Expression of wild-type and variant AT cDNA in mammalian cells. COS 7 cells were transiently transfected with pcDNA3 vector constructs containing wild-type (WT), or variant AT cDNAs encoding P, M, V, T, G, or R at position –10, or with deletion (Del) of –10L. Cell lysate (l) and culture supernatants (s) were subsequently immunoprecipitated with polyclonal AT antibody. Mock-transfected COS 7 cells (COS) indicate nonspecific binding in the cell lysates. AT is exported to the supernatant only in cells transfected with the wild-type, -10M, -10V and –10L deletion constructs. The substitution of –10L by P, T, G, or R blocks processing and export of the variant AT proteins, with cell lysates showing the presence of only unprocessed and internally initiated protein. See Results for an interpretation of band sizes.