FIG. 7.

Cleavage patterns of mutant alkaline phosphatases containing two signal peptides. The sequences of these signal peptides are further described in reference 7. Cells were radiolabeled, and alkaline phosphatase was immunoprecipitated. The various alkaline phosphatase species are identified as follows: p* is the precursor form which retains both signal peptides arranged in tandem; m′ results from cleavage after the first of these (SP1); m" results from cleavage after the second (SP2). The i marks the position of a small amount of an intermediate cleavage product. The signal peptidase cleavage sites are marked by arrows. Schematic representations of the relative locations of the two signal peptides in the precursors whose results are shown in panels A, B, and C are shown in panels D, E, and F, respectively. TSF and TLF are molecular weight markers (6); TLF corresponds to the approximate size of the fragment amino-terminal to the SP2 cleavage site, and TSF corresponds to the size of m" (6) in panels C and F.