Evidence for Functional Conservation, Sufficiency, and Proteolytic Processing of the CLAVATA3 CLE Domain

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Abstract
Arabidopsis (Arabidopsis thaliana) CLAVATA3 (CLV3) is hypothesized to act as a ligand for the CLV1 receptor kinase in the regulation of stem cell specification at shoot and flower meristems. CLV3 is a secreted protein, with an amino-terminal signal sequence and a conserved C-terminal domain of 15 amino acids, termed the CLE (CLV3/ESR-related) domain, based on its similarity to a largely unstudied protein family broadly present in land plants. We have tested the function of 13 Arabidopsis CLEs in vivo and found a significant variability in the ability of CLEs to replace CLV3, ranging from complete to no complementation. The best rescuing CLE depends on CLV1 for function, while other CLEs act independently of CLV1. Domain-swap experiments indicate that differences in function can be traced to the CLE domain within these proteins. Indeed, when the CLE domain of CLV3 is placed downstream of an unrelated signal sequence, it is capable of fully replacing CLV3 function. Finally, we have detected proteolytic activity in extracts from cauliflower (Brassica oleracea) that process both CLV3 and CLE1 at their C termini. For CLV3, processing appears to occur at the absolutely conserved arginine-70 found at the beginning of the CLE domain. We propose that CLV3 and other CLEs are C-terminally processed to generate an active CLE peptide.