Suppressors of the Chloroplast Protein Import Mutant <i>tic40</i> Reveal a Genetic Link between Protein Import and Thylakoid Biogenesis

Abstract

To extend our understanding of chloroplast protein import and the role played by the import machinery component Tic40, we performed a genetic screen for suppressors of chlorotic <i>tic40</i> knockout mutant <i>Arabidopsis thaliana</i> plants. As a result, two <i>suppressor of tic40</i> loci, <i>stic1</i> and <i>stic2</i>, were identified and characterized. The <i>stic1</i> locus corresponds to the gene <i>ALBINO4</i> (<i>ALB4</i>), which encodes a paralog of the well-known thylakoid protein targeting factor ALB3. The <i>stic2</i> locus identified a previously unknown stromal protein that interacts physically with both ALB4 and ALB3. Genetic studies showed that ALB4 and STIC2 act together in a common pathway that also involves cpSRP54 and cpFtsY. Thus, we conclude that ALB4 and STIC2 both participate in thylakoid protein targeting, potentially for a specific subset of thylakoidal proteins, and that this targeting pathway becomes disadvantageous to the plant in the absence of Tic40. As the <i>stic1</i> and <i>stic2</i> mutants both suppressed <i>tic40</i> specifically (other TIC-related mutants were not suppressed), we hypothesize that Tic40 is a multifunctional protein that, in addition to its originally described role in protein import, is able to influence downstream processes leading to thylakoid biogenesis.