| 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 14 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 CLV3-related proteins (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, Arabidopsis and BY-2 culture media that process CLV3, CLE1 and plant nematode CLEs at their C-termini. For CLV3, processing appears to occur at the highly conserved Arg70 found at the beginning of the CLE domain. We propose that CLV3 and other CLEs are C-terminally processed to generate active CLE peptides. |
