamentous.