Western blots of mouse CTL total lysates (lanes 1, 3 and 5) or immunoprecipitated with antibodies against Slp1 (lane 2), Slp2-a (lane 4) or Slac2-c (lane 6), probed with anti-Rab27a or anti-Slp/Slac2 antibody. Immunoprecipitation with anti-Slp1, anti-Slp2-a or control anti-Slac2-c IgG was performed as described in the Materials and Methods. Coimmunoprecipitated Rab27a was detected by immunoblotting with anti-Rab27a antibody (indicated by arrowheads; lanes 2, 4 and 6 in the top panel). Immunoprecipitated Slp1, Slp2-a and Slac2-c were then visualized by immunoblotting with specific antibodies (indicated by arrowheads; lanes 2, 4 and 6 in the bottom panel). Non-specific bands of IgG used for immunoprecipitation are indicated by IgG. Note that both anti-Slp1 and anti-Slp2-a IgGs, but not Slac2-c IgG, immunoprecipitated Rab27a (lanes 2 and 4 in the top panel). Under our experimental conditions, anti-Slp1 IgG immunoprecipitated Slp1 protein (and also Rab27a protein) more efficiently than anti-Slp2-a IgG did Slp2-a protein. Relative migration of molecular weight markers (in kiloDalton) is shown on the left. IP, immunoprecipitate.