Verification of the Saccharina japonica Translocon Tic20 and its Localization in the Chloroplast Membrane in Diatoms.

Zhihang Chen,Delin Duan,Jianting Yao,Xiuliang Wang,Shuang Li,Zhanru Shao

doi:10.3390/ijms20164000

Abstract

Tic20 is an important translocon protein that plays a role in protein transport in the chloroplast. The sequence of Tic20 was determined in the lower brown alga Saccharina japonica. Structural analysis of SjTic20 revealed a noncanonical structure consisting of an N-terminal non-cyanobacterium-originated EF-hand domain (a helix-loop-helix structural domain) and a C-terminal cyanobacterium-originated Tic20 domain. Subcellular localization and transmembrane analysis indicated that SjTic20 featured an “M”-type Nin-Cin-terminal orientation, with four transmembrane domains in the innermost membrane of the chloroplast in the microalga Phaeodactylum tricornutum, and the EF-hand domain was entirely extruded into the chloroplast stroma. Our study provides information on the structure, localization, and topological features of SjTic20, and further functional analysis of SjTic20 in S. japonica is needed.

Highlights

Translocons at the outer/inner envelope (OE/IE) membranes of chloroplasts are hetero-oligomeric protein complexes, regarded as the major machinery responsible for the transport of nuclear-encoded preproteins into chloroplasts [1,2]
Reduced expression of Tic20-I can result in albinism, pale leaves, defects in preprotein import into chloroplasts, and significant growth defects in Arabidopsis thaliana [7,10,11]
Tic20 was characterized by being localized to the innermost plastid membrane via the prediction of 4 α-helixes in Bigelowiella natans [12], Cyanophora paradoxa [13], Ectocarpus siliculosus, and Fucus vesiculosus [14], but there is no experimental evidence to support this prediction. van Dooren et al [15] found that Toxoplasma gondii Tic20 localized to the innermost apicoplast membrane, and its transmembrane α-helixes exhibited Nin–Cin topology, consistent with the topology observed in higher plants; downregulated expression of TgTic20 caused deficiency in growth and impairment of apicoplast preprotein import

Summary

Introduction

Translocons at the outer/inner envelope (OE/IE) membranes of chloroplasts (designated as the TOC and TIC complexes, respectively) are hetero-oligomeric protein complexes, regarded as the major machinery responsible for the transport of nuclear-encoded preproteins into chloroplasts [1,2]. Tic is an important TIC complex that localizes to the inner membrane of chloroplasts [3,4,5] This complex is believed to be the channel for preprotein translocation [6,7,8]. The canonical Tic protein contains four transmembrane α-helixes with N- and C-termini oriented into the stroma (Nin–Cin), characterized by the “M”-like membrane spanning model [9]. Van Dooren et al [15] found that Toxoplasma gondii Tic localized to the innermost apicoplast membrane, and its transmembrane α-helixes exhibited Nin–Cin topology, consistent with the topology observed in higher plants; downregulated expression of TgTic caused deficiency in growth and impairment of apicoplast preprotein import. It is necessary to verify the locations and functions of these genes in kelp

Results

Discussion

Conclusion