Functional Characterization of Sequence Motifs in the Transit Peptide of Arabidopsis Small Subunit of Rubisco

The transit peptides of nuclear-encoded chloroplast proteins are necessary and sufficient for targeting and import of proteins into chloroplasts. However, the sequence information encoded by transit peptides is not fully understood. In this study, we investigated sequence motifs in the transit peptide of the small subunit of the Rubisco complex by examining the ability of various mutant transit peptides to target green fluorescent protein reporter proteins to chloroplasts in Arabidopsis (Arabidopsis thaliana) leaf protoplasts. We divided the transit peptide into eight blocks (T1 through T8), each consisting of eight or 10 amino acids, and generated mutants that had alanine (Ala) substitutions or deletions, of one or two T blocks in the transit peptide. In addition, we generated mutants that had the original sequence partially restored in single- or double-T-block Ala (A) substitution mutants. Analysis of chloroplast import of these mutants revealed several interesting observations. Single-T-block mutations did not noticeably affect targeting efficiency, except in T1 and T4 mutations. However, double-T mutants, T2A/T4A, T3A/T6A, T3A/T7A, T4A/T6A, and T4A/T7A, caused a 50% to 100% loss in targeting ability. T3A/T6A and T4A/T6A mutants produced only precursor proteins, whereas T2A/T4A and T4A/T7A mutants produced only a 37-kD protein. Detailed analyses revealed that sequence motifs ML in T1, LKSSA in T3, FP and RK in T4, CMQVW in T6, and KKFET in T7 play important roles in chloroplast targeting. In T1, the hydrophobicity of ML is important for targeting. LKSSA in T3 is functionally equivalent to CMQVW in T6 and KKFET in T7. Furthermore, subcellular fractionation revealed that Ala substitution in T1, T3, and T6 produced soluble precursors, whereas Ala substitution in T4 and T7 produced intermediates that were tightly associated with membranes. These results demonstrate that the transit peptide contains multiple motifs and that some of them act in concert or synergistically.