Functional FTSH4 complexes in <i>Arabidopsis</i> mitochondria: a megacomplex with SLP1 and SLP1-free smaller complexes

The FTSH4 protease is a major component of the Arabidopsis mitochondrial protein quality control system. It has both a proteolytic and a chaperone-like activity and forms complexes anchored in the inner mitochondrial membrane. Here, we show that FTSH4 assembles into two distinct forms: a dominant high-molecular-weight megacomplex with stomatin-like protein 1 (SLP1), and smaller SLP1-free assemblies. In the slp1-1 mutant, the FTSH4-SLP1 megacomplex is absent, while the abundance of SLP1-free FTSH4 assemblies is nearly doubled. Despite this, slp1-1 maintains wild-type levels of FTSH4 substrates, TIM17-2 and NAD9, indicating that the SLP1-free assemblies retain proteolytic activity. Furthermore, slp1-1 mitochondria accumulate fewer detergent-resistant HSP23.6 aggregates under elevated temperature than ftsh4-1 and even wild-type. Consequently, the mitochondrial unfolded protein response reported in ftsh4-1 is not induced in slp1-1. Although slp1-1 plants display morphological changes previously associated with ftsh4-1, such as shorter inflorescence stems due to premature arrest of the shoot apical meristem, these are less pronounced. Taken together, the increased abundance of SLP1-free FTSH4 assemblies is sufficient to support general mitochondrial proteostasis, providing effective protection against heat-induced aggregation of mitochondrial proteins. In contrast, the FTSH4-SLP1 megacomplex more effectively fulfils the meristem-specific functions of FTSH4.