Fig. 1. The HSC associates with membranes.

A, following hypotonic lysis and fractionation of Drosophila embryos, total cellular membranes were sequentially extracted by homogenization with a Dounce in lysis buffer containing the indicated concentrations of NaCl or finally 1% Nonidet P-40 (NP40). The cytosol and sequential extractions were normalized to protein, separated by SDS-PAGE, and immunoblotted for the indicated proteins. B, total embryo membranes were washed in lysis buffer containing 0.15 m NaCl and then extracted with lysis buffer containing 0.5 m NaCl (producing the S4 fraction). This salt extraction was immunoprecipitated with antibodies to Fu or Cos2 or with rabbit IgG or mouse IgG₁ as controls (Ctrl.). The immunoprecipitates were separated by SDS-PAGE and immunoblotted for the indicated proteins. C, the cytosolic fraction (top panel) and S4 extraction of embryo cellular membranes (middle and bottom panels) were fractionated by size on a Superose 6 column. The size exclusion column was equilibrated in lysis buffer containing 0.15 m NaCl (top panel), 0.5 m NaCl (middle panel), or 1 m NaCl (bottom panel). Every other fraction was immunoblotted for Fu. Fractions where standard size markers elute are indicated. Peaks A and B of Fu elution are referred to in the text.