The characteristic shapes and sizes of organs are established by cell proliferation patterns and final cell sizes, but the underlying molecular mechanisms coordinating these are poorly understood. Here we characterize a ubiquitinactivated peptidase called DA1 that limits the duration of cell proliferation during organ growth in Arabidopsis thaliana. The peptidase is activated by two RING E3 ligases, Big Brother (BB) and DA2, which are subsequently cleaved by the activated peptidase and destabilized. In the case of BB, cleavage leads to destabilization by the RING E3 ligase PROTEOLYSIS 1 (PRT1) of the N-end rule pathway. DA1 peptidase activity also cleaves the deubiquitylase UBP15, which promotes cell proliferation, and the transcription factors TEOSINTE BRANCED 1/CYCLOIDEA/ PCF 15 (TCP15) and TCP22, which promote cell proliferation and repress endoreduplication. We propose that DA1 peptidase activity regulates the duration of cell proliferation and the transition to endoreduplication and differentiation during organ formation in plants by coordinating the destabilization of regulatory proteins.

The characteristic shapes and sizes of organs are established by cell proliferation patterns and final cell sizes, but the underlying molecular mechanisms coordinating these are poorly understood. Here we characterize a ubiquitin-activated peptidase called DA1 that limits the duration of cell proliferation during organ growth in Arabidopsis thaliana. The peptidase is activated by two RING E3 ligases, BB and DA2, which are subsequently cleaved by the activated peptidase and destabilized. In the case of BB, cleavage leads to destabilization by the RING E3 ligase PRT1 of the N-end rule pathway. DA1 peptidase activity also cleaves the de-ubiquitylase UBP15, which promotes cell proliferation, and the transcription factors TCP15 and TCP22, which promote cell proliferation proliferation and repress endoreduplication. We propose that DA1 peptidase activity regulates the duration of cell proliferation and the transition to endoreduplication and differentiation during organ formation in plants by coordinating the destabilization of regulatory proteins.

Osteoarthritis (OA) is a debilitating degenerative joint disease that results in chronic pain and inflammation due to progressive mechanical and proteolytic cartilage degradation. Articular chondrocytes, the main cell type present in cartilage, are responsible for the deposition and maintenance of the cartilage extracellular matrix (ECM). However, following damage and inflammation, chondrocytes undergo hypertrophy, apoptosis, and contribute to inflammation and ECM degradation. NF-κB signaling is known to be dysregulated in OA. TRAPPC9, a vesicle trafficking protein, is known to directly activate NF-κB signaling in neuronal and bone cells, however, the biological significance of this protein in chondrocytes has yet to be elucidated. Here, we demonstrate that TRAPPC9 enhances pro-inflammatory gene and protein expression in murine primary articular chondrocytes. Furthermore, we show that TRAPPC9 elicits these responses via phosphorylation of P-100 that activates non-canonical NF-κB signaling. Taken together, these findings suggest TRAPPC9 may be a potential therapeutic target to decrease inflammation and matrix degradation during OA pathology.