ROLE OF ENDOPLASMIC RETICULUM CHAPERONE ENDOPLASMIN IN THE SYNTHESIS AND ACTIVITY OF CLAVATA3

Regulation of root and shoot apical meristems is tightly regulated by CLAVATA (CLV) genes. These genes encode for three proteins that form an active complex on the cell surface: CLV1 and CLV2 form a plasma membrane receptor complex, activated by an extracellular short peptide derived from the soluble secretory protein CLV3. Endoplasmin/GRP94 is the endoplasmic reticulum (ER)-located member of the HSP90 protein class, but little is known about its function and its protein clients in plants. It has been shown that mutated Arabidopsis thaliana plants expressing highly reduced amounts of endoplasmin show a Clavata-like phenotype, indicating a role for endoplasmin in the formation of the CLV complex. By transient expression in tobacco protoplasts, we show that the synthesis of a functional fusion between CLV3 and GFP is supported by endoplasmin and that this support is stronger than the one performed by the chaperone BiP, the ER-located HSP70. This provides biochemical evidence that CLV3 is an endoplasmin client. We also suggest that the proteolytic maturation of CLV3, leading to the formation of the active peptide, starts early in the cell, possibly within the ER