Abstract
The protein import translocon at the inner envelope of chloroplasts (Tic complex) is a heteroligomeric multisubunit complex. Here, we describe Tic40 from pea as a new component of this complex. Tic40 from pea is a homologue of a protein described earlier from Brassica napus as Cim/Com44 or the Toc36 subunit of the translocon at the outer envelope of chloroplasts, respectively (Wu, C., Seibert, F. S., and Ko, K. (1994) J. Biol. Chem. 269, 32264-32271; Ko, K., Budd, D., Wu, C., Seibert, F., Kourtz, L., and Ko, Z. W. (1995) J. Biol. Chem. 270, 28601-28608; Pang, P., Meathrel, K., and Ko, K. (1997) J. Biol. Chem. 272, 25623-25627). Tic40 can be covalently connected to Tic110 by the formation of a disulfide bridge under oxidizing conditions, indicating its close physical proximity to an established translocon component. The Tic40 protein is synthesized in the cytosol as a precursor with an N-terminal cleavable chloroplast targeting signal and imported into the organelle via the general import pathway. Immunoblotting and immunogold-labeling studies exclusively confine Tic40 to the chloroplastic inner envelope, in which it is anchored by a single putative transmembrane span.
Highlights
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AJ243758
In this paper we report that a 44-kDa polypeptide from pea is a bona fide component of the Tic complex
The results show that Tic40 is almost exclusively detected in the inner envelope membrane fractions as is Tic110, whereas Toc34 is largely confined to the outer envelope membrane fraction (Fig. 5A)
Summary
Translocon at the outer envelope membrane of chloroplasts; ␣, antiserum; Cim, chloroplast inner envelope membrane protein; Com, chloroplast outer envelope membrane protein; DSP, dithiobissuccinimidyl propionate; Hsp, heat shock protein; PAGE, polyacrylamide gel electrophoresis; preSSU, precursor of ribulose-bisphosphate carboxylase/oxygenase small subunit; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; Tic, translocon at the inner envelope membrane of chloroplasts. A certain population of joint Toc and Tic translocation sites is present even in the absence of precursor proteins It is not clear, if contact sites between the outer and the inner envelope membranes, which can be seen in electron microscopic pictures (5, 26 –28), are identical with or related to joint protein translocation sites. Isolated Toc complexes from purified chloroplastic outer envelopes still contain the prominent Tic components and vice versa [20, 21, 23, 31]. It is, important to establish the exact localization of a new translocon component.
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