YlmG1 is localized exclusively to the chloroplast envelope membrane and is involved in preprotein translocation in Arabidopsis thaliana

In green lineages, chloroplast protein import is mediated by conserved TOC and TIC translocons at the outer and inner envelope membranes, respectively. A recent structural study on TIC from Chlamydomonas reinhardtii revealed an additionally associated YGGT family membrane protein YlmG. Its homolog in Arabidopsis thaliana, YlmG1, is essential but was reported to be a thylakoid-localized membrane protein involved in nucleoid distribution. Thus, it remains unclear whether YlmG1 participates in chloroplast protein import. Here, we show that YlmG1 is exclusively localized to the chloroplast envelope membrane where it behaves like a monomer. However, while YlmG1 is not firmly associated with TIC, it is recruited to the preprotein-translocation intermediate complex together with other TIC components, suggesting it has an important role in chloroplast protein import.