Essential for plastoquinone-9 (PQ-9) biosynthesis through its interaction with the solanesyl diphosphate synthase SPS2 (PubMed:28751900). Binds to the hydrophobic solanesyl moiety, which is generated by SPS2, in FBN5-SPS homodimeric complexes to stimulates the enzyme activity of SPS2 (Probable). Interacts with SPS2. Reduced growth and sometimes seedling lethality (PubMed:28751900). Strong reduction of levels of plastoquinone-9 (PQ-9) and complete loss of plastochromanol-8 (PC-8) in leaves (PubMed:28751900). Severe increase of photoinhibition at high light intensity (PubMed:28751900). Belongs to the PAP/fibrillin family.