Haptophytes are abundant phytoplankton that possess 4 membrane-bound chloroplasts. These chloroplasts originated from a red alga that was taken up by a eukaryotic host cell. No previous work has reported on the protein targeting of haptophytes, except for computational analyses. We isolated the two genes encoding chloroplast proteins AtpC1 and FtsZ from Pavlova pinguis and analyzed their molecular structure. These proteins had a bipartite sequence at the N-terminus, which consisted of an endoplasmic reticulum (ER) signal sequence followed by a chloroplast transit peptide. To demonstrate the functionality of the ER signal sequences and the chloroplast targeting sequences in vivo, we fused the predicted ER signal sequence, chloroplast transit peptide, and bipartite sequence of AtpC1 and FtsZ individually to the N-terminus of green fluorescent protein (sGFP). This was then introduced into cultured tobacco cells. Microscopic observation revealed that the predicted ER signal sequences of AtpC1 and FtsZ had the ability to localize to the ER, and the following amino acids worked as a chloroplast transit peptide in the tobacco cells. We concluded that the AtpC1 and FtsZ of P. pinguis have a bipartite sequence at the N-terminus and a molecular structure in common with other independently established secondarily endosymbiotic algae.
(2010), The Japan Mendel Society