A SHELL OF F-ACTIN SURROUNDS THE BRANCHED NUCLEI OF SILK GLAND-CELLS

The branched nuclei from silk gland cells of larvae of Calpodes ethlius label with antibodies to actin and myosin and with rhodaminyl-phalloin, which is specific for f-actin. Optical sectioning localizes this actin and myosin to the nuclear periphery. Residual nuclear-associated fractions prepared from these cells contain sheets of nuclear lamina-like structures that bind heavy meromyosin and gold-tagged antibodies to actin and myosin. The results suggest that both actin and myosin, or a myosin-like protein, are components of a layer at the nucleocytoplasmic boundary that we call the nuclear shell. The nuclear shell appears to be associated with the nuclear envelope and may correspond to a zone on the cytoplasmic face of the envelope seen in electron micrographs of unextracted cells. The residual nuclear-associated fraction has a unique isoform of actin (43 kD, pl 6.45) that might allow the nuclei to associate with an actin network structurally and developmentally distinct from that of the cytoplasm.