The biological functions of FtsH in plant organelle protein homeostasis

Both mitochondria and chloroplasts are capable of driving the electron transport chain reactions for ATP production that are essential for plant growth, development, and stress resilience. The maintenance of organelle protein homeostasis relies on the coordinated assembly of membrane protein complexes and the degradation of unassembled or damaged subunits. FtsH proteins, containing an ATPase domain and a protease domain, are a highly conserved ATP-dependent protease family in photosynthetic organisms. In plants, different FtsH family members are targeted to specific organelle membranes to orchestrate protein homeostasis. This review provides an updated overview of the functions of plant FtsHs in mitochondrial and chloroplastic protein homeostasis, focusing on their protease activity, chaperone-like functions, and substrate processing capabilities. Additionally, it highlights the regulatory mechanisms currently known to modulate the activity and stability of FtsH. Furthermore, we summarize the genetic modifier loci of AtFtsH2/VAR2 encoding the AtFtsH2 subunit of the thylakoid FtsH complex in Arabidopsis and also propose a potential role for the thylakoid FtsH in the degradation of unassembled photosynthetic proteins, particularly under conditions in which the chlorophyll biosynthetic pathway is disrupted.