Identification of a chloroplast division mutant coding for ARC6H, an ARC6 homolog that plays a nonredundant role

A chloroplast division mutant arc6h was identified. ARC6H, a single Arabidopsis gene, is a homolog of ARC6 (accumulation and replication of chloroplasts mutant 6). arc6h produces only 1–13 enlarged chloroplasts/cell, with a median of 7. ARC6H rescued the loss of chloroplast division phenotype. Although arc6h appeared normal in tissue culture, plants were smaller in soil. Significant structural differences indicate that ARC6H and ARC6 are not functionally redundant. In contrast to ARC6, ARC6H lacks a J-domain characteristic of Hsp70 partner proteins, and has two transmembrane segments instead of one, indicating a different membrane topology. The arc6h mutation produces a stop codon deleting the sequence coding for a C-terminal region of ARC6H predicted to extend into the stroma. In transient expression assays a preprotein reporter with a putative transit peptide and the N-terminus of ARC6H fused to YFP was indeed imported into wild type chloroplasts. In some cells, YFP fluorescence was concentrated at the ends, or poles, of chloroplasts, similar to proteins required late in chloroplast division. Our results indicate that upon preprotein import, an N-terminal 10 amino acid region of ARC6H plays a key role in determining this novel localization pattern that reflects a specialized function for ARC6H as chloroplasts divide.