Arabidopsis BAG proteins regulate cellulose synthase stability
Cellulose synthase complexes (CSCs) synthesize cellulose at the plasma membrane, and their activity and trafficking are critical for maintaining cell wall integrity during plant growth. Clathrin-mediated endocytosis (CME) regulates CSC internalization and has been implicated in their rapid stress-induced removal from the plasma membrane. Stress adaptation, instead, requires the maintenance of a subset of CSCs at the plasma membrane, yet the mechanisms underlying this homeostasis remain poorly understood. The Arabidopsis Bcl-2-associated athanogene4 (BAG4) was identified as an interactor of the adaptor protein 2 complex (AP-2) and the TPLATE complex (TPC), two key components of plant CME. Here, we show that AP-2 and the TPC associated with four closely related BAG proteins, BAG1-BAG4. A quadruple mutant exhibited abnormal growth, increased sensitivity to salt stress, and reduced endocytic flux. However, the abundance, localization and dynamics of CME machinery was largely unaffected, suggesting that BAG proteins are not core regulators of CME. Instead, BAG1-BAG4 deficiency caused hypersensitivity to cellulose biosynthesis inhibitors and impaired hypocotyl elongation in darkness, consistent with defective cellulose-dependent growth. BAG1-BAG4 also interacted with CESA6, and salt-induced CESA6 degradation and ubiquitination was enhanced in the quadruple mutant. Together, these findings identify BAG1-BAG4 as redundant proteostasis factors that safeguard CESA6 stability during salt stress, thereby maintaining cellulose synthesis, cell wall integrity, and plant stress tolerance.