KDEL-tailed cysteine endopeptidases involved in programmed cell death, intercalation of new cells, and dismantling of extensin scaffolds
Am J Bot 2008 95: 1049-1062
Helm M, Schmid M, Hierl G, Terneus K, Tan L, Lottspeich F, Kieliszewski MJ, Gietl C

Abstract
KDEL-tailed cysteine endopeptidases are a group of papain-type peptidases found in senescing tissue undergoing programmed cell death (PCD). Their genes have so far been cloned and analyzed in 12 angiosperms. They are synthesized as proenzymes with a C-terminal KDEL endoplasmatic reticulum retention signal, which is removed with the prosequence to activate enzyme activity. We previously identified three genes for KDEL-tailed cysteine endopeptidases (AtCEP1, AtCEP2, AtCEP3) in Arabidopsis thaliana. Transgenic plants of A. thaliana expressing beta-glucuronidase (GUS) under the control of the promoters for the three genes were produced and analyzed histochemically. GUS activity was promoter- and tissue-specific GUS activity during seedling, flower, and root development, especially in tissues that collapse during final stages of PCD, and in the course of lateral root formation. KDEL-tailed cysteine endopeptidases are unique in being able to digest the extensins that form the basic scaffold for cell wall formation. The broad substrate specificity is due to the structure of the active site cleft of the KDEL-tailed cysteine endopeptidase that accepts a wide variety of amino acids, including proline and glycosylated hydroxyproline of the hydroxyproline rich glycoproteins of the cell wall.
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