Alb4 of Arabidopsis Promotes Assembly and Stabilization of a Non Chlorophyll-Binding Photosynthetic Complex, the CF1CF0-ATP Synthase

All members of the YidC/Oxa1/Alb3 protein family are evolutionarily conserved and appear to function in membrane protein integration and protein complex stabilization. Here, we report on a second thylakoidal isoform of Alb3, named Alb4. Analysis of Arabidopsis knockout mutant lines shows that Alb4 is required in assembly and/or stability of the CF1CF0-ATP synthase (ATPase). alb4 mutant lines not only have reduced steady-state levels of ATPase subunits, but also their assembly into high-molecular-mass complexes is altered, leading to a reduction of ATP synthesis in the mutants. Moreover, we show that Alb4 but not Alb3 physically interacts with the subunits CF1 beta and CF0II. Summarizing, the data indicate that Alb4 functions to stabilize or promote assembly of CF1 during its attachment to the membrane-embedded CF0 part.