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© The Rockefeller University Press, 0021-9525/1999/10/33/ $5.00
The Journal of Cell Biology, Volume 147, Number 1, October 4, 1999 33-44

Original Article

Stromal Processing Peptidase Binds Transit Peptides and Initiates Their ATP-dependent Turnover in Chloroplasts

Stefan Richtera and Gayle K. Lamppaa
a Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois 60637

Correspondence to: Gayle K. Lamppa, Department of Molecular Genetics and Cell Biology, University of Chicago, 920 East 58th Street, Chicago, Illinois 60637. Tel:(773) 702-9837 Fax:(773) 702-3172 E-mail:gklamppa{at}midway.uchicago.edu.

A stromal processing peptidase (SPP) cleaves a broad range of precursors targeted to the chloroplast, yielding proteins for numerous biosynthetic pathways in different compartments. SPP contains a signature zinc-binding motif, His-X-X-Glu-His, that places it in a metallopeptidase family which includes the mitochondrial processing peptidase. Here, we have investigated the mechanism of cleavage by SPP, a late, yet key event in the import pathway. Recombinant SPP removed the transit peptide from a variety of precursors in a single endoproteolytic step. Whereas the mature protein was immediately released, the transit peptide remained bound to SPP. SPP converted the transit peptide to a subfragment form that it no longer recognized. We conclude that SPP contains a specific binding site for the transit peptide and additional proteolysis by SPP triggers its release. A stable interaction between SPP and an intact transit peptide was directly demonstrated using a newly developed binding assay. Unlike recombinant SPP, a chloroplast extract rapidly degraded both the transit peptide and subfragment. A new degradative activity, distinguishable from SPP, was identified that is ATP- and metal-dependent. Our results indicate a regulated sequence of events as SPP functions during precursor import, and demonstrate a previously unrecognized ATP-requirement for transit peptide turnover.

Key Words: chloroplast, metallopeptidase, protein degradation, stromal processing peptidase, transit peptide


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